CA1128873A - Loudspeaker coupler - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
An electrically driven loudspeaker is coupled to the atmos-phere, which is to carry sound to the human ear, through a hol-low coupler the inlet end of which has a cross sectional area com-parable to the effective area of the loudspeaker, said cross sect-ional area decreasing progressively from said inlet end to an outlet end of substantially smaller cross sectional areas. In a specific embodiment of the invention, an exponential horn is form-ed within the confines of a loudspeaker cabinet enclosure, and the inlet end of the horn is coupled to the electrically driven loudspeaker by means of the hollow coupler also formed within the cabinet enclosure. The outlet end of the coupler has the same cross sectional area and shape as the inlet end of the horn.

Description

3~73 LOUDSPEAKER COUPLER
BACKGROUND OF THE INVENTION
This invention relates to loudspeakers, and more particu-larly to novel means by which to couple the driven component of a loudspeaker to the atmosphere.
Loudspeakers and other types of sound reproducers hereto-foxe have been coupled directly to the atmosphere using such devices as Helmholtz resonators, acoustic suspensions, infinite baffles, tuned ports and others ~o alter the out-of-phase sound emanating from the rear of the driving unit. These devices pro-vide a "system resonance" intended to reinforce low frequency 10 drivers so as to achieve a "flat" response curve. Such devices introduce sounds that are not present in the original music and they also cause distortions created by their sharply defined boundaries. These artifîcial sounds and distortions are ~urther amplified when they are fed through exponential hornsO
Exponential horns are recognized as very effective devices for coupling sound reproducers to the air which is to carry the sound to the human ear. The primary disadvantage of an exponen-tial horn resides in the excessive length, from its inlet to its outlet, required to transport without distortion, those sound 20 frequencies at the low end of the audible spectrum.
Historically, it has been recognized that an exponential horn designed to produce an undistorted audible note of 30 Hertz from a 15 inch diameter woofex, requires that the loudspeaker be confined in a cham~er of no less than 5200 cubic inches and hav-ing an outlet of 75 square inches matching the inlet opening (throat) of the horn, and that the horn must exceed 16 feet in length and terminate in a mouth opening equivalent to 127 square feet, or about 11 feet squareO Further~ if the inlet end of the horn is provided with a larger cross sectional area~ for example 3~ to at least match the effective cross sectional area o~ a larger loudspeaker diaphragm, the outlet end of the exponen~ial horn is even more unreasonably large. In any event, the folding of such a path requires an unacceptably large cabinet, at least for residential usage.
In order to utilize at least some of the benefits of an exponential horn, it has been the practice heretofore to couple the disphragm of a loudspeaker to the inlet of the horn by means of a "slot" forLned by a chamber which communicates with the loud-speaker and which also has an outlet "slot" or passageway, of smaller dimensions than the loudspeaker disphra,m but matching 10 the inlet end of the horn. The cross sectional area of the cham-ber changes at random from the diaphragm to the slot. The cross sectional area of this outlet passageway generally is greater than about one-fourth the cross sectional area of the loud-speaker diaphragm. Neverthelessg such a reduction in cross sectional area of such a "slot" relative to the loudspeaker diaphragm, allows the dimensions of the exponential horn to be reduced to a degree that allows the horn to be folded within a cabinet of overall dimensions which render it practicable for use at least in large rooms. On the other hand, the size o~ ~ch 20 a cabinet is unsuitable for use in the average home, and further size reduction, through further reduction in the dimensions of the coupling slot diminishes the quality of sound reproduction to an unusable level~
SUMMA~Y OF THE INVENTION
In its basic concept, this invention provides for the coup-ling of an electrically driven sound xeproducer to the atmosphere by means of a hollow coupler the cross sectional area of which decreases progressively from its inlet end to its outlet end.
It is by virtue of the foregoing basic concept that the principal objective of this invention is achieved; namely, to overcome the distortions resulting from the resonances which characterize the coupling devices of the prior art described hereinbefore.

~ 3 Another ~mportant objective is to provide a coupler of the class described which maybe associated with an exponential horn and which permits the olding of such a horn within a sound reproducer cabinet of such minimum volume as to render it pract-icable for use in conventionally sized residential rooms.
Another important objective of this invention is the pro-vision of a sound reproducer coupler of the class described which, in association with an e~ponential horn, allows the in-let end of the horn to be reduced in cross sectional area many times smaller than has been possible heretofore, while maintain-ing maximum efficiency of sound transferO
Still another important objective of this invention is the provision of a sound reproducer coupler of the class described which, in association with an exponential hoxn, provides for matching the acoustic re~istance of the inlet of the exponen-tial horn and the acoustic impedance of ~e sound reproducer as-sociated therewith.
A further specific objecti~e of this invention is the pro-vision of a loudspeaker couplex of the class described in which minimiæation of the cross sectional area of the outlet end of the coupler serves beneficially to reduce subs~an~ially the loudspeaker diaphragm excursions required to produce a given sound level in the air, thereby correspondingly reducing dis-tortions of the reproduced sounds.
~ still further important objective of ~his invention is the provision of a sound reproducer coupler of the class des-c~ibed which is free of sound absorbing, throa~ choking material, wkereby all of the sound frDm the loudspeaker is heard in sub-stantially unda~ped condition throughout ~he audible spectrum, thereby contributing beneficially to an output characterized by crisp, lifelike sounds.
~ further important objective of this invention is the pro-vision of a loudspeaker coupler of the class described which is ~$ ~ ~ ~

capable of utilizing the backwave sounds with minimum distortion and which may incorporate therewith a plurality of tweeters and other speakers arrayed in any desired manner.
The foregoing and other objectives and advantages of this învention will appear from the ~ollowing detailed descrip~ion, taken in connection with the accomanying drawings of preferred embodiments.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a schematic representation, in longitudinal sect-10 ion~ of a loudspeaker having associated therewith a coupler em-bodying the features of this inventionO
Fig. 2 is a schematic representation, in plan view, illus-trating the manner in which the diaphragm of a loudspeaker is coupled to the inlet of an exponential horn in accordance with this invention.
Figs. 3, ~ and 5 are schematic representations, in plan view, illustrating various structural arrangements accommoda~ing the coupling concept of this invention, and Figs. 3a, 4a and 5a are electrical diagrams illustrating the equivalent electrical 20 circuits representing the structural configura~ions of ~igs. 4, 4 and 5, respectively.
Fig. 6 is a vertical cross section through a loudspeaker enclosure embodying the features of this invention.
Fig. 7 is a sectional view taken on the line 7-7 in Fig.6.
Fig. 8 is a sec~ional view taken on the line 8-8 in Fig.7.
Fig. 9 is a vertieal cross section through a loudspeaker enclosure illustrating a second embodiment of this inventionO
Fig. 10 is a sectional view taken on the line 10-10 in Fig. 9.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Fig. 1 illustrates a loudspeaker 10 supported in a housing 12, prefera~ly perforated for exposure to atmospheric pressure.
The ele~trically driven diaphragm 14 of the loudspeaker is coup-led to the atmosphere through a hollow coupler 16 which~ inaccordance with this invention, provides an acousti-¢ally mat-ched coupling between the loudspeaker and the environment, thereby minimizing distortions as are created by the resonant chambers and pipes which characterize the coupling devices of the prior art. As illustrated, the inlet end 18 of the coupler has a cross sectional area comparable to the effective cross sectional area of the diaphragm 14. Further, it is to be noted that the inlet end 18 is closed by the diaphragm and is sealed thereby against the passage of air or other fluid pressure.
~Q The outlet end 20 of the coupler, sometimes hereinafter refer-red to as the "bore" is of substantially smaller cross section-al area, as discussed mo~ fully hereinafter.
The coupler may take a variety of shapes. Thus, the coup-ler in Fig. 1 may be generated symmetrically about a common longitudinal axis as is the case in Fig. 2 described hereinafter.
It may be rectangular in cross section, with the sides 22 ccn-verging progressively from the inlet end 18 to the outlet end 20 as illustrated, and~ith the opposite sides 24 being disposed either parallel to each other or converging progressively from 20 the inlet end 18 to the outlet end 20~
The sides of the coupler may be constructed of a variety of materials. They may be made of materials such as Firtex~, or other particle boards and constructed so that ~hey flex and thereby respond to the internal air pressures generated by the loudspeaker or other form of sound reproducer. The responsive surfaces of such materials radiate the sounds to the atmosphere devoid of the usual resonances described hereinbefore, and the progressive change of cross sectional area eliminates the dis-tortions also referred to hereinbefore. When made o these types 30 of materials, the coupler may serve as the only medium by which the sound reproducer is coupled to the atmosphere, by providing a larger sur~ace for radiating the sound than the spea~er dia-~ c~ r ~

phragm itself.
On the other hand, the coupler may function to couple aloudspeaker or other sound reproducer to the inlet of an expon-ential horn. In such instances~ it is preferred that the sides of the coupler be made so that they are substantially inflexible and thereby do not respond efficiently to the internal pres-sures generated by the reproducer. Materials for this type of coupler include a variety of synthetic thermosetting and ther-moplastic resins, wood, ceramic, and others.
Referring now to Fig. 2 of the drawings, there is illus-trated schematically an exponential horn 26 the inlet end of which is generally referred to as the "throat" and is coupled to the electrically driven diaphragm 14 of the loudspeaker 10 through a hollow coupler 16. As explained hereinbefore, the inlet end 18 of the coupler preferably has a cross sectional area comparable to the effective cross sectional area of the diaphragm 14 and is closed and sealed by the latter. The out-let end, or "bore" 20 of the coupler and the throat 20'of the horn are connected at the same ~ransverse plane and therefore obviously have the same cross-sectional area and shape.
The arrangement illustrated in Figo 2 achieves a funda-mental objective of this invention; namely, the acoustic re-sistance of the bore 20 matches the reflective impedances of both the coupler 16 and horn 26~ whereby to effect cancelling of acoustic resonances in either. This is analogous to the termination of an electrical transmission li~ by a resistor equal in value to the characteristic impedance of the trans-mission line, whereby to effect cancelling of reflected waves.
Thus, referring to Figs. 3 and 3a of the drawings, wherein Fig. 3 is the same as Fig. 2, the transformer 26 in Fig. 3a 30 represents the electrical equivalence of the exponential horn 25 and the transformer 16 represents the electrical equivalance of the coupler 16. The resistor 28 represents the electrical equivalance of the throat resistance and the resistor 30 rep-resents the electrical equivalence of the room resistance.
This arrangement substantially improves the acoustic impedance matching of ~he speaker 10 through ~he exponential compression transformer 16 to a resistive load 28 at the junc~ion 20 be-tween the coupler 16 and the exponential horn 26. This re-sistive load 28 matches the impedance of the input end of the exponential horn 26 with the open end of the horn matching the very high impedance oE the sound-carrying air within a room.
Accordingly, it will be apparent tha~ it i9 primarily the ratio of the cross sectional areas of the closed inlet end 18 of the coupler 16 and the outlet end 20 thereof that deter-mines whether the magnitude of acoustic resistance is suffi-cient to render the hollow coupler non-resonant. The effective cross ~ectional area of the outlet end 20 is reduced somewhat by forming the coupler walls 22 and 24 of such sound absorb-`-- ing materials as Firtex and others, as previously mentioned.
Thus, the condition of non-resonance is established by measure-ments of sound levels at given frequencies attributable to coup-20 lers of different materials having different inlet and outletratios, the condition of non-resonance being exhibited by the lack of variations in sound levels at given frequencies.
Fig. 4 illustrates a modification of Fig. 3 by the incor~
poration of one or more resonant chambers, two such chambers 32 and 34 of difEexen~ ~ol~unes being illustrated, each communi-cating with ~he throat 20 through a radial opening 32', 34' therein.
In Fig. 5~ one or more tuned pipes 26 are interposed between the bore 20 and throat 20'. Although only one such 30 tuned pipe i9 illustrated, it will be understood that a plur-ality of differently tuned pipes mayke disposed side-by-side within the transverse dimensions o:E a loudspeaker enclosure, as will become appaxent hereinafter.
'~ Tr~7GJ~ rk In Fig. 2, the combination of loudspeaker diaphragm 14, coupler 16 and exponential horn 26 is shown to be generated symmetrically about a common longitudinal axis 38. In practice, however, loudspeaker cabinets generally arerectangular in shape, as illustrated in Figs. 6-10. Accordingly, since loudspeaker diaphragms are usual~y circular in cross section, it is nec-essary that the transition from the circular cross section of the diaphragm to the rectangular cross section of the cabinet be made in such a manner that the rectangular cross sectional area immediately adjacent the diaphragm is compara~le to the effective cross sectional area of the diaphragm. Ideally, it is desirable that the cabinet be provided with a transition volume by which the circular cross section of the cabinet open~
ing registering with and matching the effective cross section-al area of the loudspeaker diaphragm, be converted to an out-let opening of rectangular cross section of equal area. This ideal arrangement is provided in the embodiment illustrated in Figso 9 and 10 and described in detail hereinafterO
Referring first to the embodiment illustrated in Figs.
20 6, 7 and 8, the cabinet is formed of side walls 40, back wall 42, bottom wall 44, top wall ~6 which supports the loudspeaker 10, and cover 48, which conceals the loudspeakPr.
The top wall 46 is provlded with a circular opening 50 having a cross sectional area equal to the effective cross sectional area of the diaphragm of the loudspeaker. It is through this circular opening that the loudspeaker communicates with and closes the inlet end 18 of thP coupler 16, the outlet end, or bore 20 of which communicates with the inlet end, or throat, of the exponential horn 26.
The coupler 16 and exponential horn 26 may be provided within the cabinet by various construction techniques. In the embodiment illustrated in Figs 6, 7 and 8, the interior of t~

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cabinet is fitted with preformed structural sections 52, 54 and 56 which cooperate with the ~a~inet walls to provide be-tween them the hollow coupler 16 and exponential horn 26. The sections may be formed of any desired material, sucha, wood, rubber, paper, fiberboard, synthe~ic thermoplastic or thermo-setting resins, or other suitable material, and by any well known procedure, such as molding, stamping, fabricating, or other desired techniques.
In the emb~diment illustrated, the sections 52 and 54 10 are formed of a mixture of rubber and synthetic thermoplastic resin and supported in properly spaced-apart relation by means of the end support plates 58 and the laterally eætending dowel pins 60.
The integrated assembly of sections 52 and 54 with structural plates 58 may be secured withi~ the cabinet by any suitable means, such as adhesive, screws, etc.
It is to be noted, particularly from Fig. 8 of the draw-ings, that the inlet end 18 (Fig4 6) of the coupler 16 is of rectangular cross section and somewhat larger in area than the ~ cross sectional area of the circular opening 50 in the top wall ; 46. Although some loss of fidelity of sound transfer xesults from this less than ideal arrangement~ it has been found that the loss is more technical than apparent in the human ear, and therefoxe the axrangement, in which the cross sectional areas of the opening 18 and diaphragm 14 are comparable, is qute ade-quate for all practicable purposes.
It is to be noted from Fig. 6 that the coupler 16 ex-tends from its inlet end 18 downward to its outlet at point 20 of minimum cross sectional dimension. This '~ore" also is 30 the inlet or "throat" of the exponential horn 26 which, in its folded condition, extends upward toward the rear of the cabinet between the forming sections 52 and 54 and thence downward betw-een the back wall 40 of the cabinet and forming section 52, _9.

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~and 5~ and thence downward between the back wall 40 of the cab-~inet and forming section 52, thence forward to the open front of the cabinet. Typical dimensions for this cabinet are 38" high, 17" wide and 18" deep, accommodating a 12" diameter cone speaker.
Experiments conducted with the illustrated arrangement have shown that the size of the bore 20 has been reduced to as small as 0.2" in height by 15" in width and has performed succes-sfully with a 12" diameter speaker. This cross sectional area is 1/38 that of the speaker. Of particular interest is the observa-tion that speakers of smaller diameter but of comparable qualitymay be utilized in this cabinet, with no noticeable loss in performance.
Further, such experiments have also shown that frequencies well below the theoretical cut-off limit of the horn itself are passed without large attenuation, as expected by cut-off calcu-lations. For example, a horn and coupler assembly calculated to provide undistorted transmission of a minimum 65 Hertz note, actually passed an undistorted 12 Hertz note.
The internal section 56 is illustrated as ~eing formed of a curved section o wood secured to the walls of the cabinet by any unsuitable means. It may be formed of the same material as the sections 52 and 5~. If desired, it may be formed in segments, in which case the support plates 58 are extended downward to the bottom wall 44 of the cabinet to support the segments of section 56, in the same manner as previously described.
Figs. 9 and 10 illustrate a s~ructural arrangement in which the coupler 16 extends from its inlet downward and thence rearward and upward to the point 20 of minimum opening. Thi~ is the bore of the coupler 16 and the throat o~ the exponential horn 26, as previously explai~ed. The horn progresses upward and rearward, in surpentine fashion, and thence downward and forward to the open front of the cabinet. The arrangement illus-trated provides, within a cabinet 40" high, 15" wide and 22"

deep, housing a 12" diameter loudspeaker, an exponential horn having an over-all length of 10'. This provides for the undistort-ed transmission of sound frequencies as low as 30 Hertz.
The embodiment of Figs. 9 and 10 also illustrates means providing a transition volume 62 by which the circular cross sect-ion of the opening 50 in the top wall 46, registering with, closed by and matching the effective cross sectional area of the diaph-ragm of the loudspeaker 10, is converted to rectangular cross section of equal area. This transition volume is provid~d by asso-10 ciating with the upper portions of the segments 52 and 54, pre-formed sections 64 which span the space be~ween the upper portions of the sections 52 and 54. These sections 64 have semi-circular inner edges at their top end and a straight edge at their bottom end, merging wi~h the side walls 40 of the cabinetc This transu-tion volume is an in~egral par~ of the coupler 16 and provides the ideal structural arrangement reerred to hereinbefore, sinc~
it converts the circular cross section of opening 50 to the rect-angular cross section of equal area.
As noted hereinbefore, the preferred coupler 16 of this in-20 vention is characterlzed by having an inlet 18 closed and sealedby diaphragm 14 and of cross sec~ional area comparable to the effective cross sectional area of the loudspeaker 10. I~ also has an outlet 20, or bore, of cross sectional shape generally match-ing the cross sectional shape of its inle~, but much smaller in dimensions. The shape and cross sectional area of the bore mat-ches the cross sectional shape and area of the inlet, or throat of the exponential horn~
Additionally, the coupler of this invention has a cross sectional area which decreases progressively from its inlet to 30 its outlet. This progressi~e decrease in cross sectional area may be uniform, providing the coupler with the shape of a trun-cated cone -~ig. 2~ or wedge (Fig. 6). As previously mentioned, all four sides may decrease in cross sectional area progressively from its inlet to its outlet. In the preferred form of coupler, ~he cross sectional area decreases exponentially.
Thus, in the embodiment illustrated in Figs. 6 and 9, al-though the lateral sides of the coupler are parallel, being de-fined by the lateral sides ~0 of the cabinet, the front and back walls defining the depth dimensions of the coupler are curved ex-ponentially. Although a variety of configurations is suitable for this purpose, the hyperbolic curve has been found to provide opti-mum results.
The coupler 16 of thi5 invention provides many advantages:
It functions to provide acous~c damping upon any remote tenden-cy_toward resonance within the coupler, by virtue of the true acoustic resistance quality of the bore 20. With loudspeakers of the movable diaphragm type, it reduces the excursions required to produce a given sound level in the air. By thus reducing Hook's law forces, corresponding reduction in sound reproduction distortions also is achieved, from both front and ~ack sides of the speaker.
The damped horn characteristics o the coupler render it 20 useful for driving resonant chambers for special ef~ects.
-rt is by virtue of the provision of the hollow coupler that the inlet end of the exponential horn may be reduced to minimum cross sectional area without introducing acoustic distortions and o~her deleterious factors. Minimization of the cross section-al axea of the inlet end of the horn beneficially affords minimiz-ing the size of the cabinet in which to contain the horn, by allowing the hown to be folded in a variety of ways to minimize the volume containing it.
The provision of the coupler functions to match the acous-30 tic resistance of the bore with the reflected impedances of boththe coupler and exponential horn, whereby to provide for maxi-mum transfer of energy from the speaker to the environmentO By collecting the sound through purposeful compression in the coup-~ 3 ler into a high acoustical impedance and then guiding ~he sound expansion exponentially through the horn, the ~irtual radiating surface is enlarged many times, with a marked improve-ment in ~he ability to reproduce low frequency sounds, as well as the higher requency sounds throughout the audible spectrum.
Many variations of the structural arrangements illustrat-ed in the drawings may be made. For example, the outlet end of the coupler may communicate with the inlet end of a passageway of uniform cross section, the outlet end of which communicates 10 with the inlet end o~ the exponential horn. This passageway may be one or more tuned pipes, as illus~rated in Fig. 5 and des-c~ibed hereinbefore. The outlet end of the coupler may communi-cate both with the inlet end of the eæponential horn and with one or more resonant chambers, as illustrated in Fig. 4, if it is desired to alter the characteristics of the loudspeakPr such as to alter its resonant peaks or to e~tend its frequency-ampli-tude performance. In all instances, the coupler performs its function of impedance matching as described hereinbe~ore~
In the embodiment illustrated in the drawings, the loud-20 speaker is shown arranged with the front side of the diaphragmfeeding the couplerO It will be understood, of rourse~ that the disposition of the s~eaker may be xeversed, with the back side of the diaphragm feeding the coupler. Further, sound reproducers other than the electromagnetically driven diaphragm type illus-trated and de$cribed hereinbefore, are intended to be included in the general term "loudspeaker", as utilized in the specifica-tion and claims. Such other sound reproducers include electro-static speakers, the Keff woofer, the Magnaplaner, and othersO
Figs. 6-10 show the cabinet provided with a cover 48 which 30encloses the loudspeaker. It is preferred that the cover be per-forate, as illustrated by openings ~9 maintaining atmospheric conditions within it. Such an arrangement allows for the utiliza-tion of the back wave sounds from the sp~aker. This enhances i -~3-~stereo imaging. Such a cover may serve to conceal within it a plurality of tweeters and other speakers arrayed to provide most effective dis~ribution of sounds in the medium and higher fre-quency portion of the spectrum.
on the other hand, the cover may be imperforate and de-signed to provide a resonant chamber ~or the purpose of alter-ing speaker perormanc~ as a horn driver, also as is well known in the art.
The cabinet illustrated and described h~reinbefore may be formed in a variety o ways and in a variety o~ shapes and sizes. Indeed, the assembly of intexnal sections 52 and 54, in-tegrated by plates 58 to form the coupler and horn channels, together with the top wall 46 supporting the loudspeaker 10, may be associated in a variety of ways wi~h surfaces o tables and ceiling, ~loor and wall components of a room effectively to provide an enclosure or the assembly, in the manner of the cab-inet described hereinbefore by which to complete the hollow coupler and exponential horn.
The cabinets illustrated in Figs. 6 and 9 may be ~ormed 20 in sections to facilitate portability. For example, the portion below the section 52 may be formed separately, for detachable connection to the upper section.
It will be understood, of course, that the cabinets may be turned upside down for use. If the cabinet is sectioned, the section including member 56 may be omitted and replaced by sur-faces of furniture, walls, etc D
A plurality of couplers may be utilized as a coupler sys-tem with one or more loudspeakers as a loudspeaker system, and a single coupler as a coupler system maybe utilized with a plur-30 ality of loudspeakers as a loudspeaker system, provided thetotal cross sectional area of the common inlet of the coupler system and the total ef~ective cross sectional area of the as-sosiated loudspeaker system are comparable.

1~73 It will be apparent to those skilled in the art that various other changes may be made in the size, shape, type, num-ber and arrangement of parts described hereinbefore, without departing from the spirit of this invention.

Claims (15)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. In combination with a loudspeaker, a hollow, non-reson-ant coupler having an open outlet end and an open inlet end com-municating with and closed by the loudspeaker, the coupler having substantially inflexible walls that are substantially unrespon-sive to the internal air pressure generated by the loudspeaker, to inhibit radiation of sounds to the atmosphere, the cross sec-tional area of the hollow coupler decreasing progressively from its inlet end to its outlet end, and a horn having an inlet end communicating with and substantially matching the cross section-al area and shape of the outlet end of the hollow coupler.

2. The combination of claim 1 wherein the loudspeaker sys-tem comprises a single loudspeaker and the coupler system com-prises a single hollow coupler having an open outlet end and an open inlet end communicating with and closed by the loudspeaker, the cross sectional area of the hollow coupler decreasing pro-gressively from its inlet end to its outlet end.

3. The combination of claim 2 wherein the loudspeaker has a round cross sectional shape and the inlet end of the coupler has a rectangular cross sectional shape, and including means form-ing a transition volume at the inlet end of the hollow coupler for converting the cross sectional shape of the loudspeaker to the cross sectional shape of the inlet end of the coupler, with substantially the same cross sectional area.

4. The combination of claim 2 wherein the progressive de-crease in cross sectional area of the hollow coupler from its inlet end to its outlet end is exponential.

5. The combination of claim 2 wherein the hollow coupler has a generally rectangular cross section of substantially uni-form width and progressively decreasing depth from its inlet end to its outlet end.

6. The combination of claim 5 wherein the shape of the sur-faces defining said progressively decreasing depth in hyperbolic.

7. The combination of claim 1 including a cabinet having side, rear, top and bottom walls, and transverse partitions in the cab-inet cooperating with the walls of the latter to form said horn and said hollow coupler.

8. The combination of claim 7 wherein the transverse partitions form with the cabinet walls a hollow coupler extending downward from its inlet to its outlet end, and a folded horn extending from its inlet end at the outlet end of the coupler upward and rearward and thence downward and forward to the front side of the cabinet.

9. The combination of claim 7 wherein the transverse parti-tions form with the cabinet walls a hollow coupler extending downward from its inlet end to an intermediate portion and thence upward therefrom to its outlet end, and a folded horn extending from its inlet end at the outlet end of the coupler upward and rearward in surpentine manner, and thence downward and forward to the front side of the cabinet.

10. The combination of claim 7 wherein the top of the cabinet enclosing the loudspeaker is a perforate cover.

11. The combination of claim 2 wherein the cross sectional area of the inlet end of the coupler is substantially the same as the effective cross sectional area of the loudspeaker.

12. For use with a loudspeaker and an enclosure having side, back, top and bottom walls, wherein the top wall has an opening therethrough registering with a loudspeaker mounted on the top wall, a unitary assembly of transverse partitions arranged for installation within the enclosure and forming with the latter a hollow, non-resonant coupler extending from its inlet end regis-tering with and closed by the loudspeaker to its outlet end registering with the inlet end of a horn also formed by the enclosure and partition assembly, the inlet end of the coupler having a cross sectional area approximately equal to the effect-ive cross sectional area of the loudspeaker and the outlet end of the coupler and inlet end of the horn having the same cross sectional area and shape, the coupler having substantially in-flexible walls that are substantially unresponsive to the inter-nal air pressure generated by the loudspeaker, to inhibit radia-tion of sounds to the atmosphere, the cross sectional area of the coupler decreasing progressively from its inlet end to its outlet end.

13. The unitary assembly of claim 12 wherein the transverse partitions include a plurality of laterally disposed segments interleaved with support plates, the segments and plates being secured together as an integral unit.

14. For use with structures forming spaced side and rear sur-faces of the outlet portion of a horn, a loudspeaker, a unitary assembly of transverse partitions secured in spaced apart relation-ship and contoured to form a hollow, non-resonant coupler and an inlet portion of a horn, the coupler having an inlet end register-ing with and having a cross sectional area approximately equal to the effective cross sectional area of and closed by the loud-speaker and an outlet end registering with and matching the shape of the inlet end of the horn inlet portion, the coupler having substantially inflexible walls that are substantially unrespons-ive to the internal air pressure generated by the loudspeaker, to inhibit radiation of sounds to the atmosphere, the cross section-al area of the coupler decreasing progressively from its inlet end to its outlet end.

15. In combination with a loudspeaker, a hollow, non-resonant coupler having an open outlet end and an open inlet end communi-cating with and closed by the loudspeaker, the coupler having substantially inflexible walls that are substantially unresponsive to the internal air pressure generated by the loudspeaker, to in-hibit radiation of sounds to the atmosphere, the cross sectional area of the hollow coupler decreasing progressively from its in-let end to its outlet end.