US3393764A - Loudspeaker systems - Google Patents

Description

July 23, 1968 c. R. scr-:AFER

LOUDSPEAKER SYSTEMS Filed Dec. 27. 1966 IN VEN TOR.

Caer/55 R. ScHA FER EBV Attorney United States Patent O 3,393,764 y LOUDSPEAKER SYSTEMS Curtiss R. Schafer, RED. 1, Poverty Hollow Road, Newtown, Conn. 06470 Continuation-impart of application Ser. No. 311,721, Sept. 26, 1963. This application Dec. 27, 1966, Ser. No. 604,667

11 Claims. (Cl. 181-31) ABSTRACT OF THE DISCLSURE The disclosed loudspeaker system involves a pair or multiple pairs of oppositely directed, matched loudspeakers in a largely sealed enclosure, the back faces of the speaker diaphragms of each pair being directly exposed to each other and excited to move concurrently inward and concurrently outward so that the direct acoustic coupling between the rears of each pair of speakers inside the enclosure provides dynamic damping for correcting distortion, the whole compact unit simulating a large high-quality sound source.

This is a continuation-impart of my application Ser. No. 311,721, filed Sept. 26, 1963, now abandoned.

The present invention relates to loudspeaker systems, and is primarily concerned with providing enhanced sound reproduction, lbetter than what might be expected from the particular loudspeakers used. p

An object of the invention is to provide a novel loudspeaker system that will afford faithful SOun-d reproduction, being characterized particularly with improved transient response, reduced harmonic and intermodulation distortion, and improved bass response, as cornpared with the performance of loudspeakers of like quality in know batiie arrangements. A further object resides in the provision of a novel loudspeaker system that is operative over a Awide frequency range, particularly in the bass range, Without depending on a large or elaborate baiiie structure.

Many diferent forms of Ibaille structure have been used with loudspeakers. Wall mounting (true infinite baie) gives good frequency response butpoor transient response. A loudspeaker mounted in a fiat baiie of large but limited size is less adequate in respect to low-frequency performance than wall mounting. The familiar open-back cabinet is still less desirable. A loudspeaker in a sealed box of limited yet substantial volume (a simulated infinite batiie) gives a boxed-in or muffled effect. A loundspeaker in a cabinet formed with a folded or labyrinth passage lroughly proportioned `as an acoustic horn tends to become bulky, and has :poor damping in the lowfrequency range.

Loudspeaker syste-ms made pursuant to the present invention represent a distinct improvement in many respects over these and other known baille arrangements. Using relatively inexpensive loudspeakers in the novel systems, excellent loudspeaker performance can be obtained rivalling that of much more expensive loudspeakers in conventional baffles. In the novel system, a pair of loudspeakers is contained in a confined enclosure, the loudspeakers being directe-d oppositely or Iat least in roughly opposite directions, and the cones of the loudspeakers being driven either simultaneously outward relative to the enclosure or simultaneously inward relative to the enclosure.

The invention is concerned with correcting faulty reproduction due to the transient response of a loudspeaker, and due to other types of distortion. These effects are widely recognized, and a variety of electrodynamic arrangements to correct them have been devised. In such arrangements, the signal generated in the moving coil that drives the cone has been fed back into the audio amplifier, phased for negative feedback. However, such an arrangement would seem effective only where the diaphragm .and the coil move alike, i.e., where the cone moves as a unit, like a piston. Such operation occurs only for the lowest frequencies. For other frequencies, different portions of the cone move differently so that the coil motions can not be used to sample and to correct faulty cone operation by negative electrical feedback. Pursuant to the present invention, there is direct, phaseopposed acoustic feedback from each cone to the other, thus providing dynamic damping that has proved highly effective for correcting distortions, especially for improving transient response.

A further object of the invention resides in the provision of a very compact loudspeaker unit which, nevertheless, performs very much like a relatively large source of sound.

To best advantage, the enclosures of the present invention are compact. However, the impression to the listener is that of a very large source of sound. This effect may perhaps be due to the fact that, while one loudspeaker of a pair is `directed generally toward the listener, the sound compression waves developed by the other loudspeaker of the pair tend to spread laterally before ljoining the sound energy of the first loudspeaker traveling toward the listener. The combined energy distribution simulates a large sounce of sound.

The nature of the invention and its further objects and features of novelty will be more fully understood from the illustrative embodiments shown in the accompanying drawings, and from the detailed description thereof, below. In the accompanying drawings:

FIGURE l is a vertical cross-section of a presently preferred embodiment of a novel loudspeaker system illustrating certain features -of the invention;

FIGURE 2 is a diagrammatic illustration of another illustrative embodiment, the enclosure being shown in vertical cross-section;

FIGURES 3 and 4 are front elevation and top plan views, respectively, of another modification shown diagrammatically;

FIGURE 5 is a lateral elevation of still another embodiment shown diagrammatically;

FIGURE 6 is a lateral elevation, partly in section and largely diagrammatic, showing a modification of the embodiment in FIG. 5;

FIGURE 7 is the top plan view of a `further embodiment; and

FIGURE 8 is the front elevation of another embodiment of the invention.

FIG. 1 shows a spherical enclosure 10 having a pedestal 12, and two open-mesh spherical grilles 14. Two loudspeakers 16 and 16 of the moving-coil type are disposed with their cones 18 and 18' facing in opposite directions behind grilles 14. Each loudspeaker has a voice coil 20 (20') that operates in the air gap of permanentmagnet pole structure 22 shown diagrammatically. Framework 24 supports the pole structure 22 and the corrugated and compliant periphery of cone 18. Framework 24 has openings 24a that expose the rear surface of cone 1S freely to the interior of the enclosure. Sounddeadening material 26 such as felt or cork-neoprene aggregate cover the rear ixed surfaces of the framework, and such material is interposed between the pole structures 22 and 22 of the oppositely directed loudspeakers, these covered surfaces otherwise being sound-reflecting.

The rear portions of the two loudspeakers are close to each other, with ample space for the described sounddeadening material. With the arrangement of loudspeakers shown, the volume within the enclosure is limited, being usually little more than that necessary to enclose the two loudspeakers. A vent or constricted opening 28 is shown, the effect of the constriction being enhanced by a short tube forming a passage from the interior space of the enclosure to the exterior.

Voice coils 20 and 20' are connected to a common -source of audio signals. The connections are phased so that coils 20 and 20 move in opposite directions in response to input signals, i.e. both coils move toward each other at some moments while both coils move away from each other during other moments.

During operation, the closeness and directness of the acoustic coupling between the rear surfaces of cones 13 and 18 produces a mutually opposing or degenerative effect. The loudspeakers being in `an enclosure that provides an effective baffle (in this embodiment and the others to be described), this relationship has the important effect of leveling-off resonances that tend to develop at different parts of the frequency spectrum, and it tends to cancel transients which are produced by a faulty tendency of either or both cones to overshoot. Such overshoot tends to produce an acoustic pressure that is relatively greater than the electrical signal prducing it. This relationship is especially effective where the loudspeakers are of the same type and size and are made by the same manufacturer, being therefore practically identical.

The opposing effects in the unit of FIG. 1 insofar as signal drive is concerned, can be taken into account in the signal source, in estimating the effective impedances of the coils and the drive required. Where the motion of the cone is spurious, that is, not proportional to the signal, each spurious acoustic signal tends to suppress the other, dynamically.

The vent opening 28 is small, being 1/2 inch to 2 inches in diameter in enclosures for l2-inch diameter cones, and a smaller range of vent sizes rfor smaller cones. The limited size of vent allows some air to escape during low-frequency large displacements of the two cones, thereby avoiding `what could be considered a stifling effect at low-frequencies. On the other hand, the smaller sizes of the vents used, the better is the suppression of resonant responses, transients, etc.

The sound-absorbing material 26 suppresses direct refiections from each cone to the frame structure of the other loudspeaker and other opposite sound-reflecting surfaces.

Various other forms of enclosure are contemplated. Thus, in FIG. 2, a pair of speakers 16a, 16a are within a rectangular, nearly cubic enclosure a. Soundabsorbing material 26a is provided, as before, to suppress refiections. Enclosure 10a has flat faces at which the cones of loudspeakers 16a and 16a `are exposed. A constricted vent opening 28a is also provided.

In FIG. 2, the cones of the loudspeakers are directly aligned and close together, consistent with practical limitations. Accordingly, there is a direct and effective acoustic coupling between them. The dual enclosed speaker system of FIGS. 3 and 4 also involves close acoustic coupling between the cones of two loudspeakers which are alike or Substantially identical. Here, the two loudspeakers 16b and 16h -are offset laterally from each other. A side view would show one loudspeaker overlapping the other. However, the close proximity between the back surfaces of the cones and the minimal volume of enclosure 10b create effective acoustic coupling between the cones, approaching the close coupling between the cones in FIGS. 1 and 2. The different configuration of FIGS. 3 and 4 has the advantage of greater design fiexibility, in. that varied pleasing proportions of the enclosure can be used without sacrificing the improved performance that can be realized with directly opposed loudspeakers.

In FIG. 5, an enclosure 10c has three pairs of loudspeakers 16C and 160'; 16d and 16d; and 16e and 16e. The speakers of each pair are matched, being preferably units of like design from the same manufacturer. These pairs of loudspeakers are specially proportioned for performance in the lowest frequency band of the audio range, the middle register, and the high-frequency range. This is called a three-way system, improved in the several respects discussed above. FIG. 6 shows a modification of FIG. 5. A two-way system is illustrated including vertically aligned pairs of loudspeakers. Bass loudspeakers'16f' and lof have an enclosure 10d providing close coupling between the rear surfaces of their cones, and treble loudspeakers 16g and 16g have a much smaller enclosure 10e providing close acoustic coupling between the rear surfaces of their cones. Loudspeakers lof and lof have a nominally fiat response range of l5 to 1,000 cycles and loudspeakers log and 16g havea range of 300 to 20,000 cycles.

FIG. 7 shows two pairs of matched loudspeakers 16h and 16h', and 161' and 161", in a common pedestal-type enclosure lflf.

FIG. 8 is a multiple-speaker system in which a group of matched loudspeakers 16j are assembled in one fiat panel of an enclosure 10g, .and a second group of loudspeakers (not shown) are exposed at the back of the enclosure. The illustrated arrangement of loudspeakers can be modified for particular effects, as by disposing the loudspeakers of FIG. 7 in vertical alignment. The speakers are matched to each other, in front-and-back pairs, and the speakers of each group are alike.

In each of the examples shown, it is understood that a constricted vent opening is provided in the enclosure, for example a round hole whose diameter is less than a quarter of the diameter of the cone, where cone-type loudspeaker-s are used. Also, in each embodiment there is used a layer or body of sound-absorbing material (eg. 26 and 26a) covering the hard surfaces in the enclosure, especially those hard surfaces directly to the back of the cone.

The foregoing loudspeaker systems are useful in providing greatly improved performance when using inexpensive loudspeakers, as compared to that which could be expected with known bathe structures. The improvement is most marked when using inexpensive or mediumpriced ($25 to $75 each) loudspeaker units, but is also quite noticeable when relatively expensive units to each) are used. In FIG. 6 (and in each of the other embodiments) there is a pair of oppositely directed loudspeakers that are closer to each other than the maximum transverse dimension of one such speaker, resulting in a compact construction. The compactness is an advantage in that little space is needed for the speaker unit; and yet the operation simulates a large-sized sound source, particularly in the low-frequency audio range.

Various modification of the illustrative embodiments may be readily devised by those skilled in the art, and therefore the invention should be construed broadly, oonsistent with its full spirit and scope.

What is claimed is:

1. A loudspeaker system including an enclosure and a pair of substantially matched loudspeakers substantially sealed in the enclosure, said loudspeakers having cones whose rear surf-aces are exposed to the inte-rior of the enclosure and whose front surfaces are exposed to the exterior of the enclosure, said exteriorly exposed surfaces being directed approximately in opposite directions and said interiorly exposed surfaces having substantially unobstructed and direct acoustic feedback therebetween, said pair of loudspeakers being connected together for connection to a common source and phased relative to each other so that both cones move inward concurrently and outward concurrently.

Z. A loudspeaker system in accordance with claim 1, wherein fixed interior sound-reflecting surfaces directly opposite to the cone of each of said pair of loudspeakers are covered with sound-absorbing material.

3. A loudspeaker system in accor-dance with claim 1, wherein the pair of loudspeakers are closer to each other than the maximum transverse dimension of either loudspeaker.

4. A loudspeaker system in accordance with claim 3, -wherein said pair of loudspeakers are in back-to-back alignment.

S. A loudspeaker system in accordance with claim 3, wherein said pair of loudspeakers have mutually off-set cent-ral axes, and wherein the lateral projected View of one of said pair of loudspeakers overlaps the lateral projected view of the other of said pair of loudspeakers.

6. A loudspeaker system in accordance with claim 1, including multi-ple pairs of loudspeakers, all of said loudspeakers being substantially matched, and one loudspeaker of each pair being disposed in a respective one of a pair of oppositely directed surfaces of the enclosure.

7. A loudspeaker system in accordance -with claim 1, including multiple pairs of loudspeakers, each pair being specially proportioned for performance in a particular portion of the audible frequency range, and at least the pair of loudspeakers for the lowest portion of the audible frequency range being disposed closer to each other than the maximum transverse dimension of one such loudspeaker.

8. A loudspeaker system in accordance with claim 1, wherein said enclosure includes a constricted vent.

9. A loudspeaker system in accordance with claim 3, wherein said enclosure is sealed except for a constricted vent.

10. A loudspeaker system in accordance with claim 1, including multiple pairs of loudspeakers, each pair being proportioned for optimum performance over a particular portion of the audible frequency range, and each said pair of loudspeakers having its separate close-coupling rear enclosure.

11. A loudspeaker system in accordance with claim 1, wherein the loudspeakers of the system are directed only in said opposite directions.

References Cited UNITED STATES PATENTS 2,114,680 4/ 1938 Goldsmith 179--1 2,544,742 3/1951 Volf 181-31 2,768,702 10/'1956 L-owry 181-31 3,054,856 9/ 1962 Arany 179-1 y31,0745 03 1/ 1963 yCool 181-'3'1 3,136,382 l6/ 1964 Thaler 181--31 FOREIGN PATENTS 975,222 10/ 1961 Germany. 1,075,666 2/ 1960 Germany. 1,196,858 6/-1959 France.

STEPHEN J. TOMSKY, Primary Examiner.

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