US2852087A - Sound reproducing devices - Google Patents

Description

nited States Patentfiice 2,852,087 Patented Sept. 16, 1958 SOUND'RE-PRODUCING DEVICES Dolph W. Ruschhaupt, Fresno, Calif.

ApplicationFe'bruary 15, 1955, Seril N0. 488,350

-9Claims. =(Cl. 18131) 'The present inventionxelates to wund reproducing dev1ces anti more particularly to a speaker anti endlosure therefor having improved sound reproducing character- 1st1cs.

The desirability cf high fidelity wund reproduction -has lang been recognized and much efiort has been expended in improwiing Ihe erformance characteristics of radios, phonographs, audioportions of television sets, am- .plifiers and the like. While marked progress has been made in improving the electrical .systems =involved so as to deliver current to speakers With a minimum of distorti0n cand interference, lt has now been discovered that .the s peakers and their enclosures vthemselves a-re sources of considerable distortion andinterference precluding the attainment o-f high fidelity no matter how perfected the associated electrical systcms.

The final.step in sound reproduction usually involves Ihe vibration at 'control-led .-frequencies and amplitudes,

0f a sound producing mernber such as a diaphragm,

speaker cone, er Ihe like. Whether the associated ele'ctrical -systems -are amplitude er ifrequency modulated in response to intelligence -impressed ujgon a canrier wave, Ihe sound producing memoer is -vibr=ated by clecttomagnetic aetuation, with the volume regulated -by control-ling the extent of vibration of fl1e =sound producing memben, and the -pitch of the reproduced sound regulated hy Cont101'0f the frcquency cf vibration. The actual movement of The sound producing member is tnus a complex, coutinually changing composite of peri0dic and aperioclic waves cf individually varied magnitude dependent upon the sound to be reproduced.

The vibrated sound producing members are frequently frusto-coxiical in forrn 10 give direction to the reproducad sound projected from within the frusto-conical member *and incid'entally to disperse sound fronn Ihe -opposite side thereof. The two projected sounds, the desired frorn the front o'f the rnember and Ihe undesired 'frem =the rear there of, i-nterfere to such an =eXtent tha't high fidelity has heretofere been -diflicult -'01 imposs'ilale 10 achieve.

Previous eiforts to correct 'the descfibed sound interferences and dist0rtion haVe generally involved either of two approa'ches to the problem. Effort has bcen made to create substantially equa"l 'and o'pposite sound waves and to utili ze such waves "(o destroy fhe undesir'able sound wavcs emanating fr0m th'e rearward side of sound members. Devices of this type are intricate and complex an have not succeeded in minimizing to any major extent interference and distortion 0f the character described. Additionally, in the infrequently encountered environments in which abundant *space is available, en ilosures have been made for speakers having vibrated sound reproducing members, which completely enclosed the speakers except for the forward side thereof. Such enclosures have been air tight and provided Wit-h baffles er sound absorbing materials so as to reduce to substantial inaudib'ility sound coming from th'e back of the speal ers.

Such dcvices have been somewhat successful laut are limitefl "t0 use where space is a'bundant. It is generally considered that tobe fully ellective a chamber containing approximatly 1.,000 cubic Ieet of dead air space is required. Obviously, Inere are very few operational environments in w'hich a cubical chamber ten feet long on each side can be io'leratecl. If the closed chambers are much smaller than that described, the associiated spcakers have very poor transient response, being unable to accommoda-te s-udden large excursions cf the vibratory member-o'f the speaker. The pressure or vacuum created by the vibrating mernber of Ihe speaker dampens or restricts the free actuation thereof.

Sound de-adening chambers capable of absorbingsubstantially all 1carwardly directed sound waves emanating :from speakers have bcen referred to as infinite bafiles am] -as noted above under known practices req1iire vgrcat sizeorcler toabsorb Ihe undesirable sounds from speakers ofsizes popuiar fnrhousehcd use. When such cham- -laers.have heretofore been employed, but 0f a size practb cal for household use, they have been unsatisfactory. F0r example, they usually are subject to one er more :points =0f resonance, have inadequate low frequency Yesponse, impedetransient responseby back pressure an the speaker, incl-uce severe phase -distontion, are subject t0 serious harmonic distortion, Jack smeoth frequency response, und/er generate spurieus frequencies which a-re -neither 0dd ;nor even harmenics cf frequencies -w'hose reproducti0n :is desired.

The essence cf t=he present invention vresides in the discovery that a relatively srnall com act enjected from one side o'f a vibrated sound producing meinber without interfering With sound produced by the opposite side thereof.

An'oth'er object is to at tain tl1e appmximate operational characteristics ofan infii1ite baffle in a speaker enclosure which is compact, simple, economical, -and ful-ly effe'ctive in performing i'ts ii1tended function.

Further objects and avantages will bec-0rne appar-ent in the subsequent description in the specification.

In the raw'ings: k

Fig. 1 is 'a pe-rspec'tive of a speaker and enclo'sure =emlacrdying th'e principles of -the prcse'nt invention.

Fig. 2 i's a horizontal section of the structure shown in Fig. 1 ta-ken on a plane representcd by the in'temecting li-ne therein.

Fig. '3 is a transverse section of the structure taken 'a1: he positi'on represented by line 3-3 in Fig.

Fiig. 4 is a sectional perspective of the s=tructure divided in a plane 0f referenee corresponditxg to the -secti'oning plane o'f Fig. '=l and viewed obliqu'ely- 'from the man.

'Referring in=greeier particularity to the drawings:

A speaker cf well-known 'form is indicated gener-a-lly at 10 mounted in an encl'o'sure 131 crnbodying the rpirin 'ciples of tlie pnesent invention.

Cbnsideri'rrg first the elements cf 1he speaker 10, it will be no'ted that .a frusto-conical sound produ-cillg membez, or cone 15 2is employed. The cone is vibrated axially in the well-.known manner by a 1ow .frequency speaker unit 16 containi'ng actuating electromagnetic means, not shown, responsive to compamtively low frequency pulses of electrical current. A tweeter er high frequency horn 17 is optionally provided concentrically of the cone and vibrated axially of the cone by a high frequency actuating unit 18. The present invention is not concerned with the high frequency horn, its actuating unit, nor other specific detailed speaker construction, these elements being shown as optional auxiliary equipment usually utilized where high fidelity is desired. In rearwardly spaced relation to the cone 15, a frusto-conical bracket 19 is provided having an end 20 of major diameter concentric to the end of major diameter of the cone 15 and an end 21 of minor diameter connected in supporting relation to the 10W frequency speaker unit 16 also in concentric relation to the cone. lt will be noted that air passages 22 are provided through the bracket 19 which have an aggregate area substantially one-third of the crosssectional area of the end of major diameter of the cone 1.5. While the described speaker and relationship of the air passages 22 to the end of major diameter of the cone 15 are well known and detailed consideration of speaker design beyond the province of this description, the existence 'of the bracket 19 and of the air passages 22 is of such significance to the subsequent description as to Warrant comment.

If no passages are provided through the bracket 19, an air pocket 23 is formed which so cushions the cone that movement thereof is impeded and srnooth frequency response and realistic transient response are precluded. Conversely, if no such bracket is employed and the rearward side of the cone entirely exposed, a desirable minor dampening effect of the air cushion 23 is avoided and distortion incurred usually of harmonic nature although oftentimes including spurious frequencies. At any rate, the total area cf the air passages 22 usually approximates one-third the cross-sectional area of the. end of major diameter of the cone 15 and it is sufficient at this point to observe that the present invention bears a significant relationship to the column or columns of air set in motion rearwardly of the cone 15, which columns are of crosssectional area corresponding to the aggregate crosssectional areas of the passages, whatever they may be.

The enclosure 11 takes the form of a housing for the speaker of any desired external form er dimensions consistent with the following description of the parts employed. The enclosure has a forward panel or Wall 25 providing a circular opening, 26 therein of substantially the same diameter as the end of major diameter of the speaker cone 15. 011 opposite sides of the opening 26 are providecl venting slots 27. F01 reasons subsequently described, the slots have a total area substantially equal to the total area of the air passages 22.

The forward panel 25 is rigidly mounted on a base panel 30 which conveniently takes the form of a support for the enclosure 11, a sl1elf in a cabinet, or the like. A pair of side panels 31 are rigidly mounted on the base panel 30 outwardly adjacent to the slots 27 and upwardly extended in substantially parallel relation adjacent to the slots. The side panels abut and are secured to the forward panel. A molding 32 is preferably mounted about the marginal edge of the forward panel for decorative eifect and for a functional purpose subsequently described. A substantially impervious rearward panel 33 is rigidly mounted 011 the base panel and upwardly extended therefrom along rearward edges of the side panels 31 to which the rearward panel is securely affixed. A top panel 34 is fitted betWeen upper edges of the forward panel 25, side panels 31 and rearward panel 33. In the forn of the enelosure shown, the base panel 30, side panels 31, and top panel 34 are in rectangular relation in a reference plane parallel to the forward panel 25. Somewhat sirnilarly, the base panel 30, forward panel 25, top panel 34, and rearward panel 33 are in rectangular relation in a reference plane parallel to the side panels 31. Also, the forward panel 25, side panels 31, and rear- Ward panel 33 are in rectangular relation in a reference plane parallel to the base panel 30. While such rectangular relationship is generally preferred for ease of construction and convenience of embodiment in a console 01' other article of furniture, it Will readily occur to those skilled in the art that the enclosure may take innumerable other forms witl1out departing from the spirit or the scope of the present invention.

A pair 0f substantially parallel partitions 38 interconnect the base panel 30 and the top panel 34 inwardly acljacent to the slots 27, have forward edges 39 connected to the forward panel, and rearward edges 40 in spaced parallel relation to the rearward panel 33. It will be noted that the partitions define a divided air duct 41 extended rearwardly from the speaker 10 to form a first charnber of the duct as defined by the partitions 38, base panel 30, and top panel 34, and which is return bent about the rearward edges 40 of the partitions and forwardly extended t-o form second chambers of the duct as defined by each partition, its adjacent side panel 31, base panel 30, and top panel 34, to the venting slots 27. As shown, the surfaces of the partitions and panels defining the air duct 41 disposed inwardly of said duct are coated With any suitable sound absorbing material 42 such as rock W001, Fiberglas, cellulose, padding, or other suitable material, many forrns of which are Well known for sound absorbing purposes. The sound absorbing material is adhesively afl1xed or otherwise secured to the surfaces coated.

It Will be noted that the sound absorbing material 42 is provided in increased thickness at the rearward edges of the partitions and in the corners defined between the rearward panel 33 and the side panels 31. Such ation of the material provides a constriction, indicated at 44, oriented so that the cross secti0n thereof is obliqueto both the rearwardly and forwardly extended portions of the air duct 41. The constrictions are of substantially fihe sarne height as the slots 27 and approximately the same width thereof. Thus, at no point in the duct 41 is the total cross-sectional area of the duct, whether in its unitary rearwardly extended portion er in its divided forwardly extended portion, appreciably less than the area of the air passages 22 in the bracket 19. The crosssectional area of each eonstriction 44 is, 0f course, less than the cross-sectional area cf the first duct chamber while each second duct chamber is greater in cross-sectional area than its associated slot. the first charnber and constriction and each second charmber and associated slot constitute a pair of inverted, series connected megaphcnes which provide a continuous unimpeded path throug*h which the rearward side of the speaker 10 exhausts.

The speaker 10 is mounted in the enclosure 11 in the usual manner, as by bolts 46 extended through the forward panel 25 and the forward portion of the bracket 19. Nuts 47 screw-threadably mounted on the bolts serve dependably to hold the brac ket and speaker in place. As is the usual custom, an annular resilient cushion 48 is provided between the end of major diameter of the cone 15 and the forward panel to accomrnodate axial vibration of the cone. The cone is juxtapositioned to the opening 26 and for purposes of appearance, the opening overlaid with a porous screening cloth 49 or the like. The molding 32 overlies the screening cloth and holds the same in osition.

Operation The operation of the device cf the present invention is believed t0 be clearly apparent and is briefly summarized at this point. As the cone 15 and the ln'gh frequency horn 17 are actuated in the well-known manner, desired sound waves are projected through the opening 26. The undesirable and interfering sound waves Which are projected rearwardly frorn the cone are to a minor extent darnpened by the bracket 19 but in major extent result in the Vibration of columns of air extended through In effect, therefore,

the passages 22. These are the sound waves .previously referred to as normally precluding high fidelity sound reproduction. The instant invention makes possible the re- .duction of these sound waves to inaudibility without the concomitant disadvantage of subjecting the cone to back ;pressure and resultant distortion.

.As the sound waves passing rearwardly through the passages 22 move through the duct 41 they are pro- :gressively reduced in magnitude by sound energy absor.ption by the side Walls and upon striking the sound ab- .sorbing material-42 on the rearward panel 33, absorbed .to a major extent. Further, the material precludes the reflection of the sound forwardly to interfere with the desired tones resulting from co ne 01' hornvibration. The .undesired sound waves passing obliquely Jfrom the rearward panel 33 through the constrictions 44 are further reduced in magnitude and effectively rendered inaudible as they pass to the venting slots 27. In addition to absorbing substantially all of the undesired sound waves, no appreciiable back pressure is imposed upon the cone and even air displaced by major exeursions of the cone moves with unimpeded freedom to or from the venting s"lots 27. In this manner, undesired dampening of the movement of the cone isavoided.

In actual practice, the device f the present invention has made possible the redu=ction to inaudibility, and thus the obViation cf interference f1orn, rearwardly projected sound waves emanatingfrom the speaker. Fo r example, enclosures of popular 'household size in which the length of the rearward panel is approxirnately 24 in ihes the height of the side panels 20 inches, and the Widthof the base panel 20 inches, the excellenee of sound reproduc tion has equaled or surpassed the emp'loyment of the same speker in sound deadening charnbers of cubical 'forrn feet in height. Thus, high fidelity sound reproduction has been made possible in pract'ical 'househ'old sizes o-f speaker enclosures as good as the best results previously attainable where 110 space limitationswere encountered.

Although the invention has been herein shownand:de-

scribed in what is eonc'eived 10 be the most practicaland preferred embodiment, it is recagnized =that depantures maybe made therefrom Within the scope of the 'invention, Which is not to be limited to the detailsdisclosed herein but is to be accorded the full scope=of the clairns so.as .to embrace any and all equivalent devices and apparatus.

Having described rny inyention, what 1 cla'nn as new and desire t0 secure by Letters Patent ls:

1. 111 eombination with a speaker having avibratory sound producing mernber providing a .forward side and a rearward side; a housing having a panel providing a speaker opening therein; means mounting the speaker in the housing with the forward side cf the sound producmg member juxtapositioned to the speaker opening of the housing, said means being in spaced adjacent relat1on to the rearward side of the sound producing member and including a wall having air passages of predetermmed cross-sectional area therethrough; means within t he hansing cooperating with the housing to -form a condu1t hav.mg a rearwardly extended end connected to the rearward side.of the sound producing rnember, enclosing the speaker mounting means, and having reversely bent forwardly extended open opposite ends opening :adjacent to the speaker opening, and sound abso'rbent materral mternal ly covering the Walls cf the conduit and form1n g :constr1ctions therein between the rearwardly -extended end and the forwardly extended ends, the conduit at the constrictions providing air passages of a total cross sectional area approxirnating the total cro=ss-sectional area of the air passages cf the speaker mountingmeans, "the cross-see- -tional area of the forwardly extended ends cf the condu1t being greater than the cross-sectional area of the constrictions but less than the cross-sectional area of sa1d rearwardly extended end.

2. The combination of a housing having a Wall providing a speaker.opening and vent openings therein, aspeaker havmg a sound producing member in registry with the spea k er opening mounted f01 vibratory movernent in a .rectrlmear path normal to the wall cf the speaker opening, the sound producing 1nember having substantially the same area projcted in a plane normal t0 the rectilinear path 0f movement thereof as said opening, rneans opposite t0 the speaker from the speaker opening mounting the speaker in the housing, said rneans being in spaeed adjacent relation to the sound producing rnember and defining a vented air .cushion therebetween, the mounting means having air passages therethrough 0f a cross-sectional area approxirnating one-third the area cf the speake1 opening, means Within the housing in cooperation With the housing forrning a conduit having an inner end in registry with the passages in the.mounting means and extended from the passages in alignrnent With the rectilinear path of Vibration of the sound pro=ducing member oppositely from the opening in the .housing and a pair of return bent unter ends individually extended from the end in registry with the passages to the vent openings, and sound .in sulating material lining said conduit and forming cou- Strictions therein between the unter ends of the conduit and the inner ends, the conduit being o-f such size that air paths areprovided therethrough frorn the sound produeingmernber to the atmosphere which are of mini1nurn eross-sectional area at the constrictions approximating the cros&sectional area of the pasasges in the mounting means the return bent.ends of the .conduit having a crosssectional area greater than the cross-sectional area of the respectively adjacent constrictions but substantially reduced from that cf the .inner end cf the conduit.

3. In combination with a .speaker having a vibratory sound .producing member characterized by the prcje-ction of -.desir.ed sound waves .from one side thereof and the projection of undesired sound waves from the opposite .sideihereof; a.housing for the speaiker having a forward wall, .a rearward Wall, side Walls interconnecting the forward wall and the rearward Wall, a top 'WE111 interconnecting the forward wall, the rearward Wall, and the side Walls, and a bottorn wall interconnecting the forward Wall, the rearward Wall, and the side Walls, the forward Wall having a speaker opening therein and vent openings on opposite sides of the speaker opening of total crosssectional area approximately one-third of the cross-sectional area of the speaker opening; means.rnounting the speziker within the housing With the side of the sound pro-ducing rnember thereof from which desired sound waves are projected facing the opening and the opposite side thereof disposed inwardly ofthe housing; partitions individually connected t0 the forward Wall between the speaker opening and the vent openings and extended therefrorn toward the rearward wall in closely adjacent spaced relation to the side Walls terminated in spaced relation to said rearward wall,'said partitions interconnecting the top and bottom Walls cf the housing and in association with the housing defining a divided air duct from the rearward side of the sound producing rnember to the vent openings; and sound absorbing material on the rearward Wall, opposite sides of the partitions, o-n the top and bottomwalls, between the partitions and their respectively adjacent s1de Walls of the h0using and 011 the side Walls, providing corresponding constrictio-ns in the divided air duet frorn the sound producing member to the vent openmgs, co-rresponding constrictions in the divided duct be1ng of a c ross sectional area approxirnately that of the cross-sectxonal areaof.the vent openings. V

4.1ncombination with a speaker having a speaker cone, a housing having a Wall providing a speaker op.enmg o;f substantially the same size as the major transverse d1- mension of the speaker cone and apair o-f vent opemngs on opposite sides of the speaker opening of total area substantially one-third that of the speaker opemn means mounting the speker With the end of major diameter of the cone thereof juxtapositioned to the speaker opemng;

an annular resilient means interconnecting the speaker cone and the wall about the speaker opening; partitions Within the housing in association therewith defining a divided return bent air passage within the housing having an enlarged central portion connected to the sound producing member and a pair of return bent portions rednced in size from the central portion connected to the vent openings; and sound absorbing material coating the Walls of the air passage f1om the speaker cone to the vent openings, said sound absorbing material forming norrespondingly positioned constrictions in the air passage in planes oblique to the passage at the return bent portions thereof; the minirnum combined total cross-sectional area of the corresponding constrictions in the divided passage being approxirnately the crosssectional area of the vent openings and being lass than the cross-sectional area of either the central pnrtion or the retu1n bent portions of the conduit.

5. In comoination witn a speaker providing a vibratory sound producing mernber having a forward side from which a1'e projected desired sound waves and a rearward side from which are projected undesired sound waves; a housing having pairs of spaced substantially parallel Walls interconnected 110 form an enclosure of substantially rectangular cr-oss-section, one of the Walls having a speaker opening therein and one cf the Walls having a vent slot therein, means mounting the speaker in the housing With the forward side f the sound producing member directecl toward the speaker opening and With the rearward side disposed inwardly 0f the housing; partitioning n1eans connected to the Wall having the speaker opening, adjacent to the speaker opening, extended toward the wall opp0sice to the wall with the speaker opening, defining With the walls an air duct having an and porti0n cf maximum cross-sectional area in comn1unication witl1 the rearward side of the sound producing rnemher, an opposite and pcrtion opening outwardly of the l1ousing thr0ugh the vent slot and for-ming an open and unimpecled path from the rearward side cf the sound produclng member to the outside cf the housing througl1 the slot; and means circurnscribing said path f0rrning a cc nstriction in the air duct between said end portions, the cross-sectional area of the end portion cf the duct opening through the slot being of cross-sectional area greater than the constriction but substantially lass than the cross-secti0nal area cf the end portion comrnunicating with the sound producing member.

6. In combination Witl1 a speaker having a speaker cone provicling a forward side from which are projected desired sound waves and 21 rearward sicle from whicl1 are projected undesired sound waves; a housing for enclosing the speaker having top, bottom, side, and forward Walls interconnected to forrn an enclosure of substantially rectangular cross-section, the forward wall having a speaker opening and elongalerl substantially rectangular vent openings on opposite sides cf the speaker opening of a total cross-sectional area approximately one-third cf the cross-sectional area of the speaker opening; a bracket mounting the speaker in the housing and being in rearwardly spaced relation to the speaker cone, the bracket having a plurality of air passages therein cf an aggregate cross-sectional area substantially one-third of the cross-sectional area of the speaker opening; a pair of partitions individually connected t0 the forward Wall between the speaker opening and the vent openings, ext'ended rearwardly therefrom toward the rearward wall, interconnecting opposed Walls 0f the housing, and having rearwarcl edges in adjacent spaced relation to'the rearward wall, the partiti ons in association with the housing defining an air duct having a rearwardly extended cenrral ortion cf maximum crosssectional area between said opposecl walls of the housing and the partitions, and in circumscribing relation to the rearward side cf the speaker cone and the air passages in the bracket, the

air duct further having a pair of forvvardly extended side portions between the partitions and the respectively adjacent side Walls of the housing interconnecting said cantral portion and the vent opening; and sound absorbing material forming constrictions between the rearward edges 0f the partitions and the respectively adjacent corners of the housing defined by the intersection of the side and rearward walls, the constrictions being 0f a total cross-sectional area substantially equal to the total cross-sectional area of the vent openings, and the crosssectional area of the side p0rtions of the air duct being greater than the cross-sectional area of the vent openings but substantially less than the cross-sectional area 0f said central portion.

7. In combination with a speaker having a speaker cone having a forwardly disposed major diameter providing a forward side from which are projected desired sound waves and a rearward side from which are projected undesired sound waves; a housing for enclosing the speaker having a forward wall, a rearward wall in spaced relatition to the forward Wall, spaced parallel side Walls interconnecting the forward and rearward walls, and spaced parallel top and bottom Walls interconnecting the forward, rearward, and side walls, the forward Wall having a speaker opening therein of substantially the same diameter as the major diameter of the cone and elongated substantially rectangular vent openings of a total cross-sectional area approximately one-third of the cross-sectional area of the speaker opening, the vent openings being substantially vertically positioned between the speaker opening and adjacent side Walls of the housing; a frusto-conical bracket mounting the speaker in the housing and being in rearwardly spacec l substantially concentric relation to the speaker cone, the bracket having a plurality of air passages therein of an aggregate cross-sectional area substantially one-third of the cross-sectional area of the speaker opening; a pair 0f substantially vertical partitions individually connected t0 the forward Wall between the speaker opening and the vent openings, extended rearwardly therefrorn toward the rearward Wall, interconnecting the top and bottom Walls of the honsing, and having rearward edges in adjacent spaced relation to the rearward wall, the partitions in association with the housing defining an air duct hav ing a central portion 0f maximum cross-sectional area between the top and bottorn Walls cf the honsing and the partitions, and in circumscribing relation to the rearward side cf the speaker cone and the air passages in the bracket, the air duct further having a pair of side portions between the partitions and the respectively adjacent side Walls 0f the housing providing rear ends connected said central portion and forward ends opening outwardly of the housing through the vent openings; and sound absorbing material 011 the opposite sides of the partitions, interiorly of the housing 011 the top, bottom, side, and rearward walls, the sound absorbing material providing constrictions between the rearward edges of the partitions and the respectively adjacent corners of the housing defined by the intersection of the side and rearward walls, the constrictions being of a total cross-sectional area substantially equal to the total cross-sectional area of the vent openings, and the cross-sectional area of the side portions of the air duct being greater than the crosssectional area of the vent openings but substantially lass than the cross-sectional area of said central portion.

8. In combination With a speaker having a vibratory sound producing member providing a forward side and a rearward side; a housing having a panel providing a speaker opening therein; means mounting the speaker in the housing with the forward side of the sound producing mernber juxtapositioned to the speaker opening of the housing, said means being in spaced adjacent relation to the rearward side of the sound producing member and including a wall having air passages cf predetermined cross-sectional area therethrough; means Within the housing cooperating with the housing to form a conduit having a reawvardly extended and connected to the rearward side of the sound producing member, enclosing the speaker mounting means, and having a reversely bent forwardly extended pen opposite end opening adjacent to the speaker opening; and sound absorbent material internally covering the walls of the conduit and forming a constriction therein between the rearwardly extended end and the forwardly extended and, the conduit at the constriction providing an air passage of a cross-sectional area approximately ans-half of the total cross-sectional area of the air passages of the speaker mounting means, the cross-sectional area of the forwardly extended and of the conduit being greater than the cross-sectional area of the constriction but lass than the cross-sectional area of said rearwardly extended and.

9. In combination With a speaker providing a vibratory sound producing member having a forward sicle from which are projected desired sound waves and a rearward side from which are projected undesired sound waves; a housing having pairs of spaced substantially parallel Walls interconnected to form an enclosure of substantially rectangular cro-ss-section, one 0f the walls having a speaker opening therein and one of the Walls having a vent slot therein; means mounting the speaker in the housing with the forward side of the sound producing member directed toward the speaker opening and With the rearward side disposed inwardly of the housing; par-' titioning means connected to the Wall having the speaker opening, extended toward the Wall opposite to the wall With the speaker opening, and defining with the walls an elongated air duct circumscribing an open and unimpeded path in communication with the rearward side of the sound producing member and opening outwardly of the ho using through the vent sl0t; and rneans circumscribing said path forming a constrlction in the air duct and dividing the air duct into a first duct charnber of maximum cross-sectional area extended between the rearward side of the sound producing member and the coustriction forming means and a second duct chamber of substantially uniform cross-sectional area With the crossse-ctional area of the constriction being less than the cross-sectional area of said first chamber and With the cross-sectional area of said second charnber being greater than either the cross-sectional area of the slot er the cross-sectional area of the constriction whereby said undesired sound waves are dampened by passage through said duct and whereby back pressures 011 the sound producing member are minimized because of said open and unimpeded path provided by the duct.

References Cited in the file of this patent UNITED STATES PATENTS 1864308 Kolster et a1 Inne 21, 1932 1,869178 Thuras July 26, 1932 2,031500 Olney Feb. 18, 1936 2315896 Dumas Apr. 6, 1943 FOREIGN PATENTS 696,671 Great Britain Sept. 9, 1953

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