Tone and Voice - A Derivation of The Rules of Voice-Leading From Perceptual Principles PDF

2001 BY THE
Music Perception
Fall 2001, Vol. 19, No. 1, 164
REGENTS OF THE UNIVERSITY OF CALIFORNIA

ALL RIGHTS RESERVED.
Tone and Voice: A Derivation of the Rules of VoiceLeading from Perceptual Principles
D AV I D H U R O N
School of Music, Ohio State University

The traditional rules of voice-leading in Western music are explicated
using experimentally established perceptual principles. Six core principles
are shown to account for the majority of voice-leading rules given in
historical and contemporary music theory tracts. These principles are
treated in a manner akin to axioms in a formal system from which the
traditional rules of voice-leading are derived. Nontraditional rules arising from the derivation are shown to predict formerly unnoticed aspects
of voice-leading practice. In addition to the core perceptual principles,
several auxiliary principles are described. These auxiliary principles are
occasionally linked to voice-leading practice and may be regarded as compositional options that shape the music-making in perceptually unique
ways. It is suggested that these auxiliary principles distinguish different
types of part writing, such as polyphony, homophony, and close harmony. A theory is proposed to account for the aesthetic origin of voiceleading practices.
Received November 20, 1996, accepted June 25, 2001
N the training of Western musicians, the canon of harmony and voiceleading rules has been considered one of the essential foundations of the
art-music craft. Of course not all composers accede to the norms of traditional harmony or voice-leading. Nor should they. Rules ought to be followed only when the composer agrees that the goal or goals embodied by
the rules form worthy musical objectives, and when the rules themselves
provide an effective means by which the goal(s) may be achieved. We can,
therefore, ask three questions of any procedural rule: (1) What goal is served
by following the rule? (2) Is the goal worthwhile? and (3) Is the rule an
effective way of achieving the purported goal?
The practice of Western harmony and voice-leading has been the subject
of extensive theoretical attention and elucidation (Aldwell & Schachter,
Address correspondence to David Huron, School of Music, Ohio State University, 1866
College Rd., Columbus, OH 43210.
ISSN: 0730-7829. Send requests for permission to reprint to Rights and Permissions,
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David Huron
1989; Berardi, 1681; Ftis, 1840; Fux, 1725; Hindemith, 1944; Horwood,
1948; Keys, 1961; Morris, 1946; Parncutt, 1989; Piston, 1978; Rameau,
1722; Riemann, 1903; Schenker, 1906; Schoenberg, 1911/1978; Stainer,
1878; and many others1). Over the centuries, theorists have generated a
wealth of lucid insights pertaining to harmony and voice-leading. Earlier
theorists tended to view the voice-leading canon as a set of fixed or inviolable universals. More recently, theorists have suggested that the voice-leading canon can be usefully regarded as descriptive of a particular, historically encapsulated, musical convention. A number of twentieth-century
theorists have endeavored to build harmonic and structural theories on the
foundation of voice-leading.
Traditionally, the so-called rules of harmony are divided into two broad
groups: (1) the rules of harmonic progression and (2) the rules of voiceleading. The first group of rules pertains to the choice of chords, including
the overall harmonic plan of a work, the placement and formation of cadences, and the moment-to-moment succession of individual chords. The
second group of rules pertains to the manner in which individual parts or
voices move from tone to tone in successive sonorities. The term voiceleading originates from the German Stimmfhrung, and refers to the direction of movement for tones within a single part or voice. A number of
theorists have suggested that the principal purpose of voice-leading is to
create perceptually independent musical lines. This article develops a detailed exposition in support of this view.
In this article, the focus is exclusively on the practice of voice-leading.
No attempt will be made here to account for the rules of harmonic progression. The goal is to explain voice-leading practice by using perceptual principles, predominantly principles associated with the theory of auditory
stream segregation (Bregman, 1990; McAdams & Bregman, 1979; van
Noorden, 1975; Wright, 1986; Wright & Bregman, 1987). As will be seen,
this approach provides an especially strong account of Western voice-leading practice. Indeed, in the middle section of this article, a derivation of the
traditional rules of voice-leading will be given. At the end of the article, a
cognitive theory of the aesthetic origins of voice-leading will be proposed.
Understood in its broader sense, voice-leading also connotes the dynamic quality of tones leading somewhere. A full account of voice-leading
would entail a psychological explanation of how melodic expectations and
implications arise and would also explain the phenomenal experiences of
tension and resolution. These issues have been addressed periodically
throughout the history of music theory and remain the subject of continu1. No list of citations here could do justice to the volume and breadth of writings on this
subject. This list is intended to be illustrative by including both major and minor writers,
composers and noncomposers, dydactic and descriptive authors, different periods and nationalities.
The Rules of Voice-Leading
ing investigation and theorizing (e.g., Narmour, 1991, 1992). Many fundamental issues remain unresolved (see von Hippel & Huron, 2000), and so
it would appear to be premature to attempt to explain the expectational
aspects of tone successions. Consequently, the focus in this article will be
limited to the conventional rules of voice-leading.
The purpose of this article is to address the three questions posed earlier,
namely, to identify the goals of voice-leading, to show how following the
traditional voice-leading rules contributes to the achievement of these goals,
and to propose a cognitive explanation for why the goals might be deemed
worthwhile in the first place.
Many of the points demonstrated in this work will seem trivial or intuitively obvious to experienced musicians or music theorists. It is an unfortunate consequence of the pursuit of rigor that an analysis can take an
inordinate amount of time to arrive at a conclusion that is utterly expected.
As in scientific and philosophical endeavors generally, attempts to explicate common sense will have an air of futility to those for whom the
origins of intuition are not problematic. However, such detailed studies
can unravel the specific origins from which intuitions arise, contribute to a
more precise formulation of existing knowledge, and expose inconsistencies worthy of further study.
Plan
My presentation will proceed according to the following plan. First, the
principal rules of traditional voice-leading will be reviewed. Following this
review, 10 principles of auditory perception will be described and their
musical implications identified. The perceptual principles described will
initially be limited to a core set of 6 principles that are most pertinent to
understanding voice-leading. These principles can be treated in a manner
akin to axioms in a formal system from which a set of propositions can be
derived.
In Part II, we will attempt to account for the traditional rules of voiceleading. In the process of the derivation, several novel rules will arise that
are not normally found in theoretical writings on voice-leading. These novel
rules can be regarded as theory-derived predictions. For these novel rules,
we will examine compositional practice to determine whether composers
typically write in a manner consistent with these additional rules. To foreshadow the results, we will see that voice-leading practices are indeed consistent with several nontraditional rules predicted by the perceptual principles.
Part III presents four additional perceptual principles that are occasionally linked to the practice of Western voice-leading. In essence, these auxil-
David Huron
iary principles constitute options that shape the music-making in perceptually unique ways. Composers can elect to include or exclude one or
more of these auxiliary principles depending on the implied perceptual goal.
Once again, these principles can be treated in a manner akin to axioms in
formal logic and selectively added to the core group of six central principles. Depending on the choice of auxiliary principles, we will see that
alternative voice-leading systems arise. It will be argued that the choice of
such auxiliary principles is one of the hallmarks of musical genres; these
auxiliary principles shed light on such matters as the distinction between
homophonic and polyphonic voice-leading and such unique genres as close
harmony (such as the voice-leading found in barbershop quartets).
In the concluding section, a psychological account will be advanced whose
goal is to explain why conventional voice-leading might be experienced by
many listeners as pleasing. A number of testable predictions arise from this
account. The article closes by identifying a number of unresolved issues
and posing questions for further research.
Note that the purpose of this article is not somehow to vindicate or
otherwise act as an apologist for the traditional rules of Western harmony.
Nor is the intent to restrict in any way the creative enterprise of musical
composition. If a composer chooses a particular goal (either implicitly or
explicitly), then there are frequently natural consequences that may constrain the music making in such a way as to make the goal achievable.
Musically pertinent goals might include social, political, historical, formal,
perceptual, emotional, cognitive, and/or other objectives. The purpose of
this article is merely to identify and clarify some of the perceptual and
cognitive aspects that shape music making. In pursuing this analysis, readers should not assume that other musically pertinent goals are somehow
less important or irrelevant in understanding music.
Part I: Perceptual Principles and Western Voice-Leading

RULES OF VOICE-LEADING REVIEWED
We may begin by reviewing briefly the traditional rules of voice-leading.

Of course there is no such thing as the rules of voice-leading. Theoretical
and pedagogical treatises on harmony differ in detail. Some texts identify a
handful of voice-leading rules, whereas other texts define dozens of rules
and their exceptions. Different pedagogical works emphasize 16th-century
modal counterpoint (e.g., Gauldin, 1985; Jeppesen, 1939/1963), 18th-century Bach-style counterpoint (e.g., Benjamin, 1986; Parks, 1984; Trythall,
1993), or harmony (e.g., Aldwell & Schachter, 1989; Piston, 1978). Despite this variety, there is a core group of some dozen-odd rules that ap-
pears in nearly every pedagogical work on voice-leading. These core voiceleading rules include the following:
1.
Registral Compass Rule. Harmony texts begin by specifying

the pitch range for harmonic writing. Students should write in
the region between F2 and G5. Although this region corresponds
to the combined pitch range for typical male and female voices,
it is noteworthy that students of harmony are encouraged to
write in this region, even when writing purely instrumental
music.
2. Textural Density Rule. Harmony should be written using three
or more concurrent parts or voices. The most common
harmonic writing employs four voices set in overlapping pitch
regions or tessituras: soprano, alto, tenor, and bass.
3. Chord Spacing Rule. In the spacing of chordal tones, no more
than an octave should separate the soprano and alto voices.
Similarly, no more than an octave should separate the alto and
tenor voices. In the case of the bass and tenor voices, however,
no restriction is placed on the distance separating them.
4. Avoid Unisons Rule. Two voices should not share the same
concurrent pitch.
5. Common Tone Rule. Pitches common to consecutive chords
should be retained in the same voice or part.
6. Nearest Chordal Tone Rule. If a part cannot retain the same
pitch in the next sonority, the part should move to the nearest
available pitch.
7. Conjunct Motion Rule. When a part must change pitch, the
preferred pitch motion should be by diatonic step. Sometimes
this rule is expressed in reverse:
8. Avoid Leaps Rule. Large melodic intervals should be avoided.
9. Part-Crossing Rule. Parts should not cross with respect to pitch.
10. Part Overlap Rule. No part should move to a pitch higher than
the immediately preceding pitch in an ostensibly higher part.
Similarly, no part should move to a pitch lower than the immediately preceding pitch in an ostensibly lower part.
11. Parallel Unisons, Fifths, and Octaves Rule. No two voices
should move in parallel octaves, fifths, or unisons. In the case
of many theorists, a more stringent version of this rule is used:
12. Consecutive Unisons, Fifths, and Octaves Rule. No two voices
should form consecutive unisons, octaves, fifteenths (or any
combination thereof)whether or not the parts move in parallel. Also, no two voices should form consecutive fifths,
twelfths, nineteenths (or any combination thereof).
David Huron
13. Exposed (or Hidden or Direct) Octaves (and Fifths) Rule.

Unisons, perfect fifths, octaves, twelfths, and so on should not
be approached by similar motion between two parts unless at
least one of the parts moves by diatonic step. Most theorists
restrict this rule to the case of approaching an octave (fifteenth,
etc.), whereas some theorists also extend this injunction to approaching perfect fifths (twelfths, etc.). Most theorists restrict
this rule to voice-leading involving the bass and soprano voices;
in other cases, exposed intervals between any pair of voices are
forbidden.
Note that several traditional voice-leading rules are not included in the
preceding summary. Most notably, the innumerable rules pertaining to
chordal tone doubling (e.g., avoid doubling chromatic or leading tones)
are absent. Also missing are the rules prohibiting movement by augmented
intervals and the rule prohibiting false relations. No attempt will be made
here to account for these latter rules (see Huron, 1993b).
Over the centuries, a number of theorists have explicitly suggested that
voice-leading is the art of creating independent parts or voices. In the ensuing discussion, we will take these suggestions literally and link voice-leading practice to perceptual research concerning the formation of auditory
images and independent auditory streams.
PERCEPTUAL PRINCIPLES
Having briefly reviewed the voice-leading canon, we can now identify

and define the pertinent perceptual principles. In summary, the following
six principles will be discussed: (1) toneness, (2) temporal continuity, (3)
minimum masking, (4) tonal fusion, (5) pitch proximity, and (6) pitch comodulation. Later, we will examine four auxiliary principles that contribute to the differentiation of musical genres: (7) onset synchrony, (8) limited
density, (9) timbral differentiation, and (10) source location.
1. Toneness Principle
Sounds may be regarded as evoking perceptual images. An auditory
image may be defined as the subjective experience of a singular sonic entity
or acoustic object. Such images are often (although not always) linked to
the recognition of the sounds source or acoustic origin. Hence certain bands
of frequencies undergoing a shift of phase may evoke the image of a passing automobile.
Not all sounds evoke equally clear mental images. In the first instance,
aperiodic noises evoke more diffuse images than do pure tones. Even when
noise bands are widely separated and masking is absent, listeners are much
less adept at identifying the number of noise bands present in a sound field
than an equivalent number of pure tonesprovided the tones are not
harmonically related (see below).
In the second instance, certain sets of pure tones may coalesce to form a
single auditory imageas in the case of the perception of a complex tone.
For complex tones, the perceptual image is strongly associated with the
pitch evoked by a set of spectral components. One of the best demonstrations of this association is to be found in phenomenon of residue pitch
(Schouten, 1940). In the case of a residue pitch, a complex harmonic tone
whose fundamental is absent nevertheless evokes a single notable pitch
corresponding roughly to the missing fundamental.
In the third instance, tones having inharmonic partials tend to evoke
more diffuse auditory images. Inharmonic partials are more likely to be
resolved as independent tones, and so the spectral components are less apt
to cohere and be perceived as a single sound. In a related way, inharmonic
tones also tend to evoke competing pitch perceptions, such as in the case of
bells. The least ambiguous pitches are evoked by complex tones whose
partials most closely approximate a harmonic series.
In general, pitched sounds are easier to identify as independent sound
sources, and sounds that produce the strongest and least ambiguous pitches
produce the clearest auditory images. Pitch provides a convenient hanger
on which to hang a set of spectral components and to attribute them to a
single acoustic source. Although pitch is typically regarded as a subjective
impression of highness or lowness, the more fundamental feature of the
phenomenon of pitch is that it provides a useful internal subjective label
a perceptual handlethat represents a collection of partials likely to have
been evoked by a single physical source. Inharmonic tones and noises can
also evoke auditory images, but they are typically more diffuse or ambiguous.
Psychoacousticians have used the term tonality to refer to the clarity of
pitch perceptions (ANSI, 1973); however, this choice of terms is musically
unfortunate. Inspired by research on pitch strength or pitch salience carried out by Terhardt and others, Parncutt (1989) proposed the term
tonalness; we will use the less ambiguous term toneness to refer to the
clarity of pitch perceptions. For pure tones, toneness is known to change
with frequency. Frequencies above about 5000 Hz tend to sound like indistinct sizzles devoid of pitch (Attneave & Olson, 1971; Ohgushi & Hato,
1989; Semal & Demany, 1990).2 Similarly, very low frequencies sound like
2. More precisely, pure tones above 5 kHz appear to be devoid of pitch chroma, but
continue to evoke the perception of pitch height. See Semal and Demany (1990) for further
discussion.
David Huron
diffuse rumblings. Frequencies in the middle range of hearing give rise to

the most well-defined pitch sensationsthat is, they exhibit high toneness.
The clarity of pitch perceptions has been simulated systematically in a
model of pitch formulated by Terhardt, Stoll, and Seewann (1982a, 1982b).
For both pure and complex tones, the model calculates a pitch weight,
which may be regarded as an index of the pitchs clarity, and therefore, a
measure of toneness. For pitches evoked by pure tones (so-called spectral
pitches), sensitivity is most acute in the spectral dominance regiona broad
region centered near 700 Hz. Pitches evoked by complex tones (so-called
virtual pitches) typically show the greatest pitch weight when the evoked
or actual fundamental lies in a broad region centered near 300 Hzroughly
D4 immediately above middle C (Terhardt, Stoll, Schermbach, & Parncutt,
1986).
Figure 1 shows changes of pitch weight versus pitch for several sorts of
complex tones (see also Huron & Parncutt, 1992). The pitch weights are
calculated according to the Terhardt-Stoll-Seewann model. The solid curve
Fig. 1. Changes of maximum pitch weight versus pitch for complex tones from various
natural and artificial sources; calculated according to the method described by Terhardt,
Stoll, and Seewann (1982a, 1982b). Solid line: pitch weight for tones from C1 to C 7 having
a sawtooth waveform (all fundamentals at 60 dB SPL). Dotted lines: pitch weights for
recorded tones spanning the entire ranges for harp, violin, flute, trumpet, violoncello, and
contrabassoon. Pitch weights were calculated for each tone where the most intense partial
was set at 60 dB. For tones below C6, virtual pitch weight is greater than spectral pitch
weight; above C6, spectral pitch predominateshence the abrupt change in slope. The figure shows that changes in spectral content have little effect on the region of maximum pitch
weight. Note that tones having a pitch weight greater than one unit range between about E2
and G 5corresponding to the pitch range spanned by the treble and bass staves. Recorded
tones from Opolko and Wapnick (1989). Spectral analyses kindly provided by Gregory
Sandell (Sandell, 1991a).
shows the calculated pitch weight of the most prominent pitch for sawtooth
tones having a 60 dB SPL fundamental and ranging from C1 to C7. The
dotted curves show changes of calculated pitch weight for recorded tones
spanning the entire ranges for several orchestral instruments including harp,
violin, flute, trumpet, violoncello, and contrabassoon. Although spectral
content influences virtual pitch weight, Figure 1 shows that the region of
maximum pitch weight for complex tones remains quite stable. Notice that
complex tones having a pitch weight greater than one unit on Terhardts
scale range between about E2 and G5 (or about 80 to 800 Hz). Note that
this range coincides very well with the range spanned by the bass and treble
staves in Western music.
The origin of the point of maximum virtual pitch weight remains unknown. Terhardt has suggested that pitch perception is a learned phenomenon arising primarily from exposure to harmonic complex tones produced
typically by the human voice. The point of maximum virtual pitch weight
is similar to the mean fundamental frequency for women and childrens
voices. The question of whether the ear has adapted to the voice or the
voice has adapted to the ear is difficult to answer. For our purposes, we
might merely note that voice and ear appear to be well co-adapted with
respect to pitch.
In Huron and Parncutt (1992), we calculated the average notated pitch
in a large sample of notes drawn from various musical works, including a
large diverse sample of Western instrumental music, as well as non-Western works including multipart Korean and Sino-Japanese instrumental
works. The average pitch in this sample was found to lie near D 4a little
more than a semitone above the center of typical maxima for virtual pitch
weight. This coincidence is especially evident in Figure 2, where the average notated pitch is plotted with respect to three scales: frequency, log frequency, and virtual pitch weight. The virtual pitch weight scale shown in
Figure 2 was created by integrating the curve for the sawtooth wave plotted in Figure 1that is, spreading the area under this curve evenly along
the horizontal axis. For each of the three scales shown in Figure 2, the
lowest (F2) and highest (G5) pitches used in typical voice-leading are also
plotted. As can be seen, musical practice tends to span precisely the region
of maximum virtual pitch weight. This relationship is consistent with the
view that clear auditory images are sought in music making.
Once again, the causal relationship is difficult to establish. Musical practice may have adapted to human hearing, or musical practice may have
contributed to the shaping of human pitch sensitivities. Whatever the causal
relationship, we can note that musical practice and human hearing appear
to be well co-adapted. In short, middle C truly is near the middle of
something. Although the spectral dominance region is centered more than
an octave away, middle C is very close to the center of the region of virtual
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Fig. 2. Relationship of pitch to frequency, log frequency, and virtual pitch weight scales.
Each line plots F2 (bottom of the bass staff), C4 (middle C), and G5 (top of the treble staff)
with respect to frequency, log frequency, and virtual pitch weight (VPW)-scale. Lines are
drawn to scale, with the left and right ends corresponding to 30 Hz and 15,000 Hz, respectively. The solid dot indicates the mean pitch (approx. D#4) of a cross-cultural sample of
notated music determined by Huron and Parncutt (1992). The VPW scale was determined
by integrating the area under the curve for the sawtooth wave plotted in Figure 1. Musical
practice conforms best to the VPW scale with middle C positioned near the center of the
region of greatest pitch sensitivity for complex tones.
pitch sensitivity for complex tones. Moreover, the typical range for voiceleading (F2-G5) spans the greater part of the range where virtual pitch weight
is high. By contrast, Figure 2 shows that the pitch range for music-making
constitutes only a small subset of the available linear and log frequency
ranges.
Drawing on the extant research concerning pitch perception, we might
formulate the following principle:
1. Toneness Principle. Strong auditory images are evoked when tones
exhibit a high degree of toneness. A useful measure of toneness is provided by virtual pitch weight. Tones having the highest virtual pitch
weights are harmonic complex tones centered in the region between F2
and G5. Tones having inharmonic partials produce competing virtual
pitch perceptions, and so evoke more diffuse auditory images.
2. Principle of Temporal Continuity

A second factor influencing the vividness of auditory images is their continuity. In 1971, Bregman and Campbell coined the term auditory stream
to denote the perceptual experience of a single coherent sound activity that
maintains its singleness and continuity with respect to time. A number of
factors are known to influence the perception of auditory continuity. The
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most obvious factor is temporal continuity, in which sound energy is maintained over a period of time.
Auditory images may be evoked by either real (sensory) or imagined
(purely mental) processes. Two examples of purely mental auditory images
can be found in echoic memory and auditory induction (Houtgast, 1971,
1972, 1973; Thurlow, 1957; Warren, Obusek, & Ackroff, 1972).3 In the
case of auditory induction, Warren et al. generated stimuli in which intermittent faint sounds were alternated with louder sounds. The faint and
loud sounds were contiguous but not overlapping. Nevertheless, the faint
sounds tend to be perceived as a continuous background tone against which
the loud sounds are heard to pulse. In this case, there is a mental disposition to continue a sound image even when it is physically absent.
Warren et al. were able to explain the origin of this phenomenon by
showing that the frequency/intensity thresholds for auditory induction coincide closely with the thresholds for auditory masking. In other words,
auditory induction mentally reinstates sounds that the listener would expect to be maskedeven when the sounds are truly absent. With pure tones,
robust auditory induction effects can be achieved for durations of up to
300 ms. In the case of noise bands, auditory induction may be achieved for
durations of 20 s or more.
Auditory induction may be viewed as an involuntary form of auditory
imagination. Fortunately, auditory induction is a subjective phenomenon
that readily admits to empirical investigation and measurement. Other pertinent subjective experiences are not so easily studied. Listeners (musicians
especially) are also cognizant of the existence of voluntary auditory imagination, in which the listener is able to form or sustain a purely mental
image of some sound, such as the imagined sound of a tympani roll or the
sound of a bubbling brook.
Although imagined sounds may be quite striking, in general, imagined
sounds are significantly less vivid than actual sound stimuli. Moreover,
even in the absence of sound, recently heard sounds are more vivid than
less recently heard sounds. In short, auditory images have a tendency to
linger beyond the physical cessation of the stimulus. In the case of real
sounds, the evoked auditory images decay through the inexorable degradation of the short-term auditory storewhat Neisser (1967) dubbed echoic
memory. In general, the longer a sound stimulus is absent, the less vivid is
its evoked image.
A number of experiments have attempted to estimate the duration of
echoic memory. These measures range from less than 1 s (Treisman &
Howarth, 1959) to less than 5 s (Glucksberg & Cowen, 1970). Typical
3. Thurlow referred to this phenomenon as the auditory figure-ground effect; Houtgast
used the term continuity effect. We have elected to use the terminology proposed by
Warren et al.
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measures lie near 1 s in duration (Crowder, 1969; Guttman & Julesz, 1963;
Rostron, 1974; Treisman, 1964; Treisman & Rostron, 1972). Kubovy and
Howard (1976) have concluded that the lower bound for the half-life of
echoic memory is about 1 s. Using very short tones (40-ms duration), van
Noorden (1975, p. 29) found that the sense of temporal continuation of
events degrades gradually as the interonset interval between tones increases
beyond 800 ms.
On the basis of this research, we may conclude that vivid auditory images are evoked best by sounds that are either continuous or broken only
by very brief interruptions.4 In short, sustained and recurring sound events
are better able to maintain auditory images than are brief intermittent stimuli.
Drawing on the preceding literature, we might formulate the following
principle:
2. Principle of Temporal Continuity. In order to evoke strong auditory
streams, use continuous or recurring rather than brief or intermittent
sound sources. Intermittent sounds should be separated by no more
than roughly 800 ms of silence in order to ensure the perception of
continuity.
Temporal Continuity and Musical Practice

The musical implications of this principle are readily apparent. In brief,
the implications are evident (1) in the types of sounds commonly used in
music making, (2) in the manner by which sound events succeed one another, (3) in the use of physical damping for unduly long sounds, and (4) in
the differing musical treatment of sustained versus intermittent sounds.
In the first instance, the principle of temporal continuity is evident in the
types of sounds commonly used in music making. Compared with most
natural sound-producing objects, musical instruments are typically constructed so as to maximize the duration of the sounds producedthat is, to
enhance the period of sustain. Most of the instruments of the Western orchestra, for example, are either blown or rubbedmodes of excitation that
tend to evoke relatively long-lasting or continuous sounds. Even in the case
of percussion instruments (such as the piano or the vibraphone), the history of the development of these instruments has shown a marked trend
toward extending the resonant durations of the sounds produced. For example, the history of pianoforte construction has been marked by the continual increase in string tension, resulting in tones of longer duration. Similar historical developments can be traced for such instruments as tympani,
gongs, and marimbas. In some cases, musicians have gone to extraordinary
4. The word vivid is meant to refer to the stability of an auditory image rather than its
noticeability. Intermittent sounds are more likely to capture the listeners attention, but they
may be less apt to evoke coherent auditory streams.
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lengths in order to maintain a continuous sound output. In the case of

wind instruments, the disruptions necessitated by breathing have been overcome by such devices as bagpipes, mechanical blowers (as in pipe organs),
and devices that respond to both inhaling and exhaling (e.g., the accordion, and, to a lesser extent, the harmonica). In addition, in some cultures, performance practices exist whose sole purpose is to maintain an
uninterrupted sound. Most notable is the practice of cyclic breathing
as used, for example, in the Arab shawm (zurna), and the north Australian
didjeridoo. For stringed instruments, bowed modes of excitation have been
widespread. In the extreme, continuous bowing mechanisms such as the
mechanism used in the hurdy-gurdy have been devised. In the case of the
guitar, solid-body construction and controlled electronic feedback have
become popular methods of increasing the sustain of plucked strings.
Apart from the continuous character of most musical sounds, composers tend to assemble successions of tones in a way that suggests a persistent
or ongoing existence. A notable (if seemingly trivial) fact is that the majority of notated tones in music are followed immediately by a subsequent
tone. For example, 93% of all tones in vocal melodies by Stephen Foster
and Franz Schubert are followed by another tone. The corresponding percentage for instrumental melodies exceeds 98%. The exceptions to this
observation are themselves telling. Successions of tones in vocal music, for
example, are periodically interrupted by rest periods that allow the singer(s)
time to breathe. In most of the worlds music, the duration of these rest
periods is just sufficient to allow enough time to inhale (i.e., about 1 or 2
s). (Longer rests typically occur only in the case of accompanied vocal music.)
Moreover, when music making does not involve the lungs, pitch successions tend to have even shorter and fewer interruptions. Once again, as a
general observation, we can note that in most of the worlds music making,
there is a marked tendency to maintain a more or less continuous succession of acoustic events.
In tandem with efforts to maintain continuous sound outputs, musical
practices also reveal efforts to terminate overlapping resonances. When
musicians connect successions of pitches, problems can arise when each
pitch is produced by a different vibrator. The dampers of the piano, for
example, are used to truncate each tones normal decay. Typical piano performance finds the dampers terminating one tone at the same moment that
the hammer engages the next tone. This practice is not limited to the piano,
nor is it limited to Western music. Guitar players frequently use the palm of
the hand to dampen one or more stringsand so enhance the sense of
melodic continuation when switching from string to string. In the case of
Indonesian gamelan music, proper performance practice requires the left
hand to dampen the current resonator while the right hand strikes the subsequent resonatora technique known as tutupan. In all of these examples,
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David Huron
the damping of physical vibrators is consistent with the goal of maintaining the illusion of a single continuous acoustic activity.
Of course music making also entails the use of brief sounds, such as
sounds arising from various percussion instruments like the wood block.
However, musicians tend to treat such brief sounds differently. Brief sounds
tend not to be used to construct lines of soundsuch as melodies
instead, these sounds are typically used intermittently. Those percussion
instruments having the shortest tone durations are least apt to be used for
melodic purposes. When instruments having rapid decays are used to perform melodies, they often employ tremolo or multiple repeated attacks, as
in music for marimba or steel drums. When brief tones are produced by
nonpercussion instruments (e.g., staccato), there is a marked tendency to
increase the rate of successive tones. Contiguous staccato notes are seldom
separated by more than 1 s of silence. In such cases, echoic memory serves
to sustain the illusion of an uninterrupted line of sound.
As a general observation, we can note that in most of the worlds music
making there is a marked preference for some sort of continuous sound
activity; moreover, when instrumental sounds are brief in duration, such
sounds are often assigned to nonmelodic musical taskseven when the
tones produced evoke clear pitches.
3. Minimum Masking Principle
In his Nobel-prize-winning research, Georg von Bksy showed that different frequencies produce different points of maximum displacement on
the basilar membrane of the cochlea (Bksy, 1943/1949, 1960). Specifically, low frequencies cause the greatest displacement of the membrane
near the apex of the cochlea, whereas high frequencies produce maximum
displacements toward the oval window. Bksy, and later Skarstein
(Kringlebotn, Gundersen, Krokstad, & Skarstein, 1979), carefully mapped
this relationship by measuring the distance (in millimeters from the stapes)
of the point of maximum displacement for a given frequency input.5 This
correspondence between input frequency and place of maximum displacement of the basilar membrane is referred to as a tonotopic mapping or
cochlear map.
Subsequent work by Harvey Fletcher linked the frequency-place coordinates of the Bksy cochlear map to experimental data from frequency
discrimination and masking experiments (Fletcher, 1953, pp. 168175).
5. Greenwood (1961a) has produced the following function to express this frequencyposition relationship:
F = A(10ax - k),
where F is the frequency (in hertz), x is the position of maximum displacement (in millimeters from the apex), and A, a, and k are constants: in homo sapiens A = 165, a = 0.06, and
k = 1.0.
15
Fletcher showed that there is a close correspondence between distances

along the basilar membrane and regions of masking. In pursuing this research, Fletcher defined a hypothetical entity dubbed the critical band to
denote frequency-domain regions of roughly equivalent or proportional
behavior (Fletcher, 1940). Subsequent research by Zwicker and others established critical bandwidths as an empirical rather than hypothetical construct. Most notably, Zwicker, Flottorp, and Stevens (1957) showed that
distance along the basilar membrane accounts for changes in cumulative
loudness as a function of the overall frequency spread of several tones or a
band of noise (see also Scharf, 1961).
Greenwood (1961b, 1990) extended Fletchers work by comparing psychoacoustic measures of critical bandwidth with the frequency-place coordinates of the Bksy -Skarstein cochlear map. Greenwood showed that
there is a linear relationship, with one critical bandwidth being roughly
equivalent to the distance of 1.0 mm on the basilar membrane.
Greenwood (1961b) went on to suggest that tonotopic effects might also
account for an aspect of dissonance perception now referred to as sensory
dissonance. Conceptually, the perception of consonance or dissonance is
thought to be influenced by both cultural (learned) and sensory (innate)
factors. Although little experimental research has addressed the cultural
aspects of consonance or dissonance (Cazden, 1945), considerable research
has established that sensory dissonance is intimately related to the cochlear
map. Greenwood tested his hypothesis by comparing perceptual data collected by Mayer (1894) against the critical bandwidth/cochlear map. Mayer
had collected experimental data for pure tones in which listeners were instructed to identify the smallest possible interval free of roughness or dissonance. For pure tones, this interval is not constant with respect to log frequency. Greenwood showed that there is an excellent fit between Mayers
(1894) measures and the critical bandwidth.6
The width of critical bands is roughly intermediate between a linear frequency scale and a logarithmic frequency scale for tones below about 400
Hz. For higher frequencies, the width becomes almost completely logarithmic. When measured in terms of hertz, critical bandwidths increase as frequency is increased; when measured in terms of semitones (log frequency),
critical bandwidths decrease as frequency is increased. Figure 3 shows the
approximate spacing of critical band distances using standard musical no6. Regrettably, the music perception community has overlooked the seminal work of
Donald Greenwood and has misattributed the origin of the tonal-consonance/critical-band
hypothesis to Drs. Plomp and Levelt (see Greenwood, 1961b; especially pp. 13511352). In
addition, the auditory community in general overlooked Greenwoods accurate early characterization of the size of the critical band. After the passage of three decades, the revised
ERB function (Glasberg & Moore, 1990) is virtually identical to Greenwoods 1961 equationas acknowledged by Glasberg and Moore. For a survey of the relation of consonance
and critical bandwidth to cochlear resolution, see Greenwood (1991).
16
David Huron
Fig. 3. Approximate size of critical bands represented using musical notation. Successive
notes are separated by approximately one critical bandwidth = roughly 1-mm separation
along the basilar membrane. Notated pitches represent pure tones rather than complex
tones. Calculated according to the revised equivalent rectangular bandwidth (ERB) (Glasberg
& Moore, 1990).
tation (i.e., log frequency). Each note represents a pure tone; internote distances have been calculated according to the equivalent rectangular bandwidth rate (ERB) scale devised by Moore and Glasberg (1983; revised
Glasberg & Moore, 1990).
Plomp and Levelt (1965) extended Greenwoods work linking the perception of sensory dissonance (which they dubbed tonal consonance) to
the critical bandand hence to the mechanics of the basilar membrane.
Further work by Plomp and Steeneken (1968) replicated the dissonance/
roughness hypothesis using more contemporary perceptual data. Plomp
and Levelt estimated that pure tones produce maximum sensory dissonance
when they are separated by about 25% of a critical bandwidth. However,
their estimate was based on a critical bandwidth that is now considered to
be excessively large, especially below about 500 Hz. Greenwood (1991)
has estimated that maximum dissonance arises when pure tones are separated by about 30% to 40% of a critical bandwidth. For frequency separations greater than a critical band, no sensory dissonance arises between
two pure tones. These findings were replicated by Kameoka and Kuriyagawa
(1969a, 1969b) and by Iyer, Aarden, Hoglund, and Huron (1999).
In musical contexts, however, pure tones are almost never used; complex
tones containing several harmonic components predominate. For two complex tones, each consisting of (say) 10 significant harmonics, the overall
perceived sensory dissonance will depend on the aggregate interaction of
all 20 pure-tone components. The tonotopic theory of sensory dissonance
explains why the interval of a major third sounds smooth in the middle and
upper registers, but sounds gruff when played in the bass region.
Musicians tend to speak of the relative consonance or dissonance of
various interval sizes (such as the dissonance of a minor seventh or the
consonance of a major sixth). Although the pitch distance between two
complex tones affects the perceived sensory dissonance, the effect of interval size on dissonance is indirect. The spectral content of these tones, the
amplitudes of the component partials, and the distribution of partials with
respect to the critical bandwidth are the most formative factors determin-
17
ing sensory dissonance. Note that the characterizations of relative dissonance for fixed interval sizes given by musicians may also reflect cultural
(learned) factors that have received little overt empirical attention (Cazden,
1945).
Plomp and Levelt hypothesized that in the writing of chords, composers
would typically endeavor to maintain roughly equivalent amounts of sensory dissonance throughout the span of the chord. That is, they hypothesized that composers would typically avoid chords that produced disproportionately more dissonance in one or another region of the chord. A
given sonority might be highly dissonant or consonant overall, but Plomp
and Levelt supposed that the dissonance would typically be distributed
homogeneously across a given sonority.
An alternative interpretation of this prediction may be offered without
appealing to the concept of dissonance. Spectral components that lie within
a critical band of each other cause mutual masking, which reduces the
capacity of the auditory system to resolve or apprehend all of the sounds
present. If composers were disposed to reduce the capacity for mutual masking, then it would be appropriate to space chordal tones in such a way that
roughly equivalent amounts of spectral energy would fall in each critical
band. Since critical bandwidths span more semitones in the low region
than in the higher region, the notated pitches within a chord would show a
distinctive distribution were this hypothesis true.
Plomp and Levelt carried out a study of chordal-tone spacing in two
musical works: the third movement from J. S. Bachs Trio Sonata No. 2 for
organ (BWV 526)7 and the third movement from Dvorks String Quartet
Op. 51 in E major. Their analyses demonstrated an apparent consistency
between the composers arrangements of vertical sonorities and the tonotopic
mapthus implying that critical bands significantly influence the vertical
spacing of chordal tones.
In a replication study, Huron and Sellmer (1992) showed that Plomp
and Levelts demonstration was confounded by an unfortunate artifact and
that their results could not be used to support their conclusion. However,
using a more sophisticated inferential approach and a much larger musical
sample, we went on to provide an alternative demonstration that confirmed
Plomp and Levelts original hypothesis. The effect of spectral spread on the
spacing of chordal tones is not easily summarized; however, Figure 4 can
be used to illustrate the phenomenon. Figure 4 shows the average spacing
of notated (complex) tones for sonorities having various bass pitches from
C 4 to C2. For example, the first notated sonority in Figure 4 shows the
average tenor, alto, and soprano pitches for a large sample of four-notes
sonorities having C4 as the bass pitch. (The specific sonorities notated in
7. Incorrectly cited as Trio Sonata No. 3 by Plomp and Levelt (1965).
18
David Huron
Fig. 4. Average spacing of tones for sonorities having various bass pitches from C4 to C2.
Calculated from a large sample (>10,000) of four-note sonorities extracted from Haydn
string quartets and Bach keyboard works (Haydn and Bach samples equally weighted). Bass
pitches are fixed. For each bass pitch, the average tenor, alto, and soprano pitches are
plotted to the nearest semitone. (Readers should not be distracted by the specific sonorities
notated; only the approximate spacing of voices is of interest.) Note the wider spacing
between the lower voices for chords having a low mean tessitura. Notated pitches represent
complex tones rather than pure tones.
Figure 4 should not be interpreted literally; only the approximate spacing

of voices is of interest.) As the tessitura of the chord descends, the pitch
separation between the lower notes in the sonority tends to become larger.
This is consistent with efforts to distribute spectral components in a roughly
uniform manner across the basilar membrane. Such a pattern is consistent
with both the goals of minimizing sensory dissonance and minimizing auditory masking.
As pointed out by theorist Walter Piston (1978), the spacing between
chordal tones is not related to the spacing of partials in the harmonic series
(as formerly supposed). The harmonic series exhibits the same interval spacing whatever the register of a chord. Yet composed chords show systematic
interval changes with respect to register.
While acknowledging that the extant research is consistent with both
the goals of minimzing sensory dissonance and minimizing auditory masking, for the purposes of voice-leading, we will formulate the relevant musical principle in terms of masking:
3. Minimum Masking Principle. In order to minimize auditory masking within some vertical sonority, approximately equivalent amounts
of spectral energy should fall in each critical band. For typical complex
harmonic tones, this generally means that simultaneously sounding notes
should be more widely spaced as the register descends.
4. Tonal Fusion Principle

Tonal fusion is the tendency for some concurrent sound combinations to
cohere into a single sound image. Tonal fusion arises most commonly when
the auditory system interprets certain frequency combinations as comprising partials of a single complex tone (DeWitt & Crowder, 1987). Two factors are known to affect tonal fusion: (1) the frequency ratio of the component tones and (2) their spectral content. Tonal fusion is most probable
19
when the combined spectral content conforms to a single hypothetical harmonic series. This occurs most commonly when the frequencies of the component tones are related by simple integer ratios.
The pitch interval that most encourages tonal fusion is the aptly named
unison. The second most fused interval is the octave, whereas the third
most fused interval is the perfect fifth (DeWitt & Crowder, 1987; Stumpf,
1890). Following Stumpf, many music researchers have assumed that tonal
fusion and tonal consonance are the same phenomenon and that both arise
from simple integer frequency ratios. However, the extant psychoacoustic
research does not support Stumpfs view. Bregman (1990) has noted that
the confusion arises from conflating smooth sounding with sounding
as one. As we have seen, work by Greenwood (1961a, 1961b, 1990, 1991),
Plomp and Levelt (1965), Kameoka and Kuriyagawa (1969a, 1969b), and
Iyer et al. (1999) implicates critical band distances in the perception of
tonal consonance or sensory dissonance. This work shows that sensory
dissonance is only indirectly related to harmonicity or tonal fusion.
Whether or not tonal fusion is a musically desirable phenomenon depends on the music-perceptual goal. In Huron (1991b), it was shown that
in the polyphonic writing of J. S. Bach, tonally fused harmonic intervals are
avoided in proportion to the strength with which each interval promotes
tonal fusion. That is, unisons occur less frequently than octaves, which
occur less frequently than perfect fifths, which occur less frequently than
other intervals. Of course concurrent octaves and concurrent fifths occur
regularly in music, but (remarkably) they occur less frequently in polyphonic music than they would in a purely random juxtaposition of voices.
Note that this observation is independent of the avoidance of parallel
unisons, fifths, or octaves. As simple static harmonic intervals, these intervals are actively avoided in Bachs polyphonic works. Considering the importance of octaves and fifths in the formation of common chords, their
active avoidance is a remarkable feat (see Huron, 1991b).
In light of the research on tonal fusion, we may formulate the following
principle:
4. Tonal Fusion Principle. The perceptual independence of concurrent
tones is weakened when their pitch relations promote tonal fusion. Intervals that promote tonal fusion include (in decreasing order): unisons,
octaves, perfect fifths, ... Where the goal is the perceptual independence of concurrent sounds, intervals ought to be shunned in direct
proportion to the degree to which they promote tonal fusion.
Sensory Dissonance, Tonal Fusion, and Musical Practice

Before continuing with the fifth perceptual principle, we might briefly
consider the relationship between sensory dissonance and tonal fusionin
20
David Huron
light of musical practice. As we have seen, sensory dissonance depends on

more than just the pitch interval separating two tones. It also depends on
the spectral content of the tones, as well as their tessitura. Nevertheless,
pitch distance retains an indirect influence on sensory dissonance, and it is
therefore possible to calculate the average dissonance for intervals of various sizessuch as the average dissonance of a major sixth (see Huron,
1994).
The solid line in Figure 5 shows the aggregate consonance values for
musical intervals up to the size of an octave from perceptual experiments
by Kaestner (1909). The bars show the interval prevalence in the upper
two voices of J. S. Bachs three-part Sinfonias.8 There is a fairly close fit
between these two sets of data, suggesting that Bach tends to use various
intervals in inverse proportion to their degree of sensory dissonance. There
are a few notable discrepancies, however. The unison and octave intervals
occur relatively infrequently in the Bach sample, although the sensory dissonance for these intervals is very low. In addition, the minor and major
thirds appear to be switched: the consonance data would predict that
major thirds would be more prevalent than minor thirds.
The origin of the discrepancies for the major and minor thirds is not
known. It is possible that the relative prevalence of minor thirds may be an
artifact of the interval content of common chord types. Major and minor
Fig. 5. Comparison of sensory consonance for complex tones (line) from Kaestner (1909)
with interval prevalence (bars) in the upper two voices of J.S. Bachs three-part Sinfonias
(BWVs 787801). Notice especially the discrepancies for P1 and P8. Reproduced from
Huron (1991b).
8. Technical justifications for sampling the upper two voices of Bachs three-part Sinfonias
are detailed by Huron (1994).
21
triads both contain one major third and one minor third each. However,
both the dominant seventh chord and the minor-minor-seventh chord contain one major third and two minor thirds. In addition, the diminished
triad (two minor thirds) is more commonly used than the augmented triad
(two major thirds). Hence, the increased prevalence of minor thirds compared with major thirds may simply reflect the interval content of common
chords. The relative prevalences of the major and minor sixth intervals
(inversions of thirds) are also consistent with this suggestion.
The discrepancies for P1 and P8 in Figure 5 are highly suggestive in light
of the tonal fusion principle. We might suppose that the reason Bach avoided
unisons and octaves is in order to prevent inadvertent tonal fusion of the
concurrent parts. In Huron (1991b), this hypothesis was tested by calculating a series of correlations. When perfect intervals are excluded from consideration, the correlation between Bachs interval preference and the sensory dissonance Z scores is 0.85.9 Conversely, if we exclude all intervals
apart from the perfect intervals, the correlation between Bachs interval
preference and the tonal fusion data is 0.82. Calculating the multiple regression for both factors, Huron (1991b) found an R2 of 0.88indicating
that nearly 90% of the variance in Bachs interval preference can be attributed to the twin compositional goals of the pursuit of tonal consonance
and the avoidance of tonal fusion. The multiple regression analysis also
suggested that Bach pursues both of these goals with approximately equal
resolve. Bach preferred intervals in inverse proportion to the degree to which
they promote sensory dissonance and in inverse proportion to the degree
to which they promote tonal fusion. It would appear that Bach was eager
to produce a sound that is smooth without the danger of sounding as
one. In Part III, we will consider possible aesthetic motivations for this
practice.
Musical Terminology: Types of Harmonic Intervals
The experimental results pertaining to sensory dissonance and tonal fusion may be used to illuminate traditional musical terminology. Music theorists traditionally distinguish three classes of harmonic intervals: perfect
consonances (such as perfect unisons, octaves, fourths, and fifths), imperfect consonances (such as major and minor thirds and sixths), and dissonances (such as major and minor seconds and sevenths, and tritones). These
interval types can be classified according to the criteria of sensory dissonance and tonal fusion. Perfect consonances typically exhibit low sensory
9. The word preference is not used casually here. Huron (1991b) developed an
autophase procedure that permits the contrasting of distributions in order to determine
what intervals are being actively sought out or actively avoided by the composer. The ensuing reported correlation values are all statistically significant.
22
David Huron
dissonance and high tonal fusion. Imperfect consonances have low sensory
dissonance and comparatively low tonal fusion. Dissonances exhibit high
sensory dissonance and low tonal fusion. (There are no equally tempered
intervals that exhibit high sensory dissonance and high tonal fusion, although the effect can be generated using grossly mistuned unisons, octaves,
or fifths.) It would appear that the twin phenomena of sensory dissonance
and tonal fusion provide a plausible account for both the traditional theoretical distinctions, as well as Bachs compositional practice.
5. Pitch Proximity Principle
In 1950, Miller and Heise observed that alternating pitches (such as trills)
produce two different perceptual effects depending on the pitch distance
separating the tones (Miller & Heise, 1950; see also Heise & Miller, 1951).
When the tones are close with respect to pitch, quick alternations evoke a
sort of undulating effectlike a single wavering line. However, when
the pitch separation is larger, the perceptual effect becomes one of two
beeping tones of static pitch. Musicians recognize this phenomenon as
that of pseudo-polyphony or compound melodic line, in which a single
sequence of pitches nevertheless evokes a sort of yodelling effect.
Miller and Heises observations were replicated and extended by a number of researchers including Bozzi and Vicario (1960) and Vicario (1960),
Schouten (1962), Norman (1967), Dowling (1967), van Noorden (1971a,
1971b), and Bregman and Campbell (1971). (Several of these researchers
worked independently, without knowledge of previously existing work.)
Of these pioneering efforts, the most significant works are those of Dowling
(1967) and van Noorden (1975). Over the past three decades, however, the
most sustained and significant research effort has been that of Albert
Bregman (1990).
In 1975 van Noorden mapped the relationship between tempo and pitch
separation on stream integration and segregation. Figure 6 summarizes van
Noordens experimental results. When the tempo is slow and/or the pitches
have close proximity, the resulting sequence is always perceived as a single
stream. This area is indicated in Figure 6 as region 1below the fission
boundary (lower line). Conversely, when the pitch distances are large and/
or the tempo is fast, two streams are always perceived. This condition is
indicated in Figure 6 as region 2to the left of the temporal coherence
boundary. Van Noorden also identified an intervening gray region, where
listeners may hear either one or two streams depending on the context and
the listeners disposition. Notice that the slope of the fission boundary is
much shallower than the slope of the temporal coherence boundary. We
will return to consider these different slopes later.
The importance of pitch proximity in stream organization is supported
by a wealth of further experimental evidence. Schouten (1962) observed
23
Fig. 6. Influence of interval size and tempo on stream fusion and segregation (van Noorden,
1975, p. 15). Upper curve: temporal coherence boundary. Lower curve: fission boundary. In
Region 1, the listener necessarily hears one stream (small interval sizes and slow tempos). In
Region 2, the listener necessarily hears two streams (large interval sizes and fast tempos).
that temporal relationships are more accurately perceived within streams

than between streamswhen streams are distinguished by pitch proximity
alone. This work was replicated and extended by Norman (1967), Bregman
and Campbell (1971), and Fitzgibbons, Pollatsek, and Thomas (1974).
In addition, Bregman and his colleagues have assembled strong evidence
showing the preeminence of pitch proximity over pitch trajectory in the
continuation of auditory streams. In sequences of pitches, listeners tend
not to extrapolate future pitches according to contour trajectories. Rather
they interpolate pitches between pitches deemed to be in the same stream
(Ciocca & Bregman, 1987; Steiger & Bregman, 1981; Tougas & Bregman,
1985; summarized by Bregman, 1990, pp. 417442) Similar effects have
been observed in speech perception (Darwin & Gardner, 1986; Pattison,
Gardner, & Darwin, 1986).
Further experimental work has demonstrated the perceptual difficulty
of tracking auditory streams that cross with respect to pitch. Using pairs of
well-known melodies, Dowling (1973) carried out experiments in which
successive notes of the two melodies were interleaved. The first note of
melody A was followed by the first note of melody B, followed by the
second note of melody A followed by the second note of melody B etc.
Dowling found that the ability of listeners to identify the melodies was
highly sensitive to the pitch overlap of the two melodies. Dowling found
that as the melodies were transposed so that their mean pitches diverged,
recognition scores increased. The greatest increase in recognition scores
24
David Huron
occurred when the transpositions removed all pitch overlap between the
concurrent melodies.
Deutsch (1975) and van Noorden (1975) found that, for tones having
identical timbres, concurrent ascending and descending tone sequences are
perceived to switch direction at the point where their trajectories cross.
That is, listeners are disposed to hear a bounced percept in preference to
the crossing of auditory streams. Figure 7 illustrates two possible perceptions of intersecting pitch trajectories. Although the crossed trajectories
represents a simpler Gestalt figure, the bounced perception is much more
commonat least when the trajectories are constructed using discrete pitch
categories (as in musical scales).
In summary, at least four empirical phenomena point to the importance
of pitch proximity in helping to determine the perceptual segregation of
auditory streams: (1) the fission of monophonic pitch sequences into pseudopolyphonic percepts described by Miller and Heise and others, (2) the discovery of information-processing degradations in cross-stream temporal
tasks, as found by Schouten, Norman, Bregman and Campbell, and
Fitzgibbons, Pollatsek, and Thomas, (3) the perceptual difficulty of tracking auditory streams that cross with respect to pitch described by Dowling,
Deutsch, and van Noorden, and (4) the preeminence of pitch proximity
over pitch trajectory in the continuation of auditory streams demonstrated
by Bregman et al. Stream segregation is thus strongly dependent upon the
proximity of successive pitches.
On the basis of extensive empirical evidence, we can note the following
principle:
5. Pitch Proximity Principle. The coherence of an auditory stream is
maintained by close pitch proximity in successive tones within the
stream. Pitch-based streaming is assured when pitch movement is within
van Noordens fission boundary (normally 2 semitones or less for
tones less than 700 ms in duration). When pitch distances are large, it
may be possible to maintain the perception of a single stream by reducing the tempo.
Fig. 7. Schematic illustration of two possible perceptions of intersecting pitch trajectories.

Bounced perceptions (right) are more common for stimuli consisting of discrete pitch
sequences, when the timbres are identical.
25
Pitch Proximity and Music

If a musical melody is a kind of auditory stream (Dowling, 1967), then
we might expect sequences of pitches to conform to the pitch proximity
principle. Specifically, we would expect the majority of pitch sequences to
fall below van Noordens fission boundary and certainly to fall below the
temporal coherence boundary.
The evidence concerning the prevalence of pitch proximity in music is
extensive. Several authors have observed the pervasive use of small intervals in the construction of melodies, including Ortmann (1926), Merriam,
Whinery and Fred (1956), and Dowling (1967). Figure 8 plots further data
showing the distribution of interval sizes using samples of music from a
number of cultures: American, Chinese, English, German, Hasidic, Japanese, and sub-Saharan African (Pondo, Venda, Xhosa, and Zulu). In general, the results affirm the preponderance of small intervals.
Apart from melodies, musicians know that the pitch interval between
successive notes is a crucial determinant in the construction of pseudopolyphonic passages. Dowling (1967) carried out a study of a number of
Baroque solo works in order to determine the degree to which pseudopolyphony is correlated with the use of large intervals. In measurements of
interval sizes in passages deemed pseudo-polyphonic by two independent
auditors, Dowling found the passages to contain intervals markedly larger
than the trill threshold measured by Miller and Heisea threshold similar to van Noordens fission boundary. In a sample of pseudo-polyphonic
Fig. 8. Frequency of occurrence of melodic intervals in notated sources for folk and popular
melodies from 10 cultures (n = 181). African sample includes Pondo, Venda, Xhosa, and
Zulu works. Note that interval sizes correspond only roughly to equally tempered semitones.
26
David Huron
passages by Telemann, Dowling noted that Telemann never uses intervals

less than the Miller and Heise trill threshold.
As further evidence of the primacy of small intervals in non pseudopolyphonic lines, Dowling examined the interval preferences of listeners.
Through a series of stochastically generated stimuli, Dowling found that
listeners prefer melodies that use the smaller interval sizes. Carlsen (1981),
moreover, demonstrated that musicians show a marked perceptual expectancy for proximate pitch contours. Studying musicians from three different countries (Hungary, Germany, and the United States), Carlsen discovered that although there are significant differences between musicians in
their melodic expectancies, in all cases neighboring pitch continuations of
melodic stimuli are strongly favored.
Further evidence showing the consistency between musical practice and
empirical research regarding pitch proximity is evident in the case of partcrossing. Notice that, except in the case of unisons, the crossing of parts
with respect to pitch always violates the pitch proximity principle. No matter
how the pitches are arranged, the aggregate pitch distance is always lowest
when the upper voice consists of the upper pitches and the lower voice
consists of the lower pitches. In a study of part-crossing in polyphonic
music, Huron (1991a) showed that J. S. Bach avoids part-crossing and that
he becomes most vigilant to avoid part-crossing when the number of concurrent parts is three or more.
In summary, at least five empirical observations point to the importance
of pitch proximity in musical organization: (1) the preponderance of small
pitch intervals in non-pseudo-polyphonic melodies observed by Ortmann,
Merriam et al., and Dowling, (2) the reciprocal prevalence of large pitch
intervals in pseudo-polyphonic passages found by Dowling, (3) the auditory preference for small intervals in melodies found by Dowling, (4) the
perceptual expectation for small intervals in continuations of melodic contours found by Carlsen, and (5) the avoidance of part-crossing in polyphonic music measured by Huron. Musical practice thus exhibits a notable
consistency with the pitch proximity principle.
Types of Melodic IntervalsThe Fission Boundary
Once again, we might pause and consider the relationship between the
perceptual evidence concerning pitch proximity and traditional musical
terminology. Recall van Noordens fission and temporal coherence boundaries shown in Figure 6. Not all regions in Figure 6 are equally pertinent to
musical practice. Figure 9 shows distributions for note durations for samples
of instrumental and vocal lines. Only durations for contiguous notated
pitches have been included in this sample of 13,178 notes. Notes immediately prior to rests and at the ends of works have been eliminated from the
27
Fig. 9. Distribution of note durations in 52 instrumental and vocal works. Dotted line: note
durations for the combined upper and lower voices from J. S. Bachs two-part Inventions
(BWVs 772786). Dashed line: note durations in 38 songs (vocal lines only) by Stephen
Foster. Solid line: mean distribution for both samples (equally weighted). Note durations
were determined from notated scores using tempi measured from commercially available
sound recordings. Graphs are plotted using bin sizes of 100 ms, centered at the positions
plotted.
sample in order to avoid the effect of final lengthening and to make the
data consistent with the pitch alternation stimuli used to generate Figure 6.
A mean distribution is indicated by the solid line. In general, Figure 9
shows that the majority of musical tones are relatively short; only 3% of
tones are longer than 1 s in duration. Eighty-five percent of tones are shorter
than 500 ms; however, it is rare for tones to be less than 150 ms in duration. The median note durations for the samples plotted in Figure 9 are
0.22 s for instrumental and 0.38 s for vocal works.
This distribution suggests that the majority of tone durations in music
are too long to be in danger of crossing the temporal coherence boundarythat is, where two streams are necessarily perceived. Notice that compared with the temporal coherence boundary, the fission boundary is more
nearly horizontal (refer to Fig. 6). The boundary is almost flat for tone
durations up to about 400 or 500 ms after which the boundary shows a
positive slope. Figure 9 tells us that the majority of musical tones have
durations commensurate with the flat portion of the fission boundary. This
flat boundary indicates that intervals smaller than a certain size guarantee
the perception of a single line of sound. In the region of greatest musical
activity, the fission boundary approximates a constant pitch distance of
about 1 semitone.
28
David Huron
As in the case of harmonic intervals, music theorists traditionally distinguish two main classes of melodic intervals: conjunct or step motions, and
disjunct or leap motions. In Western music, the dividing line between step
and leap motion is traditionally placed between a major second and a minor third; that is, a major second (2 semitones) is considered a conjunct or
step motion, whereas a minor third (3 semitones) is considered the smallest
disjunct or leap motion. In other cultures, such as those that employ the
common pentatonic scale, the maximum step size is roughly 3 semitones.
It is plausible that the fission boundary in effect identifies a psychoacoustic
basis for the distinction between conjunct and disjunct melodic motions.
What theorists call conjunct intervals are virtually guaranteed to evoke the
perception of stream continuation.
Melodic MotionThe Temporal Coherence Boundary
Unlike the fission boundary, the temporal coherence boundary plotted
in Figure 6 shows a marked positive slope. This means that, in order for a
sequence of pitches to break apart into two streams, tempo is the predominant factoralthough pitch interval continues to play a significant role. If
the temporal coherence boundary influences musical organization, we might
expect to see tradeoffs between pitch interval and instantaneous tempo.
For example, we might predict that large pitch leaps would be associated
with tones of long duration.
Van Noorden (1975, p. 48) and Shepard (1981, p. 319) independently
drew attention to the similarity between Miller and Heises trill results and
Krtes third law of apparent motion in vision (Krte, 1915). Krte carried
out a number of experiments using two lamps that could be alternately
switched on and off. Krte found that the sense of apparent motion depends on the distance separating the two lamps and their speed of switching. If the lamps are placed farther apart, then the rate of switching must be
reduced in order to maintain a sense of apparent motion between the lamps.
If the switching rate is too fast, or the lamps are placed especially far apart,
then the viewer sees two independent flickering lights with no sense of
intervening motion. A possible explanation for this loss of apparent motion is that it is implausible for a single real-world object to move in accordance with the presumed trajectory.
The parallel between Krtes results and Miller and Heises trills is obvious and direct. It would seem that the sense of continuation between two
tones is an auditory analog to apparent motion in vision. Note that both
Krtes results and Miller and Heises results pertain to perception. Research by Huron and Mondor (1994) suggests that the cognitive basis for
this parallel can be attributed to the real-world generation or production
of motion.
29
In Huron and Mondor (1994), it was argued that both Krtes third law
of apparent motion and Miller and Heises results arise from the kinematic
principle known as Fitts law (Fitts, 1954). Fitts law applies to all musclepowered or autonomous movement. The law is best illustrated by an example (refer to Figure 10). Imagine that you are asked to alternate the
point of a stylus as rapidly as possible back and forth between two circular
targets. Fitts law states that the speed with which you are able to accomplish this task is proportional to the size of the targets and inversely proportional to the distance separating them. The faster speeds are achieved
when the targets are large and close togetheras in the lower pair of circles
in Figure 10.
Because Fitts law applies to all muscular motion, and because vocal
production and instrumental performance involve the use of muscles, Fitts
law also constrains the generation or production of sound. Imagine, for
example, that the circular targets in Figure 10 are arranged vertically rather
than horizontally. From the point of view of vocal production, the distance
separating the targets may be regarded as the pitch distance between two
tones. The size of the targets represents the pitch accuracy or intonation.
Fitts law tells us that, if the intonation remains fixed, then vocalists will be
unable to execute wide intervals as rapidly as for small intervals.
In Huron and Mondor (1994), both auditory streaming and melodic
practice were examined in light of Fitts law. In the first instance, two perceptual experiments showed that increasing the variability of pitches (the
auditory equivalent of increased target size) caused a reduction in the fission of auditory streams for rapid pitch alternationsas predicted by Fitts
Fig. 10. Two pairs of circular targets illustrating Fitts law. The subject is asked to alternate
the point of a stylus back-and-forth as rapidly as possible between two targets. The minimum duration of movement between targets depends on the distance separating the targets
as well as target size. Hence, it is possible to move more rapidly between the lower pair of
targets. Fitts law applies to all muscle motions, including the motions of the vocal muscles.
Musically, the distance separating the targets can be regarded as the pitch distance between
two tones, whereas the size of the targets represents pitch accuracy or intonation. Fitts law
predicts that if the intonation remains fixed, then vocalists will be unable to execute wide
intervals as rapidly as for small intervals.
30
David Huron
law. With regard to the perception of melody, Huron and Mondor noted
that empirical observations of preferred performance practice (Sundberg,
Askenfelt & Frydn, 1983) are consistent with Fitts law. When executing
wide pitch leaps, listeners prefer the antecedent tone of the leap to be extended in duration. In addition, a study of the melodic contours in a crosscultural sample of several thousand melodies showed them to be consistent
with Fitts law. Specifically, as the interval size increases, there is a marked
tendency for the antecedent and consequent pitches to be longer in notated
or performed duration. This phenomenon accords with van Noordens temporal coherence boundary. In short, when large pitch leaps are in danger of
evoking stream segregation, the instantaneous tempo for the leap is reduced. This relationship between pitch interval and interval duration is
readily apparent in common melodies such as My Bonnie Lies Over the
Ocean or Somewhere Over the Rainbow. Large pitch intervals tend to
be formed using tones of longer durationa phenomenon we might dub
leap lengthening.
It would appear that both Krtes third law of apparent motion and the
temporal coherence boundary in auditory streaming can be attributed to
the same cognitive heuristic: Fitts law. The brain is able to perceive apparent motion, only if the visual evidence is consistent with how motion occurs in the real world. Similarly, an auditory stream is most apt to be perceived when the pitch-time trajectory conforms to how real (mechanical or
physiological) sound sources behave. In short, it may be that Fitts law
provides the origin for the common musical metaphor of melodic motion (see also Gjerdingen, 1994).
Further evidence in support of a mechanical or physiological origin for
pitch proximity has been assembled by von Hippel (2000). An analysis of
melodies from cultures spanning four continents shows that melodic contours can be modeled very well as arising from two constraintsdubbed
ranginess and mobility. Such a model of melodic contour is able to account
for a number of melodic phenomena, notably the observation that large
leaps tend to be followed by a reversal in melodic direction.
6. Pitch Co-moduation Principle
As early as 1863, Helmholtz suggested that similar pitch motion contributes to the perceptual fusion of concurrently sounded tones. In recent
decades, Helmholtzs suggestion has received considerable empirical confirmation and extension. Chowning (1980) vividly demonstrated how coordinated frequency modulations would cause computer-generated voices
to fuse, whereas miscoordinated modulations would cause the sounds to
segregate. (This phenomenon was used in Chownings 1979 composition,
Phone.) Bregman and Doehring (1984) experimentally showed that tonal
31
fusion is enhanced significantly when two tones are modulated by correlated changes in log-frequency. Indeed, the degree of tonal fusion is greater
for tones of changing pitch than for tones of static pitch.
In an extensive series of experiments, McAdams (1982, 1984a, 1984b)
demonstrated that co-modulations of frequency that preserve the frequency
ratios of partials promote tonal fusion. Moreover, McAdams also showed
that positively correlated pitch motions that are not precise with respect to
log-frequency also tend to contribute to tonal fusion. In other words, tonal
fusion is most salient when co-modulation is precise with respect to logfrequency and the frequencies of the two tones are harmonically related.
Tonal fusion is next most salient when co-modulation is precise with respect to log-frequency and the frequencies of the two tones are not harmonically related. Finally, tonal fusion is next most salient when co-modulation is positively correlated, but not precise with respect to log-frequency.10
Apart from the empirical evidence, the pitch co-modulation principle is
evident in musical practice as well. In traditional music theory, theorists
distinguish two types of positively correlated pitch motion: similar motion
and parallel motion. These types of positively correlated pitch motions might
be collectively referred to as semblant motions. In Huron (1989a), it was
shown that polyphonic composers (not surprisingly) avoid semblant pitch
motionsboth parallel and similar contrapuntal motions. Moreover, it was
shown that parallel pitch motions are avoided more than similar motions.
Finally, it was shown that parallel motions are most avoided in the case of
intervals that tend most to promote tonal fusion: unisons, octaves, and
perfect fifths in particular. Once again, both traditional musical terminology and musical practice are consistent with the perceptual evidence.
Drawing on this research, we might formulate the following principle:
6. Pitch Co-modulation Principle. The perceptual union of concurrent
tones is encouraged when pitch motions are positively correlated. Perceptual fusion is most enhanced when the correlation is precise with
respect to log frequency.
Part II: Derivation of the Rules of Voice-Leading

For the music theorist, the observations made in Part I are highly suggestive about the origin of the traditional rules of voice-leading. It is appropriate, however, to make the relationship between the perceptual principles
and the voice-leading rules explicit. In this section, we will attempt to de10. It should be noted that psychoacoustic research by Robert Carlyon (Carlyon, 1991,
1992, 1994) suggests that the auditory system may not directly use frequency co-modulation as the mechanism for segregating sources with correlated pitch motions.
32
David Huron
rive the voice-leading rules from the six principles described in the preceding section. The derivation given here is best characterized as heuristic rather
than formal. The purpose of this exercise is to clarify the logic, to avoid
glossing over details, to more easily expose inconsistencies, and to make it
easier to see unanticipated repercussions.
In the ensuing derivation, the following abbreviations are used:
G
A
C
D
goal
empirical axiom (i.e., an experimentally determined fact)
corollary
derived musical rule or heuristic
(traditional: commonly stated in music theory sources)
[D] derived musical rule or heuristic (nontraditional)
In formal logic, an axiom is defined as a basic proposition (a statement)

that is asserted without proof. However, in this case, each of the axioms
presented is supported by the experimental research outlined in Part I. Thus,
we might refer to these axioms as empirical axioms.
First, we can define the goal of voice-leading:
G1. The goal of voice-leading is to create two or more concurrent yet
perceptually distinct parts or voices. Good voice-leading optimizes
the auditory streaming.
Two corollaries follow:

C1a. Effective voice-leading requires clear auditory stream integration
within each of the individual parts.
C1b. Effective voice-leading requires clear auditory stream segregation
between each of the concurrent parts.
We may begin by introducing our first empirical axiom: the toneness principle:
A1. Coherent auditory streams are best created using tones that evoke
clear auditory images.
A1a. Unambiguously pitched complex tones evoke the clearest auditory
images.
From this we derive a nontraditional proposition or rule:

[D1.] Toneness Rule. Voice-leading should employ tones that evoke
strong, unique pitch sensations. This is best achieved using harmonic
complex tones.
[C2.] Voice-leading is less effective using noises or tones with inharmonic
partials.
33
In light of these nontraditional rules, a few pertinent observations may be

made about musical practice. First, most of the worlds music making relies on instruments that produce tones exhibiting harmonic spectra. Instruments that produce inharmonic spectra (e.g., drums) are less likely to
be used for voice-leading or melodic purposes. Those percussion instruments that are used melodically (e.g., glockenspiels, marimbas, gamelans,
steel drums) are more apt to produce pitched tones. Carilloneurs have long
noted that carillons are ill-suited to contrapuntal writing (Price, 1984, p.
310).
Continuing with the toneness principle:
A1b. Complex tones evoke the strongest sensation of pitch in the region
near 300 Hzbetween about 80 and 800 Hz.
From this we derive the rule regarding registral compass:

D2. Registral Compass Rule. Voice-leading is best practiced in the region between F2 and G5, roughly centered near D4.
Adding the principle of temporal continuity:

A2. In order to evoke coherent auditory streams, it is best to use continuous rather than intermittent sound sources.
This leads to a simple nontraditional rule:

[D3.]Sustained Tones Rule. In general, effective voice-leading is best
assured by employing sustained tones in close succession, with few silent gaps or interruptions.
Adding the minimum masking principle:

A3. Auditory masking is reduced when partials are evenly spaced with
respect to critical bands.
From this we derive one traditional, and one nontraditional rule of chordal
tone spacing:
D4. Chord Spacing Rule. In general, chordal tones should be spaced
with wider intervals between the lower voices.
[D5.] Tessitura-Sensitive Spacing Rule. It is more important to have
large intervals separating the lower voices in the case of sonorities that
are lower in overall pitch.
Recall that Huron and Sellmer (1992) showed that musical practice is indeed consistent with this latter ruleas predicted by Plomp and Levelt
(1965).
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David Huron
Adding the principle of tonal fusion:

A4. Effective stream segregation is violated when tonal fusion arises.
The degree to which two concurrent tones fuse varies according to the
interval separating them.
A4a. Tonal fusion is greatest with the interval of a unison.
From this we derive the traditional injunction against unisons:

D6. Avoid Unisons Rule. Avoid shared pitches between voices.
A4b. Tonal fusion is next strongest at the interval of an octave.
[D7.] Avoid Octaves Rule. Avoid the interval of an octave between two
concurrent voices.
A4c. Tonal fusion is next strongest at the interval of a perfect fifth.
[D8.] Avoid Perfect Fifths Rule. Avoid the interval of a perfect fifth
between two concurrent voices.
In general,
[D9.] Avoid Tonal Fusion Rule. Avoid unisons more than octaves, and
octaves more than perfect fifths, and perfect fifths more than other
intervals.
Recall that in Huron (1991b), Bachs polyphonic practice was shown to be

consistent with the above nontraditional rules. As simple static intervals,
unisons, octaves, and fifths occur less frequently than occurs in a random
juxtaposition of parts.
Turning now to the pitch proximity principle:
A5. The coherence of a stream is maintained by close pitch proximity in
successive notes within a voice.
A4a. The closest possible proximity occurs when there is no pitch movement.
Therefore:
D10. Common Tone Rule. Pitch-classes common to successive sonorities are best retained as a single pitch that remains in the same voice.
A5b. The next best stream cohesion arises when pitch movement is near
or within van Noordens fission boundary (roughly 2 semitones or less).
35
Hence:
D11. Conjunct Movement Rule. If a voice cannot retain the same pitch,
it should preferably move by step.
C3. Avoid Leaps Rule. Avoid wide pitch leaps.
(This last rule is merely a corollary of the Common Tone and Conjunct
Movement rules.) When pitch movements exceed the fission boundary, effective voice-leading is best maintained when the temporal coherence boundary is not also exceeded:
A5c. Wide melodic leaps most threaten auditory stream cohesion when
they exceed the temporal coherence boundary.
From this we derive a nontraditional rule:

[D12.] Leap-Lengthening Rule. Where wide leaps are unavoidable, use
long durations for either one or both of the tones forming the leap.
Leap-lengthening (consistent with Fitts law) has been observed in a crosscultural sample of melodies by Huron and Mondor.
Bregman (1990) has characterized streaming as a competition between
possible alternative organizations. It is not simply a matter that two successive pitches need to be relatively close together in order to form a stream.
The pitches must be closer than other possible pitch-time traces. That is,
previous pitches compete for subsequent pitches:
A5d. Pitches are likely to stream to the nearest previous pitch.
From this, we can derive those rules of voice-leading that relate to pitchproximity competition:
D13. Nearest Chordal Tone Rule. Parts should connect to the nearest
chordal tone in the next sonority.
D14. Part-Crossing Rule. Avoid the crossing of parts with respect to
pitch.
In a study of part-crossing in 105 works by J. S. Bach, Huron (1991a)

showed that of three competing interpretations of the injunction against
part-crossing, actual musical practice was most consistent with the principle of pitch proximity.
D15. Pitch Overlapping Rule. Avoid overlapped parts in which a
pitch in an ostensibly lower voice is higher than the subsequent pitch in
an ostensibly higher voice.
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David Huron
Once again, failure to obey this rule will conflict with the pitch proximity principle. Each pitch might be likened to a magnet, attracting the nearest subsequent pitch as its partner in the stream.11
Note that a simple general heuristic for maintaining within-voice pitch
proximity and avoiding proximity between voices is to place each voice or
part in a unique pitch region or tessitura. As long as voices or parts remain
in their own pitch territory, there is little possibility of part-crossing,
overlapping, or other proximity-related problems. There may be purely
idiomatic reasons to restrict an instrument or voice to a given range, but
there are also good perceptual reasons for such restrictionsprovided the
music-perceptual goal is to achieve optimum stream segregation. It is not
surprising that in traditional harmony, parts are normally referred to by
the names of their corresponding tessituras: soprano, alto, and so on.
Adding now the pitch co-modulation principle:
A6a. Effective stream segregation is thwarted when tones move in a positively correlated pitch direction.
[D16.] Semblant Motion Rule. Avoid similar or parallel pitch motion
between concurrent voices.
A6b. Effective stream segregation is especially thwarted when tones move
in precisely positively correlated pitch direction.
[D17.] Parallel Motion Rule. Avoid parallel motion more than similar
motion.
Huron (1989a) showed that polyphonic composers do indeed tend to avoid

positively correlated pitch motions. Moreover, parallel motion tends to be
avoided for all interval sizesnot just in the case of tonally fused intervals
such as unisons, octaves, and perfect fifths. In addition, parallel motion is
avoided more than similar motion.
Derivations From Multiple Principles
Each of the preceding voice-leading rules is deduced from a single perceptual principle. Where two or more principles pertain to the same goal,
then transgressing more than one such principle will have more onerous
consequences than transgressing only one of the principles. Conversely, the
consequences of transgressing a single such principle may be minimized by
ensuring complete conformity with the remaining principlesprovided all
principles pertain to the same goal. The six principles outlined earlier all
pertain to the optimization of auditory images and streaming.
11. This analogy is perhaps deceptive because streaming is also known to depend on
repetitive and cumulative properties as well as immediate tone successions. See Bregman
(1978).
37
There are other principles that relate to the optimization of auditory

streaming, but we will defer deriving the associated rules until Part III.
Even in the case of the six stream-related principles identified earlier, 720
(6 factorial) propositions arise from permuting all the principles. We will
present only some of these derivations.
If we link the tonal fusion and pitch proximity principles:
A4&A5. The detrimental effect of tonal fusion can be reduced by ensuring close pitch proximity within the parts.
This leads to two nontraditional rules:

[D18.] Oblique Approach to Fused Intervals Rule. When approaching
unisons, octaves, or fifths, it is best to retain the same pitch in one of
the voices (i.e., approach by oblique motion).
[D19.] Avoid Disjunct Approach to Fused Intervals Rule. If it is not
possible to approach unisons, octaves and fifths by retaining the same
pitch (oblique motion), step motion should be used.
A4&A6. Tonal fusion is further encouraged when tonally fused intervals
move in a positively correlated direction.
From this we may derive the

[D20.] Avoid Semblant Approach Between Fused Intervals Rule. Avoid
similar pitch motion in which the voices employ unisons, octaves, or
perfect fifths. (For example, when both parts ascend beginning an octave apart, and end a fifth apart.)
A4&A6a. Tonal fusion is most encouraged when tones move in a precisely positively correlated pitch direction at the interval of a unison, octave, or perfect fifth.
From this we derive the well-known prohibition against parallel fifths and
octaves:
D21. Parallel Unisons, Octaves, and Fifths Rule. Avoid parallel unisons,
octaves, or fifths.
Joining the tonal fusion, pitch proximity, and pitch co-modulation principles, we find that:
A4&A5b&A6. When moving in a positively correlated direction toward
a tonally fused interval, the detrimental effect of tonal fusion can be alleviated by ensuring proximate pitch motion.
From this we can derive the traditional injunction against exposed or hidden intervals.
38
David Huron
D22. Exposed Intervals Rule. When approaching unisons, octaves, or

fifths, by similar motion, at least one of the voices should move by step.
Note that in traditional treatises on voice-leading most theorists restrict

the exposed intervals rule to the case of approaching an octave (fifteenth,
etc.)that is, exposed octaves. Only a minority of theorists extend this
injunction to approaching perfect fifths (twelfths, etc.) as well. Since octaves engender tonal fusion more easily than perfect fifths, the perceptual
principles account for why there would be greater unanimity of musical
opinion against exposed octaves than for exposed fifths.
Observe that the principles outlined here are not able to account for the
fact that most theorists restrict the exposed intervals rule to voice-leading
involving the bass and soprano voices. Huron (1989b) has shown that outer
voices are more easily perceived than inner voices; hence one might argue
that voice-leading transgressions in outer voices are more noticeable. However, one might just as convincingly argue that since the streaming of the
inner voices is more apt to be confounded, it is more important that inner
voices conform to the voice-leading principles.
The exposed octaves rule is especially revealing. In essence, the exposed
octaves rule says: If you are going to violate principle A, and if you are
going to violate principle B, be certain not to violate principle C as well.
The exposed octaves rule inextricably links three perceptual principles together and logically implies that all three principles share some sort of common goal. Specifically, the exposed octaves rule confirms that the principle
of tonal fusion, the pitch proximity principle, and the pitch co-modulation
principle all contribute to the achievement of the same goalnamely, the
optimization of stream segregation.
Part III: Perceptual Principles and Musical Genres

As in any deductive system, changing one or two axioms will result in a
different set of derivations. As in the case of non-Euclidean geometry, it is
theoretically possible for composers to devise different voice-leading systems by adding, eliminating, or modifying one or more of the axioms. Changing the axioms will constrain the canon in distinctive ways and hence result
in different musics.
AUXILIARY PRINCIPLES
In the remainder of this article, we consider four examples of auxiliary

perceptual principles. When these auxiliary principles are added to the six
core principles, a number of additional propositions arise. As noted in the
39
introduction, these auxiliary principles in effect constitute voice-leading

options that will shape the music making in perceptually distinctive ways.
At the end of our discussion, we will identify how these auxiliary principles
may contribute to the differentiation of musical genres.
The four auxiliary principles discussed in the remainder of this article
include (7) onset synchrony, (8) limited density, (9) timbral differentiation,
and (10) source location.
7. The Principle of Onset Synchrony
Sounds are more apt to be perceived as components of a single auditory
image when they are coordinated in time. That is, concurrent tones are
much more apt to be interpreted by the auditory system as constituents of
a single complex sound event when the tones are temporally aligned. Because judgments about sounds tend to be made in the first few hundred
milliseconds, the most important aspect of temporal coordination is the
synchronization of sound onsets. Sounds whose onsets are uncoordinated
in time are likely to be perceived as distinct or separate events (Bregman &
Pinker, 1978). Onset synchronization may be regarded as a special case of
amplitude co-modulation. Apart from the crude coordination of tone onsets, the importance of correlated changes of amplitude has been empirically demonstrated for much more subtle deviations in amplitude. For example, Bregman, Abramson, Doehring, and Darwin (1985) have
demonstrated that the co-evolution of amplitude envelopes contributes to
the perception of tonal fusion.
The concept of synchronization raises the question of what is meant by
at the same time? Under ideal conditions, it is possible for listeners to
distinguish the order of two clicks separated by as little as 20 ms (Hirsh,
1959). However, in more realistic listening conditions, onset differences
can be substantially greater and yet retain the impression of a single onset.
Initial transients for natural complex tones may be spread over 80 ms or more,
even for tones with rapid attack envelopes (Saldanha & Corso, 1964; Strong
& Clark, 1967). Many sounds have much more gradual envelopes, and in the
case of echoes, more than 100 ms between repetitions may be required in
order for successive sounds to be perceived as distinct events (Beranek, 1954).
In the case of the synchronization of note onsets in music, Rasch (1978,
1979, 1988) has collected a wealth of pertinent data. Ensemble performances of nominally concurrent notes show that onsets are typically spread
over a range of 30 to 50 ms. Moreover, in experiments with quasi-simultaneous musical tones, Rasch found that the effect of increasing asynchrony
is initially to add transparency to multitone stimulirather than to evoke
separate sound images (Rasch, 1988, p. 80). Onset asynchronies have to be
comparatively large before separate sound events are perceived.
40
David Huron
Although musical notation provides only a crude indication of the temporal aspects of real performances, Raschs work suggests that synchronously and asynchronously notated events are likely to be perceived as
such. Of course tones that are notated as having concurrent onsets are by
no means performed precisely together. But in typical performances, such
tones are obviously more likely to promote the perception of tonal fusion
than tones that are notated as having asynchronous onsets. A difference of
a sixteenth-duration at a tempo of 80 quarter notes per minute produces a
nominal asynchrony of 187 ms, a time delay that is more than sufficient to
elicit the perception of separate events. In short, onsets that are deemed
asynchronous according to musical notation are unlikely to be perceived
as synchronous, whereas onsets that are deemed synchronous according
to musical notation may well be perceived as being synchronous. Thus
musical notation provides a useful (although not infallible) indication of
perceived onset synchronies and asynchronies in composed music.
7. Onset Synchrony Principle. If a composer intends to write music in
which the parts have a high degree of perceptual independence, then
synchronous note onsets ought to be avoided. Onsets of nominally distinct sounds should be separated by 100 ms or more.
In light of the onset synchrony principle, musical practice is equivocal.

Some musics conform to this principle, whereas other musics do not. Rasch
(1981) carried out an analysis of vocal works by Praetorius and showed
that onset asynchrony between the parts increases as the number of voices
is increased. Similarly, Huron (1993a) showed that J. S. Bach minimizes
the degree of onset synchrony in his polyphonic works. Figure 11 shows
onset synchrony autophases for a random selection of 10 of Bachs 15 twopart keyboard Inventions (BWVs 772-786). A mean onset synchrony function is also plotted (solid line). The values in the horizontal center of the
graph indicate the degree of onset synchrony for the work; all other values
show the degree of onset synchrony for control conditions in which the
parts have been shifted with respect to each other, measure by measure.
The dips at zero degrees are indicative of Bachs efforts to avoid synchronous note onsets between the parts.
By contrast, in nonpolyphonic works, composers routinely contradict
this principle. Figure 12 compares onset autophases for two different works.
The first graph shows an autophase for the three-part Sinfonia No. 1 (BWV
787). The second graph shows an autophase for the four-part hymn Adeste
Fideles. The y-axis values at (0,0) in each x-z plane indicate the degree of
onset synchrony for the work. All other values show the degree of onset
synchrony for control conditions in which the parts have been systematically shifted with respect to each other, measure by measure. Because there
41
Fig. 11. Onset synchrony autophases for a random selection of 10 of Bachs 15 two-part
keyboard Inventions (BWVs 772786); see Huron (1993a). Values plotted at zero degrees
indicate the proportion of onset synchrony for the actual works. All other phase values
indicate the proportion of onset synchrony for rearranged musiccontrolling for duration,
rhythmic order, and meter. The dips at zero degrees are consistent with the hypothesis that
Bach avoids synchronous note onsets between the parts. Solid line plots a mean onset synchrony function for all data.
are more than two parts, the resulting autophases are multidimensional
rather than two dimensional. The four-part Adeste Fideles has been simplified to three dimensions by phase shifting only two of the three voices
with respect to a stationary soprano voice. As can be seen, in the Sinfonia
No. 1 there is a statistically significant dip at (0,0) consistent with the hypothesis that onset synchrony is actively avoided. In Adeste Fideles by
contrast, there is a significant peak at (0,0) (at the back of the graph) consistent with the hypothesis that onset synchrony is actively sought. The
graphs formally demonstrate that any realignment of the parts in Bachs
Sinfonia No. 1 would result in greater onset synchrony, whereas any realignment of the parts in Adeste Fideles would result in less onset synchrony.
Musicians recognize this difference as one of musical texture. The first
work is polyphonic in texture whereas the second work is homophonic in
texture. In a study comparing works of different texture, Huron (1989c)
examined 143 works by several composers that were classified a priori
according to musical texture. Discriminant analysis12 was then used in or12. Discriminant analysis is a statistical procedure that allows the prediction of group
membership based on functions derived from a series of prior classified cases. See Lachenbruch
(1975).
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David Huron
Fig. 12. Three-dimensional onset synchrony autophases comparing exemplar polyphonic

and homophonic works. (a) three-part Sinfonia No. 1 (BWV 787) by J. S. Bach; (b) fourpart hymn Adeste Fideles. The vertical axis indicates the onset synchrony measured according to Method 3 described in Huron (1989a, pp. 122123). The horizontal axes
indicate measure-by-measure shifts of two of the voices with respect to the remaining voice(s).
Figure 12a shows a marked dip at the origin (front), whereas Figure 12b shows a marked
peak at the origin (back). The horizontal axes in the two graphs have been reversed in order
to maintain visual clarity. The graphs formally demonstrate that any realignment of the
parts in Bachs Sinfonia No. 1 would result in greater onset synchrony, whereas any realignment of the parts in Adeste Fideles would result in less onset synchrony.
der to discern what factors distinguish the various types of textures. Of six
factors studied, it was found that the most important discriminator between homophonic and polyphonic music is the degree of onset synchrony.
As we have noted, an auxiliary principle (such as the onset synchrony
principle) can provide a supplementary axiom to the core principles and
allow further voice-leading rules to be derived. Once again, space limitations prevent us from considering all of the interactions. However, consider the joining of the onset synchrony principle with the principal of tonal
fusion. In perceptual experiments, Vos (1995) has shown that there is an
interaction between onset synchrony and tonal fusion in the formation of
auditory streams.
43
A4&A7. When approaching a tonally fused interval, the detrimental effect of tonal fusion can be alleviated somewhat by ensuring asynchronous
tone onsets.
From this we can derive the following nontraditional rule:

[D23]. Asynchronous Preparation of Tonal Fusion Rule. When approaching unisons, octaves, or fifths, avoid synchronous note onsets.
Figure 13 shows the results from an unpublished study of the interaction

between onset synchrony and tonal fusion for polyphonic works (fugues)
and homophonic works (chorales) by J. S. Bach. In both homophonic and
polyphonic repertoires, dissonant intervals are prepared via asynchronous onsets. Imperfect intervals in both repertoires show a tendency to
favor synchronous note onsets. However, for perfect intervals, the majority
of occurrences are asynchronous in the polyphonic repertoire, whereas the
majority are synchronous in the homophonic repertoire. Moreover, for the
perfect intervals alone, there is a positive correlation between the degree to
which an interval promotes tonal fusion and the likelihood of asynchronous occurrence. In short, the above nontraditional rule is obeyed by Bach
but only in his polyphonic writing.
Fig. 13. Results of a comparative study of tonally fused intervals in polyphonic and (predominantly) homophonic repertoires by J. S. Bach. Only perfect harmonic intervals are
plotted (i.e., fourths, fifths, octaves, twelfths, etc.). As predicted, in polyphonic textures,
most perfect intervals are approached with asynchronous onsets (one note sounding before
the onset of the second note of the interval). In the more homophonic chorale repertoire,
most perfect intervals are formed synchronously; that is, both notes tend to begin sounding
at the same moment. By contrast, the two repertoires show no differences in their approach
to imperfect and dissonant intervals. From Huron (1997).
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David Huron
Whence Homophony?
The existence of homophonic music raises an interesting perceptual
puzzle. On the one hand, most homophonic writing obeys all of the traditional voice-leading rules outlined in Part I of this article. (Indeed, most
four-part harmony is written within a homophonic texture.) This suggests
that homophonic voice-leading is motivated (at least in part) by the goal of
stream segregationthat is, the creation of perceptually independent voices.
However, because onset asynchrony also contributes to the segregation of
auditory streams, why would homophonic music not also follow this principle? In short, why does homophonic music exist? Why isnt all multipart
music polyphonic in texture?
The preference for synchronous onsets may originate in any of a number
of music-related goals, including social, historical, stylistic, aesthetic, formal, perceptual, emotional, pedagogical, or other goals that the composer
may be pursuing. If we wish to posit a perceptual reason to account for this
apparent anomaly, we must assume the existence of some additional perceptual goal that has a higher priority than the goal of stream segregation.
More precisely, this hypothetical perceptual goal must relate to some aspect of temporal organization in music. Two plausible goals come to mind.
In the first instance, the texts in vocal music are better understood when
all voices utter the same syllables concurrently. Accordingly, we might predict that vocal music is less likely to be polyphonic than purely instrumental music. Moreover, when vocal music is truly polyphonic, it may be better
to use melismatic writing and reserve syllable onsets for occasional moments of coincident onsets between several parts. In short, homophonic
music may have arisen from the concurrent pursuit of two perceptual goals:
the optimization of auditory streaming, and the preservation of the intelligibility of the text. Given these two goals, it would be appropriate to abandon the pursuit of onset asynchrony in deference to the second goal.
Another plausible account of the origin of homophony may relate to
rhythmic musical goals. Although virtually all polyphonic music is composed within a metric context, the rhythmic effect of polyphony is typically
that of an ongoing braid of rhythmic figures. The rhythmic strength associated with marches and dances (pavans, gigues, minuets, etc.) cannot be
achieved without some sort of rhythmic uniformity. Bachs three-part
Sinfonia No. 1 referred to in connection with Figure 12 appears to have
little of the rhythmic drive evident in Adeste Fideles. In short, homophonic music may have arisen from the concurrent goals of the optimization of
auditory streaming, and the goal of rhythmic uniformity. As in the case of
the text-setting hypothesis, this hypothesis generates a prediction
namely, that works (or musical forms) that are more rhythmic in character
(e.g., cakewalks, sarabandes, waltzes) will be less polyphonic than works
following less rhythmic forms.
45
8. The Principle of Limited Density

Listeners do not have an unbounded capacity to track multiple concurrent lines of sound. Huron (1989b) found that as the number of concurrent
voices in a polyphonic texture increases, expert listeners are both slower to
recognize the addition of new voices and more prone to underestimate the
number of voices present. Figure 14 plots experimental data from five expert musicians. The solid columns show mean estimation errors for polyphonic textures of various textural densities; the shaded columns indicate
mean error rates for single-voice entries. For polyphonic textures employing relatively homogeneous timbres, the accuracy of identifying the number of concurrent voices drops markedly at the point where a three-voice
texture is augmented to four voices. Beyond three voices, tracking confusions are commonplace. In a study of isolated chords, Parncutt (1993) found
similar results. When asked to count the number of tones in chords constructed using tones with octave-spaced partials (Shepard tones), Parncutt
found that listeners make significant errors (underestimation) once the number of chromas exceeds three.
Limitations in the perception of auditory numerosity may be symptomatic of a broader perceptual limitation, because the results of these studies
parallel the results of similar studies in vision. A variety of counting and
estimation tasks show a marked increase in confusion when the number of
Fig. 14. Voice-tracking errors while listening to polyphonic music. Solid columns: mean
estimation errors for textural density (no. of polyphonic voices); data for 130 trials from
five expert musician subjects. Shaded columns: unrecognized single-voice entries in polyphonic listening; data for 263 trials from five expert musician subjects (Huron, 1989b). The
data show that when listening to polyphonic textures that use relatively homogeneous timbres, tracking confusions are common when more than three voices are present.
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David Huron
visual items exceeds three (Estes & Combes, 1966; Gelman & Tucker, 1975;
Schaeffer, Eggleston, & Scott, 1974). Descoeudres (1921) characterized this
as the un, deux, trois, beaucoup phenomenon. In a study of infant perceptions of numerosity, Strauss and Curtis (1981) found evidence of this phenomenon in infants less than a year old. Using a dishabituation paradigm,
Strauss and Curtis showed that infants are readily able to discriminate two
from three visual items. However, performance degrades when discriminating three from four items and reaches chance performance when discriminating four from five items. The work of Strauss and Curtis is especially significant because preverbal infants can be expected to possess no
explicit knowledge of counting. This implies that the perceptual confusion
arising for visual and auditory fields containing more than three items is a
low-level constraint that is not mediated by cognitive skills in counting.
These results are consistent with reports of musical experience offered
by musicians themselves. Mursell (1937) reported a conversation with Edwin
Stringham in which Stringham noted that no human being, however talented and well trained, can hear and clearly follow more than three simultaneous lines of polyphony. A similar claim was made by composer Paul
Hindemith (1944).
8. Principle of Limited Density. If a composer intends to write music in
which independent parts are easily distinguished, then the number of
concurrent voices or parts ought to be kept to three or fewer.
Of course most harmony writing consists of four or more parts. Indeed,

most harmony texts are based exclusively on four-part writing. So once
again, the question that arises regarding this principle is why it is rarely
followed.
As a start, we must first recognize that a surprising amount of music
actually conforms to the above principle of limited density. Consider, for
example, the density of polyphonic writing in the music of J. S. Bach. Figure 15 (Huron, 1989a) shows the estimated number of auditory streams
versus the nominal number of voices or parts for a large body of polyphonic music by Bach. Estimates of the number of auditory streams are
calculated according to a method described and tested experimentally by
Huron (1989a). The dotted line indicates a relation of unity slope. If an Npart work truly maintains a mean textural density of N auditory streams,
then the plotted value for the work would be expected to lie near the unity
slope (dotted line). In order for a value to lie above the dotted line, the
work must contain parts that manifest a considerable degree of pseudopolyphonic activity and must also refrain from much voice retirement (idle
parts). Conversely, in order for a value to lie below the dotted line, the
work must contain significant periods of voice retirement and refrain from
47
Fig. 15. Relationship between nominal number of voices or parts and mean number of
estimated auditory streams in 108 polyphonic works by J. S. Bach. Mean auditory streams
for each work calculated using the stream-latency method described and tested in Huron
(1989a, chap. 14). Bars indicate data ranges; plotted points indicate mean values for each
repertoire. Dotted line indicates a relation of unity slope, where an N-voice polyphonic
work maintains an average of N auditory streams. The graph shows evidence consistent with
a preferred textural density of three auditory streams. See also Huron and Fantini (1989).
pseudo-polyphonic writing. Figure 15 shows that as the number of nominal voices increases, Bach gradually changes his compositional strategy.
For works with just two parts, Bach endeavors to keep the parts active (few
rests of short duration) and to boost the textural density through pseudopolyphonic writing. For works with four or more nominal voices, Bach
reverses this strategy. He avoids writing pseudo-polyphonic lines and retires voices from the texture for longer periods of time. Figure 15 reveals a
change of strategy consistent with a preferred textural density of three auditory streams. In light of the extant research, it would appear that Bach
tends to maximize the number of auditory streams, while simultaneously
endeavoring to avoid exceeding the listeners ability to track the concurrent
parts.
As for works that regularly use four or more concurrent parts, once
again, the notion of musical genre may be helpful in understanding diver-
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David Huron
gent practices. In contrast to Bachs polyphonic practice, musical works

consisting of four or more concurrent parts are more likely to be perceived
as homophonic in texture. By employing such large numbers of concurrent
parts, homophonic textures may tend to obscure the component tones and
encourage synthetic perceptions such as chords.
9. The Principle of Timbral Differentiation
The influence of timbre (spectral content) on the formation of auditory
streams was demonstrated by van Noorden (1975) and Wessel (1979), and
it was tested experimentally by McAdams and Bregman (1979), Bregman,
Liao, and Levitan (1990), Hartmann and Johnson (1991) and Gregory
(1994). Wessel created stimuli consisting of a repeated sequence of three
rising pitches (see Figure 16). Successive tones were generated so that every
second tone was synthesized by using one of two contrasting timbres. Notice that although the pitch sequence is rising, each of the timbre sequences
produces a descending sequence.
If timbre-based streaming takes precedence over pitch-based streaming,
then listeners ought to hear two distinct descending pitch sequences rather
than a single ascending pitch sequence. Wessel found that as the speed of
repetition is increased, there is a greater tendency for the timbre differences to cause listeners to hear descending sequences. Also, Wessel found
that the effect is most pronounced when the two timbres used are most
contrasting (as, for example, when their spectral centroids are far apart).
Iverson (1992) has shown that timbral cues that contribute most to streaming are highly correlated with similarity judgments. That is, listeners are
most likely to link together sonic events that share similar tone colors;
conversely, listeners tend to segregate sonic events whose tone colors are
quite different.
In the case of music perception, Erickson (1975) has identified a number
of ways in which timbre is used to distinguish musical elements. Rssler
(1952) carried out an extensive analysis of the influence of organ registration on the perceptual independence of polyphonic and pseudo-polyphonic
lines. Rsslers results underscore the role of timbre in the fission and fusion of auditory streams.
Fig. 16. Schematic illustration of Wessels illusion (Wessel, 1979). A sequence of three rising
pitches is constructed by using two contrasting timbres (marked with open and closed
noteheads). As the tempo of presentation increases, the percept of a rising pitch figure is
superceded by two independent streams of descending pitcheach stream distinguished by
a unique timbre.
49

9. Timbral Differentiation Principle. If a composer intends to write
music in which the parts have a high degree of perceptual independence, then each part should maintain a unique timbral character.
Again, the question that immediately arises regarding this principle is,
why do composers routinely ignore it? Polyphonic vocal and keyboard
works, string quartets, and brass ensembles maintain remarkably homogeneous timbres. Of course, some types of music do seem to be consistent
with this principle. Perhaps the most common Western genre that employs
heterogeneous instrumentation is music for woodwind quintet. Many other
small ensembles make use of heterogeneous instrumentation. Nevertheless,
it is relatively rare for tonal composers to use different timbres for each of
the various voices. A number of reasons may account for this. In the case of
keyboard works, there are basic mechanical restrictions that make it difficult to assign a unique timbre to each voice. Exceptions occur in music for
dual-manual harpsichord and pipe organ. In the case of organ music, a
common Baroque genre is the trio sonata in which two treble voices are
assigned to independent manuals and a third bass voice is assigned to the
pedal division. In this case it is common to differentiate polyphonic voices
timbrally by using different registrationsindeed, this is the common performance practice. The two-manual harpsichord can also provide contrasting registrations, although only two voices can be distinguished in this
manner.
The case of vocal ensembles is more unusual. There are some differences
in vocal production, and hence there are modest differences in vocal character for bass, tenor, alto, and soprano singers. However, choral conductors normally do not aim for a clear difference in timbre between the various voices. On the contrary, most choral conductors attempt to shape the
sound of the chorus so that a more homogeneous or integrated timbre is
produced. Nevertheless, it is possible to imagine a musical culture in which
such vocal differences were highly exaggerated. Why isnt this commonplace?
We may entertain a variety of scenarios that might account for the avoidance of timbral differentiation in much music. Some of these scenarios relate to idiomatic constraints in performance. For example, much Western
polyphonic music is written for vocal ensemble or keyboard. On the keyboard, we have noted that simple mechanical problems make it difficult to
assign different timbres to each voice. In ensemble music, recruiting the
necessary assortment of instrumentalists might have raised practical difficulties. It might be easier to recruit two violinists than to recruit a violinist
and an oboe player. Such practical problems might have forced composers
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David Huron
to abandon timbral differentiation as a means of enhancing the perceptual

independence of the parts.
Once again, if we wish to pursue a perceptual approach to explaining
this apparent anomaly, we must posit the existence of some additional perceptual goal that has a higher priority than that of auditory streaming. One
plausible goal is that of balancethat is, the relative salience or noticeability
of the various parts. As long as the parts are performed on similar instruments or are restricted to human voices, no single part is apt to be louder
or more noticeable than another. However, once heterogeneous timbral
resources are adopted, a multitude of differences are unleashed. One or
another timbre may be more commanding of the listeners attention and
thus antithetical to maintaining an equivalent status in the auditory scene.
A French horn, for example, is no match for an oboe. Provided both instruments are equally busy, the oboe will always tend to dominate the listeners
attention. Similarly, a human voice will nearly always be more salient than
a musical instrument. Notice that timbral differentiation seems to be preferred in situations where the composer wishes to draw attention to a particular line of sound against a background of other sounds. That is, timbral
differentiation may be the primary means for choreographing foreground/
background effects.
Another plausible (and related) goal is that of achieving a homogeneous
blend of sound. Sandell (1991b) has studied a number of perceptual correlates for orchestral blend. Composers tend to prefer those instrumental
combinations that show a high degree of blend. It may be that homogeneous instrumentation merely provides the easiest way to achieve a blended
sound. Although heterogeneous instrumentation might enhance the perceptual segregation of the voices, it would seem that tonal composers have
typically found the overall timbral result less preferable to a uniform ensemble.
10. The Source Location Principle
Even in reverberant environments, listeners are adept at identifying the
physical location from which a sound originates. It is known, for example,
that reflected sound energy is suppressed by the auditory system so that
listeners make localizations judgments on the basis of the initial (unreflected)
sound (Haas, 1951). Although we have only two ears, listeners can distinguish anterior from posterior sources, as well as elevation. Even when listening with just a single ear, humans show significant localization skills.
Divenyi and Oliver (1989) have shown that spatial location has a strong
effect on streaming (although it has relatively little effect on tonal fusion).
Since one of the best generalizations that can be made about independent
sound sources is that they normally occupy distinct positions in space, one
51
would naturally expect location to provide strong streaming cues (Huron,

1991c, p. 78).
10. Source Location Principle. If a composer intends to write music in
which the parts have a high degree of perceptual independence, then it
is helpful to separate in space the various sound sources for each part.
The applications of this principle in music are largely obvious. In the

case of antiphonal writing, sound sources may be separated by 50 m or
more. Conversely, when two or more instruments combine to play the same
part, they are typically seated in close physical proximityas in the second
violin section in an orchestra.
Other spatial effects are more subtle. In the case of barbershop quartet
singing, common performance practice places the singers in very close proximity, with the singers heads positioned close to a single location in space
(Martin, 1965; Root, 1980). This implies a goal of greater perceptual homogeneitymore precisely, the avoidance of stream segregation.
A number of musical works in the late twentieth century have consciously
manipulated spatial parameters. In the case of instrumental works, spatial
effects are often associated with unorthodox seating arrangements. In extreme cases, such as Iannis Xenakis Terretektorh, the instrumentalists are
dispersed throughout the audience. In general, musicians have tended to
avoid such wide separations because listeners often find themselves unable
to hear much beyond the musician sitting next to them. In Terretektorh
however, this perceptual goal may be less important than a sociological
goal of attempting to break down the traditional dichotomy between audience and performers.
The traditional reluctance of musicians to use dispersed locations for
sounds may be readily explained by the ensuing problems of acoustic variance. As sound sources become more widely separated, the musical effect
for different audience members becomes more variable: what listeners hear
depends primarily on where they sit. Conversely, when all the sound sources
are in close physical proximity, each individual source is placed on a more
equal footing. As in the case of timbral differentiation, the traditional reluctance to employ spatial differentiation may arise because of problems in
maintaining ensemble balance.
In the case of electroacoustic music, spatial manipulation may prove
more effective. In the first instance, much electroacoustic music is experienced via stereophonic recordings in personal listening settingshence reducing the effect of listener location. In addition, spatial cues may be dynamic (trajectories) rather than static (fixed location). By using continually
moving virtual sound sources, it may be possible to maintain a relatively
homogeneous texture without inadvertent foreground/background effects.
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David Huron
AUXILIARY PRINCIPLES AND MUSICAL TEXTURES
In Part III of this article, we have argued that some perceptual principles
are auxiliary to the core voice-leading principles. It has been argued that
these principles are treated as compositional options and are chosen according to whether or not the composer wishes to pursue additional perceptual goals. If this hypothesis is correct, then measures derived from such
auxiliary principles ought to provide useful discriminators for various types
of music. For example, the degree of onset synchrony may be measured
according to a method described in Huron (1989a), and this measure ought
to discriminate between those musics that do, or do not, conform to the
principle of onset synchrony.
Figure 17 reproduces a simple two-dimensional texture space described
by Huron (1989c). A bounded two-dimensional space can be constructed
Fig. 17. Texture space reproduced from Huron (1989c). Works are characterized according
to two properties: onset synchronization (the degree to which notes or events sound at the
same time) and semblant motion (the degree to which concurrent pitch motions are positively correlated). Individual works are plotted as points; works within a single repertoire
are identified by closed figures.
53
using semblant contrapuntal motion and onset synchrony as the x and y

axes. By definition, a single monophonic line will be exactly synchronized
with itself in terms of onsets and will move in precise pitch motion with
itself. As a result, all monophonic works are necessarily plotted in the upper right hand corner. Hymns reside in the upper mid-left area. Polyphonic
works by J. S. Bach sit in the lower left area, whereas a small sample of
non-Western works are further to the right. The resulting space appears to
divide into quadrants representing three of the four principle types of musical textures: monophony, homophony, and polyphony. (A sample of
heterophonic music occupies a region closer to the center of the space).
Moreover, the conceptual utility of this texture space is evident in its placement of hybrid textures, such as Kentucky folk organum and barbershop
quartets. Barbershop quartets are plotted in a region part-way between
homophony and monophonyreflecting the greater cohesiveness of close
harmony writing compared to hymnody.
Figure 18 shows another view of similar data selected from a large musical data base. Unlike the hand-drawn boundaries plotted in Figure 17,
the distributions in Figure 18 arise from the works themselves. In Figure
18, each of 173 works was characterized according to the degree of semblant
motion and onset synchronization. From the ensuing discrete data set of x-
Fig. 18. Distribution of musical textures from a sample of 173 works; generated using
Parzen windows method. (The axes have been rearranged relative to Figure 18 in order to
maintain visual clarity.)
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David Huron
y points a Parzen windows technique (Duda & Hart, 1973) has been used
to estimate the actual distribution shapes. As can be seen, three classic
genresmonophony, polyphony, and homophonyare clearly evident.
(Heterophonic works were not sufficiently represented in the sample for
them to appear using this technique.)
By itself, this texture space does not prove the hypothesis that some
perceptual principles act to define various musical genres. This texture space
is merely an existence proof that, a priori, genres can be distinguished by 2
of the 10 principles discussed here. A complete explication of perceptually
based musical textures would require a large-scale multivariate analysis
that is beyond the scope of this article.
Conclusion
In this article, we have shown that perceptual principles can be used to
account for a number of aspects of musical organization, at least with respect to Western music. In the first instance, it was shown that six empirically established perceptual principles are able to account for the majority
of traditional rules in Western voice-leading. Moreover, the power of these
perceptual principles was evident in the fact that nontraditional rules derived from the principles were also found to be consistent with actual musical practice.
As in a formal system, it is possible to eliminate, modify, or add different
perceptual principlesin a manner analogous to axioms in formal logic.
The addition, elimination, or modification of different perceptual principles
leads to different sonic textures and so can be used to define or distinguish
various musical genres.
Of course, the account of voice-leading given here remains incomplete.
Unresolved issues include a variety of questions. First, why do theorists
typically restrict the injunction against exposed octaves to outer voices only?
Because inner voices are more difficult to trace perceptually, one might
have thought that inner voices would be more prone to stream integration
arising from exposed octaves. Second, little is known about the influence
of perceptual schemas in the formation of auditory streams. Bregman (1990)
has assembled evidence illustrating the existence and influence of learned
schemas in auditory scene analysis. It is possible that motivic and rhythmic
features of musical organization facilitate stream segregation, but no research has yet addressed this issue. For example, is it the case that imitative
polyphony facilitates or inhibits stream segregation compared with
nonimitative polyphony?
Third, the perceptual scenario outlined here does not account for the
sense of direction (leading) that attends musical pitch successions. For
55
example, the above account implies that voice-leading should be equally

effective when a work is played backward rather than forward.13 However,
embellishments (e.g., suspensions, appoggiaturas) exhibit systematic asymmetries, such as the penchant for resolution by step, but approach by either
step or leap. A plausible cognitive answer may be found in the anchoring of
unstable tones to stable ones, as demonstrated by Krumhansl (1979) and
investigated more thoroughly by Bharucha (1984).
In addition to these questions, several untested predictions arise from
the preceding account of voice-leading. The minimum masking principle
would predict that chordal-tone spacing ought to be influenced by spectral
content. Hence, the spacing of chords ought to differ in music that uses
differing instrumentation. Second, we do not yet understand the relative
importance of each of the various factors known to influence auditory
streaming. When such empirical knowledge becomes available, then we
ought to be able to measure the extent to which musical practice conforms
to the relative perceptual weights associated with each principle.
An important question is whether the principles discussed in this article
are universal aspects of human auditory perception or whether they arise
from culture-specific conditioning. Questions of nature and nurture are
important in music because they have implications for how we view music
and what we can expect of musics creative future. If perceptual principles
such as those described in this articleare primarily learned, then it suggests that human perceptual capacities have a remarkable fluidity and adaptability. Musicians might take heart in such knowledge, because it implies
that there is great potential for new creative directions in musical culture.
Conversely, if such principles are found to be physiologically determined,
then it suggests that human perceptual capacities are constrained by fixed
biological limits. Musicians might be disheartened by such knowledge, because it implies that musical cultures are destined to develop within a finite
region of musical possibilities.
Several points should be kept in mind regarding nature/nurture questions. First, beliefs are always likely to be more strongly held than is warranted by the available empirical research. Second, both extremes of belief
have possible negative repercussions. Those who believe that perceptual
mechanisms are largely fixed and immutable are in danger of underestimating human capabilities and therefore of impeding the development of
musical culture. Conversely, those who believe that perceptual mechanisms
are highly adaptable are in danger of overestimating human adaptability and
therefore of encouraging activities that are irrelevant to human needs. Although
questions concerning the adaptability of human perception may invite opinions, these questions more properly invite further empirical investigation.
13. Note onsets and note offsets will be reversed in backwards renditions, and this may
affect onset synchronies.
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David Huron
Of course, even if the rules of voice-leading were shown to be somehow immutable, their musical relevance depends entirely on the acceptance
of the underlying perceptual goals. In short, the composer must deem the
goals discussed here worth pursuing. If musicians see no need to pursue
these goals, then the principles described here are musically irrelevant. Even
when such goals are deemed worthy of pursuit, they must be balanced
against other possible compositional goalssuch as social, historical, stylistic, aesthetic, or formal goalsor other cognitive or perceptual goals
deemed to have a higher priority.
Aesthetic Origins
Given the plethora of possible compositional goals, we might rightly ask
why so much Western music appears to be consistent with the perceptual
goal discussed here. Why would so many composers endeavor to clarify
the auditory streaming of the individual parts in a musical texture?
Bregman (1990) has advanced the hypothesis that music makes use of
preexisting auditory mechanisms for parsing the environment. A useful
parallel can be drawn to animated cartoons. Although the cartoonist is free
to draw anything at all, there are advantages to drawing objects and motions that resemble real-world visual scenes. Viewers will readily apply preexisting visual mechanisms in making sense of fictitious images, even when
the objects of manipulation are nonrepresentational. According to Bregman,
composers similarly create auditory fictions populated by plausible (although largely fictional) auditory objects and events. Music listeners may
be predisposed to make use of their existing auditory expertise related to
the parsing of auditory scenes.
Absent from Bregmans account is an aesthetic discussion of why this
might be enjoyable for listeners. Why do many listeners take pleasure in
music constructed according to the rules of voice-leading?
Like all sensory systems, the auditory system arose as an evolutionary
adaptation whose function is to provide listeners with relevant information about the external world. Sensory systems are not infallible in carrying out this task, however. Sensory information can often be incomplete or
ambiguous, and deciphering the world entails a certain amount of mental
effort. Particularly when a scene is complicated, there is good reason to
suppose that the limbic system rewards the brain when perceptual systems
successfully parse the available information. By success, we do not
mean that the mental representation constructed by perception accurately
reflects the real worldsince there is no independent way of judging such
accuracy. Rather, by success we mean that the mental representation is
57
coherent, self-consistent, and congruent with information from other sensory systems and schematic expectations.
A number of phenomenal experiences accord with the view that pleasure is related to perceptual success. Watching an out-of-focus film can
engender considerable annoyance, followed by a sense of relief when the
focus is finally adjusted. A parallel auditory experience is evident when
tuning a radio to eliminate noise or static. Switching from monophonic to
stereophonic reproduction both improves the spatial resolution of the sound
sources and simultaneously evokes pleasure.
An apparent problem with this view arises from composers penchant to
create music containing several concurrent parts. If pleasure is evoked by
successful parsing of the auditory scene, then why wouldnt musicians create solely monophonic music? A plausible answer to this question is that
the amount of pleasure evoked may be proportional to the perceptual difficulty posed by the scene. Consider, by way of illustration, the experience
of random-dot stereograms. These are three-dimensional images that are
coded in a superficially confusing array of random dots. Seeing the threedimensional image can involve considerable mental effort, but if the image
is successfully apprehended, there ensues a rush of delight for the viewer.
Similarly, it is possible that some multipart music is organized to challenge
the listeners auditory parsing capacities.
This interpretation carries several implications. It suggests that early
Renaissance polyphonists discovered an aesthetic effect akin to randomdot stereograms: by challenging the listeners auditory parsing abilities, the
potential for a pleasing effect could be heightened. However, this heightened pleasure would be possible only if listeners could indeed successfully
parse the more complex auditory scenes. Having increased the perceptual
challenge, composers would need to take care in providing adequate streaming cues. Following the rules of voice-leading and limiting the density of
parts (as we have seen in Bach) might be essential aids for listeners. A
further implication is that at least some of the listeners who claim to dislike
polyphonic music may fail to get it in the same way that some viewers
fail to get random-dot stereograms.
This interpretation also has implications for how we understand sensory
dissonance. Earlier, we interpreted the congruence between chordal-tone
spacing and the cochlear map as consistent with minimizing masking. However, the results are equally consistent with the goal of minimizing sensory
dissonance. Since both sensory dissonance and auditory masking are linked
to the critical band, it may be that the phenomena are two sides of the same
coin. The central characteristic of dissonance is the negatively valenced
phenomenal experience of listenersakin to annoyance. Why would listeners find spectral components within a critical band annoying? Perhaps
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David Huron
because the auditory system recognizes the presence of masking, with the
implication that any seemingly successful parsing of the auditory scene may
be deceptive or faulty.
Note that the preceding interpretation of the aesthetic origins of voiceleading should in no way be construed as evidence for the superiority of
polyphonic music over other types of music. In the first instance, different
genres might manifest different perceptual goals that evoke aesthetic pleasure in other ways. Nor should we expect pleasure to be limited only to
perceptual phenomena. As we have already emphasized, the construction
of a musical work may be influenced by innumerable goals, from social
and cultural goals to financial and idiomatic goals. Finally, it is evident
from Figure 17, that many areas of possible musical organization remain to be explored. The identification of perceptual mechanisms need not
hamstring musical creativity. On the contrary, it may be that the overt identification of the operative perceptual principles may spur creative endeavors to generate unprecedented musical genres.14,15
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2001 BY THE

REGENTS OF THE UNIVERSITY OF CALIFORNIA

School of Music, Ohio State University

The Rules of Voice-Leading

Part I: Perceptual Principles and Western Voice-Leading

We may begin by reviewing briefly the traditional rules of voice-leading.

The Rules of Voice-Leading

Registral Compass Rule. Harmony texts begin by specifying

13. Exposed (or Hidden or Direct) Octaves (and Fifths) Rule.

Having briefly reviewed the voice-leading canon, we can now identify

The Rules of Voice-Leading

diffuse rumblings. Frequencies in the middle range of hearing give rise to

The Rules of Voice-Leading

2. Principle of Temporal Continuity

The Rules of Voice-Leading

Temporal Continuity and Musical Practice

The Rules of Voice-Leading

lengths in order to maintain a continuous sound output. In the case of

The Rules of Voice-Leading

Fletcher showed that there is a close correspondence between distances

The Rules of Voice-Leading

Figure 4 should not be interpreted literally; only the approximate spacing

4. Tonal Fusion Principle

The Rules of Voice-Leading

Sensory Dissonance, Tonal Fusion, and Musical Practice

light of musical practice. As we have seen, sensory dissonance depends on

The Rules of Voice-Leading

The Rules of Voice-Leading

that temporal relationships are more accurately perceived within streams

Fig. 7. Schematic illustration of two possible perceptions of intersecting pitch trajectories.

The Rules of Voice-Leading

Pitch Proximity and Music

passages by Telemann, Dowling noted that Telemann never uses intervals

The Rules of Voice-Leading

The Rules of Voice-Leading

The Rules of Voice-Leading

Part II: Derivation of the Rules of Voice-Leading

In formal logic, an axiom is defined as a basic proposition (a statement)

Two corollaries follow:

From this we derive a nontraditional proposition or rule:

The Rules of Voice-Leading

In light of these nontraditional rules, a few pertinent observations may be

From this we derive the rule regarding registral compass:

Adding the principle of temporal continuity:

This leads to a simple nontraditional rule:

Adding the minimum masking principle:

Adding the principle of tonal fusion:

From this we derive the traditional injunction against unisons:

Recall that in Huron (1991b), Bachs polyphonic practice was shown to be

The Rules of Voice-Leading

From this we derive a nontraditional rule:

In a study of part-crossing in 105 works by J. S. Bach, Huron (1991a)

Huron (1989a) showed that polyphonic composers do indeed tend to avoid

The Rules of Voice-Leading

There are other principles that relate to the optimization of auditory

This leads to two nontraditional rules:

From this we may derive the

D22. Exposed Intervals Rule. When approaching unisons, octaves, or