ADDIS ABABA UNIVERSITY

College of Humanities, Language Studies, Journalism and

Communication

Department of Linguistics
Experimental Phonetics
Course/ Module: Speech Production and Perception (Ling 8102)

A Review on:

Source- Filter Theory of Speech

By:

Muhammed Teshome ID No.: GSR/5633/14

Submitted to:
Dr. Abebayehu Messele
June 10, 2022
Addis Ababa

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 Table of Contents
Title Page
INTRODUCTION ........................................................................................................................................ 1
1. LARYNGEAL STRUCTURE AND FUNCTION IN SPEECH .......................................................... 1
1.1. Anatomy of Larynx in Phonation................................................................................................... 1
1.2. Physiology of Larynx in Phonation ............................................................................................... 3
2. SOURCE- FILTER THEORY OF SPEECH ........................................................................................ 3
2.1. Voice Source .................................................................................................................................. 5
2.2. Voice Filtering ............................................................................................................................... 6
2.3. Sound Resonance ........................................................................................................................... 6
CONCLUSION ............................................................................................................................................. 8
REFERENCES ............................................................................................................................................. 9

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INTRODUCTION
The source–filter theory states speech as a combination of a sound source, such as the vocal
cords, and a linear acoustic filter, the vocal tract. An important assumption that is often made in
the use of the source–filter theory is the independence of source and filter. So, when we talk, we
use our tongues and lips and other organs to produce the different speech sounds. In the majority
of speech sounds, the vibrations of the air in the passages of the mouth, throat, and nose (which
are collectively known as the vocal tract) serve as the movements that initiate the sound waves in
different forms. The vocal tract terminated at one end by the vocal folds and at the other is open
to the air beyond the lips and nostrils; thus it forms a resonating chamber of a complex shape
(Ladefoged, 1996). When the air in this chamber is set in motion by sharp tap, it vibrates in a
complex way. It is these vibrations that cause the sound waves which we hear after filtered.

1. LARYNGEAL STRUCTURE AND FUNCTION IN SPEECH

Without the structure and function of vocal apparatus, we cannot talk of source-filter theory or of
generally speech; the biological configuration is a central mechanism.

1.1. Anatomy of Larynx in Phonation

Phonation, or voicing, is the product of vibrating vocal folds, and this occurs within the larynx.
Just as we referred to respiration as the source of energy for speech, phonation is the source of
voice for speech. Respiration is the energy source that allows phonation to occur; without
respiration there would be no voicing.

Anatomically, the cavity of the larynx is a constricted tube with a smooth surface. Sheets and
cords of ligaments connect the cartilages, while a smooth mucous membrane covers the medial-
most surface of the larynx. The thyrohyoid membrane, lateral thyrohyoid ligament, and median
thyrohyoid ligament cover the space between the hyoid bone and the thyroid. The hyoepiglottic
and thyroepiglottic ligaments attach the epiglottis to the corpus hyoid and the inner thyroid
cartilage, respectively (Anthony J. S., et. al (2010, p. 172).

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Figure (above) showing the anterior and posterior view of the larynx (Anthony J. S., et. al, 2010: 167)

Figure showing cavities of the larynx (Taken from Anthony J. S., et. al , 2010, P. 176).

The vocal folds in the larynx are actually five layers of tissue, with the deepest layer being
muscle. The space between the vocal folds is called the glottis (or rima glottidis), and the area
below the vocal folds is the subglottal region. The vocal folds are located within the course of the
airstream at the superior end of the trachea. As the airstream passes between the vocal folds, they
may be made to vibrate, much as a flag flaps in the wind (Anthony J. S., et. al ,2010: 165).

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1.2. Physiology of Larynx in Phonation
The larynx is an exquisite sphincter in that the vocal folds are capable of a very strong and rapid
clamping of the airway in response to the threat of intrusion by foreign objects. As evidence of
this function, there are three pairs of laryngeal muscles directly responsible for either
approximating or tensing the vocal folds, although there is only one pair of muscles responsible
for opening them. The vocal folds are wired to close immediately on stimulation by outside
agents, such as food or liquids, a response that is followed quickly by the rapid and forceful
exhalation of a cough. This combination of actions is designed to stop intrusion by foreign matter
and to rapidly expel them from the opening of the airway (Anthony J. S., et. al (2010: 166).

2. SOURCE- FILTER THEORY OF SPEECH

A useful analytical model of how speech sounds are produced, which emphasizes the
independence of the source of sound in the vocal tract form the filter that shapes that sound to
have its own qualities. A source produces a sound, while the filter shapes the sound.
Phonation/voicing is a source of sound and a resonator is a filter. To this end, resonators pass
some frequency and attenuate other frequencies. The source of a sound functions as indicative of
periodicity, pitch, intensity, duration and phonation quality (breathy, modal, creaky). The filter
determines, for example, height, advancement and rounding in case of vowels (Ladefoged,
1996).

When we talk about speech sounds, whether vowels or consonants, there are four sound sources:
glottal (or phonation) source, aspiration source, frication source, and transient source. The source
of the sound may vary. With vowels, the source will always be phonation in normal speech. With
consonants, other sources will include the turbulence of frication or combinations of voicing and
turbulence. Thus, the source-filter theory can be expanded and applied not only to vowels, but to
any speech sound, including consonants (Stevens, K. N. (2005). In all cases, we produce a noise
source and pass it through the filter of the oral cavity that has been configured to meet our
acoustic needs.

Speech sounds are the response coming from a vocal tract system where the sound source is fed
into and filtered by the resonance characteristics of the vocal tract. Source-Filter Theory explains

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the manner in which the vocal tract filters speech sounds. It includes a source function, transfer
function and output function (Stevens, K. N., 2005).

Flohberger, M. (2003)

In the source function, the source of all speech sounds is the glottis. The fundamental frequency
(F0) occurs here. In other words, when producing the sound, say the vowel /a/, the glottis
produces the sound at a certain amplitude and as this sound source goes through the vocal tract,
the amplitude of the source will increase. The sound source is modified by the vocal tract, which
acts as a sound filter. Again, the size and shape of the vocal configuration determines how the
sound is shaped.

In the transfer function, vocal tract cavities (laryngeal, pharyngeal, oral and nasal) and valves
(labial, lingual, velopharyngeal and laryngeal) involved. Different cavities filter the sound with
varying amplitudes (formants). This can be tied to the process of valving, manner and place of
articulation.

In the output function/ lip radiation, we hear a sound that emerges from lips. The frequency and
manipulation of vocal tract determines how sound is filtered. The fundamental frequency (F0)
remains the same, but the harmonics can be modified. Cavities are filters and the combinations
of cavities and valves gives the quality of a sound (Ladefoged, 1996).

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 2.1. Voice Source
Voicing is a periodic source produced by modulation of the airflow from the lungs by the vocal
folds. The vocal folds are muscular folds located in the larynx. The space between the vocal folds is
the glottis. A source is an input of acoustic energy into the speech production system. There are two
basic types: noise and voicing.

If the vocal folds are close together, then air pressure from the lungs can cause them to vibrate
periodically, generating voicing. Vocal fold vibration produces a complex periodic wave whose
spectrum contains energy at the fundamental frequency of laryngeal vibration and multiples of the
fundamental frequency (harmonics).

Air turbulence generated at an obstruction involves random (aperiodic) pressure fluctuations over a
wide range of frequencies. Noise generated at the glottis is called aspiration, while noise generated
elsewhere is called frication (Fry, D.B. 1979: 82).

Johnson, K. (1997)
A sound may involve more than one source: E.g. a voiced fricative combines voicing and
frication noise; voiceless fricatives can have noise generated at the glottis and at the
supralaryngeal constriction, and breathy voice can combine voicing and aspiration source.

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2.2. Voice Filtering
The vocal tract acts as a filter, modifying the source waveform. An acoustic filter is a device
which passes certain frequencies and attenuates others. An important characteristic of a filter is
its transfer function. The sound wave at some distance from the speaker is the result of filtering
the source with the vocal tract filter, plus the radiation characteristics of the lips/nose. The
column of air in the vocal tract is a resonator, hence a filter. The characteristics of the filter
depend on the shape of the vocal tract - we will explore the relationship by considering simple
cases (Flohberger, M., 2003).

2.3. Sound Resonance

According to Ladefoged (1996), resonance is the tendency to which one of the four cavities
(oral, nasal, pharyngeal and laryngeal) vibrate with greatest amplitude in response to a frequency
close to their natural frequency.

Ladefoged (1996: 65)

2.3.1. Resonators
A body, like a mass of air in a tube, naturally vibrates at one or more frequencies. If a pulse of
energy is imparted to the air, it will vibrate at these natural frequencies.

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If a source (driving force) which is vibrating at a natural frequency of the body is applied to it
(e.g. a tuning fork is held over the tube of air), the body will resonate with the source, i.e. vibrate
strongly at the same frequency. If a source of a different frequency is applied to the body, it will
vibrate with less amplitude. We can plot the response of a resonator to a range of input
frequencies. Peaks in spectrogram mark the resonant frequencies (Fant, G., 2001).
Resonators vary in the range of frequencies; they will respond strongly to a sharply tuned
resonator only responds strongly to frequencies very close to the resonant frequency (it has a
narrow bandwidth). And, a resonator with a wide bandwidth will vibrate strongly in response to
a wider range of input frequencies.

2.3.2. Resonators as an Acoustic Filters

A resonator acts as an acoustic filter in cases as in:
 The vibration of a column of air driven by a vibrating source.
 The source is the input to the filter.
 This input produces vibration in the column of air.
 This vibration produces a sound wave in the external air - the output of the filter.
 The strength with which an input frequency is output from the filter will depend on
the resonance characteristics of the air column - if it is near a resonant frequency, it
will be passed through to the output at full strength, if it is not near a resonant
frequency, it will have low amplitude in the output.

Ladefoged, 1996: 73

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In speech the input has energy at a range of frequencies (voicing or noise, or both) and the
resonator has multiple frequencies of varying bandwidths. The amplitude in the output wave of
each frequency component of the source depends on the extent to which the air in the vocal tract
resonates at that frequency. Thus the vocal tract filters the source. The resonances of the vocal
tract are called formants.

The resonant frequencies (formants) of air in the vocal tract depend on how the cross-sectional
area of the vocal tract varies over its length. The relationship between vocal tract shape and
transfer function is complex (Ladefoged (1996).

2.3.3. Resonance Occurrence

The vocal tract in a vowel can be approximated by a tube which is closed at one end (the glottis)
and open at the other (the lips). A compression or rarefaction travelling down a tube is reflected
off the ends of the tube. There is reflection even at the open end of a tube, but with a change of
phase - a compression is reflected as a rarefaction, and vice versa. For resonance to occur,
compressions must reach the closed end at the same time as the excitation source produces
another compression peak, and rarefactions must reach the closed end at the same time as the
source produces a rarefaction (Ladefoged 996, Fry, D.1979, Stevens K. N., 1998).

CONCLUSION
The source-filter theory states that speech is the product of sending an acoustic source, such as
the sound produced by the vibrating vocal folds, through the filter of the vocal tract that shapes
the output. The ever-changing speech signal is the product of moving articulators. Sources may
be voicing, as in the case of vowels, or the product of turbulence, as in fricatives.

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REFERENCES

Anthony J. S., Douglas W. K., & David G. D. (2010). Anatomy & Physiology for Speech,

Language, and Hearing, 4th edition. Delmar: Cengage Learning. pp. 165-377.

Flohberger, M. (2003). Source Filter–Model: Acoustic Tube Models, Linear Prediction &

Formant Synthesizer. Graz, 19.11.

Fry, D.B. (1979). The Physics of Speech. Cambridge: Cambridge University Press.

Johnson, K. (1997). Acoustic and Auditory Phonetics. Malden, MA: Blackwell Publishers.

Ladefoged, P. (1982). A Course in Phonetics, 2nd edition. New York: Harcourt Brace

Jovanovich.

Ladefoged, P. (1996). Elements of Acoustic Phonetics. Chicago, University of Chicago Press.

Stevens, K. N. (1998). Acoustic Phonetics. Cambridge, MA: MIT Press.

Stevens, K. N. (2005). The acoustic/articulatory interface. Acoustical Science and

Technology, 26(5), 410-417

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