US 3617654 A
Description (OCR text may contain errors)
United States Patent Inventor Appl. No. Filed Patented ELECTROACOUSTIC TRANSDUCER  References Cited UNITED STATES PATENTS 3,324,966 6/1967 Heidrich l81/31(.1)
Primary Examiner- Kathleen H. Claffy Assistant Examiner-Thomas L. Kundert Attorney-Ernest G. Montague ABSTRACT: An electroacoustic transducer comprising a fluidtight housing at the rear of an acoustic diaphragm. A cartridge, including at least one wall enclosing a rear chamber 15 Claims 5 Drawing Figs and compressing a density optimizing damping means therein, US. Cl ..l79/115.5 R, is inserted in the housing dividing the latter into a front 181/31 B, 179/180 chamber adjacent the diaphragm and an air duct communicat- Int. Cl. H04r l/28 ing the front chamber with the rear chamber, providing an op- Field of Search 179/ l 79, timized acoustic resistance-compliance network providing op- 180; 181/31, 31.1 timum acoustic response comparable to the electrical input.
5' r *L 3* 12'' t 5 r 1 e t, e I9 r 1 i 9 i j 09" 7 I 28 I7 I9o- I0" 33 f f 2 3 s i 1 Q I8a, '4 $7 I t 1 7 t i 9 9 l8 i 7 i i 2 L L f i, Z 1 g l t 25 i r 2 v 44 26 25a. 24 2/ PATENTEDwuy 2 1971 I8 26 25 INVENTOR STEPHEN L. HElDRlCI l BY 4 1 L; B U 4;
ATTORNEY ELECTROACOUSTIC TRANSDUCER The present invention relates to sound translating devices, in general, and to electroacoustic high impedance transducers which consist of an operable acoustic diaphragm with operating means in a housing enclosing air or fluid containing suitable acoustic properties wherein the enclosed fluid provides an acoustical load to the operations of the diaphragm which opposes the applied power variations more or less as determined by the acoustical properties of the enclosed fluid, in particular.
Loudspeaker enclosures commonly used are basically of two types: the completely enclosed type known as the infinite baffle enclosure; and the vented type known as the reflex enclosure. The present invention relates to the infinite bafile or completely enclosed type. A characteristic of the infinite baffle is that the low-frequency response capability is increased with an increase in size of the enclosed space. For a satisfactory response at 50 cycles this requires an enclosed space equivalent to approximately times the cubed nominal diameter of the acoustic diaphragm. If the enclosed space is packed with fiber glass, as disclosed in US. Pat. No. 2,775,309 to E. M. Villchur, an enclosed space equivalent to approximately 2% times the cubed nominal diameter of the acoustic diaphragm will produce an equally satisfactory response at 50 cycles, thus attaining a substantial reduction in size of the speaker enclosure. These devices are relatively low acoustic impedance devices because of the low impedance of large chambers.
The sound translating device disclosed in the Villchur patent may be regarded as utilizing an acoustic capacitance and an acoustical resistance couple or network, effective at low frequencies, in which the capacitance acoustically charged and discharged from applied fluid compressions and decompressions generated from the back of the related acoustic diaphragm is retarded by 'the acoustical resistance of a fiber glass packing therein effecting a time lag that may be considered analogous to certain RC timing circuits used in certain electronic devices. The nature of the fiberglass resistance, however, is such that it is not effective as a means of further lowering the frequency response by means of increasing its density beyond a certain optimum for a given volume of enclosure.
The quest is for more compact and better speakers.
Accordingly, the present invention provides very compact loudspeakers or other electroacoustic translating devices having a satisfactory wide frequency range of response with an enclosure that may be generally less than one times the nominal diameter of the acoustic diaphragm cubed. The present invention, due to high pressures and small volumes, is herein regarded as a high-impedance acoustic device capsule, or sound cell because of the high acoustic impedance of the small chamber or chambers, and the high acoustic impedance of the duct.
It is one object of the present invention to provide an electroacoustic high acoustic impedance transducer, wherein a satisfactory wide range of audio .response from a suitable operable acoustic diaphragm mounted in a very compact housing or enclosure is obtained, in which small air chambers functioning as air capacitance are interconnected through an extraordinary high acoustic resistance due to viscosity in an air duct conductor defined by contiguously opposing walls of great area as compared with the cross-sectional area of the duct and in which the first capacitance in cooperation with the second of the air capacitances or, in other words, the acoustic regeneration chamber, acts as an acoustic resistancecapacitance network couple that reacts favorably to lowfrequency cycles of compression and decompression that are impressed via the first or acoustic pressure generation chamber from the back of a related acoustic diaphragm.
The present invention is also directed to electroacoustic transducers in general, of the completely enclosed or infinite baflle type containing a resurgent high pressure acoustic system in which a high acoustic resistance is coupled with relatively small acoustic capacitance for response to lower frequencies of the acoustic spectrum of the general type as disclosed in my copending US. Pat. applications, Ser. No. 738,192 now US. Pat. No. 3,543,827 filed June l9, 1968, entitled Electroacoustic High Impedance Transducer, Ser. No. 629,623 filed Apr. 10, 1967, entitled Electroacoustic Transducer," and Ser. No. 6l9,658, filed March I, l967, entitled Electroacoustic Transducer," and in my US. Pat. No. 3,324,966, granted June 13, 1967, entitled Electroacoustic Transducer, and No. 3,3l7,000, granted May 2, I967, entitled Electroacoustic Transducer."
In the aforementioned there are disclosed enclosed acoustical systems in which the related acoustic diaphragm is predominantly controlled by the acoustic characteristics of the enclosed air or fluid in the housing according to the properties of a system of acoustic elements consisting of fluid compliance, resistance and mass contained in an arrangement of chambers and connecting duct or ducts, which improves the fidelity of the reproduced sound corresponding to the variations of electrical input.
As disclosed therein, the principal feature is in the utilization of the resurgence of acoustic energy from a highly compressed fluid in a small secondary chamber controlled through a high fluid flow resistant duct back to the primary chamber or enclosure, in which the acoustic pressure variations are generated on the back of the related acoustic diaphragm, thereby causing the fluid resurgence to react effectively on the back of the acoustic diaphragm increasing the efficiency of operation; that is, an acoustic resistance-compliance network designed to reproduce optimum acoustic response comparable to the electrical input.
In particular, the low-frequency response capability of the infinite baffle is increased, with a decrease in the acoustic capacitance heretofore contained in the enclosure, for very compact loudspeakers or receivers. This is achieved as indicated, with an operative acoustic diaphragm mounted in a small housing in which two small air chambers, functioning as air capacitances, are interconnected by an extraordinarily high acoustic resistance in the form of a high resistance air duct conductor of substantial area relative to its width between walls, which in cooperation with one of the two air capacitances acts an an acoustic resistance-capacitance network couple, that reacts favorably to low-frequency cycles of compression and decompression that are impressed via the first chamber from the back of a related acoustic diaphragm.
The specific embodiments disclosed in US. Pat. No. 3,317,000 consist of an arrangement of contiguous walls forming an air conductor passage which is generally peripheral to the unit and axially disposed thereto, with the second or outer chamber located entirely rearwardly of the inner chamber. The specific constructions disclosed in US. Pat. No. 3,324,966 consist of an arrangement of contiguous walls forming an annularly radial fluid medium passage disposed perpendicularly and radially to the unit; and wherein further, two annular chambers are formed about the magnetic structure of the unit, permitting compact loudspeaker systems, with the second chamber located entirely rearwardly of the inner chamber adjacent the diaphragm with an outer chamber positioned rearwardly and/or at least in part annularly around the inner chamber. The other copending patent applications generally disclose an arrangement of wall means disposed in an enclosed space at the back of the diaphragm, which wall means may also include the fluid tight enclosure, or be separate therefrom, and which wall means forms contiguous radial walls defining a narrow radial or annular space therebetween, which narrow space communicates with the fluid medium in the enclosed spaces, and with the second chamber generally smaller than the first chamber, and located entirely rearwardly and substantially radially inwardly relative to the inner chamber.
It is one object of the present invention to provide an improved electroacoustic transducer of the above-mentioned type,
It is another object of the present invention to provide an electroacoustic transducer of the above mentioned type comprising a single dynamic unit encapsulated in a compact airacoustic network controlling the diaphragm inertia.
It is another object of the present invention to provide an electroacoustic transducer of the above-mentioned type comprising a compact fiber glass or the like cartridge readily secured to the magnet structure of the loudspeaker unit and positioned at the back of or annularly around the magnet structure dividing the transducer into interconnected chambers and baffles and includes an optimum density damping material, such as fiberglass or the like, pressed therein. The cartridge may be economically mass-produced and designed for a balanced wide range audiofrequency response independent of size, and may be made in all sizes from extremely small to extremely large. Applications include, for example, loudspeakers, telephone receivers, hearing aids, and the like.
With these and other objects in view which will become apparent in the following detailed description, the present invention will be clearly understood in connection with the accompanying drawings in which:
FIG. 1 is an axial section of a loudspeaker embodying the features of the present invention;
FIG. 2 is a section along lines 2-2 of P16. 1, and partly broken away;
FIG. 3 is a perspective view of the fiber glass rings;
FIG. 4 is a partly broken away perspective view of the fiber glass cartridge; and
FIG. 5 is an axial section of another embodiment of the loudspeaker with the cartridge mounted behind the magnetic structure.
Referring now to the drawing and in particular to FIGS. 1, 2 and 4, the electroacoustic transducer comprises a conventional dynamic loudspeaker unit of a conical acoustic diaphragm 11 is suspended by a compliant peripheral flange 12 cemented to a supporting rim 13 of a diaphragm supporting basket 14 of a magnetic structure 15 actuated by a voice coil 16 which operates in a magnetic gap 17 in the magnetic structure 15. The dynamic loudspeaker 10 is integrated in a housing or enclosure according to this acoustic device.
The housing comprises an outer cylindrical member 20, which may be made of overlappingly joined spirally wound heavy cardboard. The cylindrical member is covered at its rear end by a rear disc 19 sealed fluidtight to the edges 20a of the cylindrical member 20 by any conventional sealing cement or glue. The rear disc 19 is formed by any suitable material, such as, for example, a plastic hard board material. The housing members 19 and 20 may also be fabricated as a single cylindrical member with one open end, namely the front end 20b. The front end 20b of the cylindrical member 20 is sealingly pressed coaxially against the rim 13 of the diaphragm support basket 14 and held thereto by a screw 33 extending through a central bore 190, countersunk in the rear disc 19, into a threaded washer 34 bounded to the magnetic structure 15.
A cartridge unit 31 is disposed coaxially within the housing 20 about the magnetic structure 15 and comprises a substantially cylindrical wall 23 made preferably of plastic or metal or the like, and having at its rear end an inwardly directed annular flange portion 44. A perforated annular disc 18 is freely positioned inside the cylindrical wall 23 of the cartridge unit 31 abutting the flange portion 44. The disc 18 is formed of a material, such as, for example, plastic, and has a plurality of perforations 18a communicating with the interior of the cartridge. A spacer disc 28, covering most of disc 18, leaves a narrow exposed portion for fluid passage through the perforations, and is joined to the innermost portion of the perforated disc 18. When the cartridge is positioned inside the housing 20, the spacer member 28 abuts the rear disc 19 of the housing, spacing the cartridge unit 31 therefrom.
The cartridge includes a front annular disc formed of wood or fiber board or the like and having rearwardly bevelled outer peripheral edges 25a which are sealed fluidtight by a sealing cement or the like to the front end 230 of the cylindrical member 23 ofthe cartridge. The front end 230 of the cylindrical wall 23 is formed with notches 26 defining tabs 260 which are bent inwardly against the annular disc 25, further securing the disc 25 to the cylindrical wall 23 of the cartridge 31, as shown in FIGS, 2 and 4.
A damping material, such as, for example, a fiber glass packing 30 or the like, is compressed between the annular front and rear disc, 25 and 18, respectively, of the cartridge unit under a pressure of approximately 2 lbs. per square inch, for frequencies of about 50 c.p.s. to optimize density for high fidelity operation of the transducer. Responsiveness for upper bass frequencies require a lesser compression and for lower frequencies a greater compression. The fiber glass packing comprises about 8 to l0 more or less compressed layers cut from a conventional insulation fiber glass blanket. The layers are collectively compressed to one-eighth inch or one-tenth inch or less, each, according to the desired response.
The fiber glass packing under pressure presses the rear disc 18 outwardly against the flange portion 44, and the fiber glass fills the space completely within the discs 25 and 18 and the cylindrical wall 23 of the cartridge unit 31 up to the magnet structure 15, which it surrounds, having a central cylindrical opening through which the magnet structure 15 axially extends, with the cartridge unit 31 positioned concentrically about the magnet structure 15.
FIG. 3 discloses fiber glass rings 30 cut from, for example, a l-inch fiber glass blanket insulating material, which are then stacked in the cartridge and compressed collectively to, for example, about one-eighth inch each under a pressure of several lbs. per square inch to optimum density (FIG. 3 illustrating the rings compressed, but not showing the cartridge walls). The compressed fiber glass rings are illustrated in FIG. 1 within the cartridge unit, which cartridge unit when positioned within the housing 20 divides the housing into a front chamber 32 and a rear, or cartridge chamber 21 filled with the density optimized, damping material. The filling of the cartridge chamber with compressed fiber glass extends the lower range of response of the transducer.
The cartridge unit 31 is inserted into the housing, dividing enclosed space into the front and cartridge chamber 32 and 21, respectively. The screw 33 passes through an opening centrally formed in the spacer member 28 and is screwed into the washer 34 bounded on the magnetic structure 15, substantially fluidtightly pressing the inner annular portion 144 of the basket 14 against the inner periphery of the front annular disc 25 of the cartridge 31, substantially sealing the chambers 21 and 32 from fluid communication at the inner periphery of the front disc 25. The cartridge 31 is clamped securely between the annular flange 14a of the basket 14 ofthe loudspeaker unit 10 and the disc 19 at the rear of the housing. Simultaneously, the bore in the spacer member 28 and the screw 33 substantially orients the cartridge 31 coaxially within the housing 20, and spacing the cylindrical walls 20 and 23 of the housing and the cartridge, respectively, from one another to define a narrow cylindrically annular passageway 24 between the front chamber 32 and the rear or cartridge chamber 21.
Felt strips or the like are provided spaced from each other circumferentially around and between the periphery of the cylindrical wall 23 of the cartridge unit 31 and the wall 20 of the housing. The strips 27 are glued against the rear of the disc 18 and prevent the cartridge from shifting out of axial alignment. In this manner the spacing of the passageway or duct 24 can be maintained without requiring the bore in the spacer member 28 to be formed to precise tolerances.
The front and rear chambers 32 and 21, respectively, communicate with each other by the peripheral air duct 24 and via the openings 18a in the rear annular disc 18, substantially constituting the only communication between the two chambers, since the annular portion 14a of the basket 14 is sealingly pressed against the front wall 25a of the chamber unit, thereby substantially isolating chambers 32 and 21 from communication directly.
The movements of the diaphragm 11, impelled by the energized voice coil 16, generate compressions and decompressions of air in the chamber 32 which causes plus and minus pressure to attain exceptionally high degrees above and below the atmospheric pressure and, thereby provides an air stiffness and control over the movements of the diaphragm 11, which predominates over the mechanical stiffness of the compliances in the diaphragm 11. The air stiffness thereby takes over much of the inertia of the moving parts.
The air duct 24 retards the release of the higher pressures in the chamber 32, which are generated increasingly with decrease of frequency to the chamber 21, where plus and minus pressures are regenerated again but with a time lag as compared with the pressures prevailing in the first chamber 32 and reach a peak when the incoming air pressures through the air duct 24 becomes approximately equal to the air pressures in the second chamber 21. The regenerated pressures in the chamber 21, which acts as a low frequency pressure accumulater, become immediately resurgent with the reversal of airflow adding the released power of resurgence through this air duct 24 to the back of the acoustic diaphragm 11 at a frequency compatible to that in the acoustic resistance-capacitance couple of the air duct 24 with the air chamber 21.
Because of the small housing sometimes desired for this loudspeaker, the acoustic capacitance of the chambers 32 and 21 is comparatively minute and complementary acoustic resistance necessary to achieve a low frequency resurgence of 50 cycles or less is extraordinarily great. The high acoustic resistance is achieved in accordance with the present invention by the extremely close contiguous spacing, that is, in the order of a few thousandths of an inch, extending over a range of about one-half of a thousandth for telephone receivers and such to about one hundred-thousandths of an inch, depending on desired tonal balance and upon the size of the loudspeaker, between the cylindrical walls and 23, which define the air duct 24 and the pressure packing of fiber glass thus providing the means for attaining an extraordinarily high acoustic resistance in the great area of surface viscosity provided as compared to the small cross-sectional area of the duct 24. The duct 24 is approximately 0.05 inch more or less in width for a 4 inch unit, for example.
It is known that loudspeakers have heretofore employed acoustical loading systems acting on the back of a related acoustic diaphragm which consists of two air chambers cooperating through an acoustic resistance of one kind or another, but these known structures have employed relatively large low-pressure air chambers or capacitances acoustically coupled to each other through a relatively low acoustic resistance means and in which the acoustic resurgencies were dominated by the properties of the acoustic chambers or capacitances rather than by the acoustic resistance which has been heretofore utilized merely as adamping element. These are essentially low-pressure capacitance controlled systems.
The present invention, however, features the opposite concept, that is, a high pressure by virtue of the small capacitances, resistance controlled acoustic loading system in which the acoustic resistance is very great and is an effective retarding element to the interchanging flow of air between two relatively small chambers. The means of attaining such a high acoustic resistance is the utilization in a thin air duct conductor of the viscosity effect of extensive surface area as compared to the cross-sectional area of such air ducts and the optim um density packing ofthe fiber glass damping element.
The present invention features, therefore, a high-pressure resistance controlled regenerative acoustic loading system acting on the back ofa related acoustic diaphragm, in which a high acoustic resistance is provided in the great viscosity residing in a thin air duct formed intermediate contiguous walls, as compared to the cross-sectional area of such air duct.
The cartridge in effect constitutes a low-frequency pressure accumulator. The compacted compressed fibrous material within the cartridge constitutes a dead end low-frequency filter accumulator responsive to low-frequency pressures applied thereto presenting a high acoustic impedance to the applied pressures.
Accordingly, the present invention provides a single dynamic unit encapsuled in a compact air-acoustic network which controls the diaphragm inertia. Within this network are means for designing a balanced wide range audiofrequency response, independent of size. The construction constitutes the cartridge assembly forming a specially designed interior baffle and chambers locked together with a single axial screw to the unit. Such a cartridge unit may be easily mounted in any loudspeaker housing and may be designed accordingly for economical mass production, is applicable to all sizes, from extremely small to extremely large loudspeakers, and has application for improved telephone receivers, hearing aids and the like. A long sought practical audio reproducer is thereby available for essentially all applications.
Referring now again to the drawings, and in particular to FIG. 5, an embodiment is disclosed, which follows substantially the previously disclosed embodiment.
The electroacoustic transducer comprises again a conventional dynamic loudspeaker unit 16 consisting of a conical acoustic diaphragm ll suspended by a compliant peripheral flange 12 cemented to a supporting rim 13 ofa basket 14 of a magnetic structure 15' actuated by a voice coil 16 which operates in a magnetic gap 17 in the magnetic structure 15'. The dynamic loudspeaker 10 is integrated in a housing or enclosure according to this acoustic device.
While the housing 20, and the other unprimed portions not again herein described are substantially identical with the arrangement in the first embodiment, the magnetic structure 15' is extended diametrically, and the cartridge 31' is formed substantially the same as the cartridge illustrated in FIG. 1, however, with rear disc 18' of the cartridge being illustrated as annular. The securing screw 33' is longer than that of FIG. 1, and extends completely and axially through the cartridge 31' within the central opening formed by the fiber glass rings 30. The cartridge 31' is held by screw 33' completely behind the magnetic structure 15', the screw 33' passing through a bore in the spacer member 28, axially through the cartridge 31', and into a threaded bore in washer 34 axially bonded to the rear of the magnet structure. The tightening of the screw 33', clampingly presses the magnetic structure 15' against the front disc 25, effectively sealing front and rear chambers 32' and 21 respectively, formed within the housing, from direct fluid communication thereat. The cylindrical housing wall 20 is axially longer than the housing wall of FIG. 1, since the cartridge 31 is located behind the magnet structure 15'.
The operation of this embodiment is substantially as described above with the fluid resurgence between the chambers occuring through the duct 24 between the cylindrical walls 20 and 23.
Although the dead end low-frequency pressure filter and accumulator cartridge as herein described is shown attached to the magnet structure with a screw, it may also be mounted at the back of a conventional loudspeaker enclosure free of the magnet.
This cartridge is intended to be an independent damping device suitably and conveniently to be mounted in any conventional and appropriate speaker enclosure.
The volume of the cartridge unit 31, the degree of compression of the fiber glass, the width of the gap taken radially between the intermediate members 28 and 23 and the width of the narrow duct 24, each and collectively having a bearing on the design for such tonal quality of reproduction as may be desired.
Whereas the present invention constitutes an improvement in the construction of a small loudspeaker and preferred embodiments of the construction have been disclosed, it is to be understood that other embodiments are possible and that the scope of the present invention includes any acoustic device employing an operative acoustic diaphragm, the back of which acts on a system of two small interconnected chambers and made of any suitable materials in which the generated and regenerated acoustical resurgencies from one to the other one, controlled by a dominatingly high acoustic resistance in an air duct defined by wall surfaces of an extensive area, as compared with the cross-sectional area of the duct. it is further understood that the term air is analogous with any fluid medium and the use of lighter or heavier than air fluid mediums in the resistance-capacitance system described, are within the scope of the present invention, which is defined by the objects and the claims.
While I have disclosed several embodiments of the present invention, it is to be understood that these embodiments are given by example only and not in a limiting sense.
1. An electroacoustic transducer comprising a housing,
an axial operating means including an acoustic diaphragm, a diaphragm supporting basket and a magnetic structure operatively connected, generating acoustic pressure variations to a free fluid medium on one side of said acoustic diaphragm and for generating corresponding acoustic pressure variations to a fluid medium enclosed in said housing on the other side of said acoustic diaphragm,
said housing on the other side of said acoustic diaphragm being fluidtight and defining an enclosed space,
a cartridge unit including wall means disposed within said enclosed space dividing said enclosed space into a first chamber adjacent said acoustic diaphragm and a second chamber enclosed by said wall means,
said cartridge unit positioned within said housing defining a narrow air duct in said enclosed space in fluid communication between said first and second chambers,
damping means comprising compacted fibrous material compressed within said second chamber in said cartridge unit,
said cartridge unit perforated at least at one portion thereof, communicating with said narrow air duct, and permitting passage of pressure variations into and out of said second chamber of said cartridge unit through said narrow air duct to and from said first chamber,
said housing is cylindrical, and
said cartridge unit is a cylindrical unit axially mounted in said housing defining said air duct peripherally therebetween.
2. The electroacoustic transducer, as set forth in claim 1, further comprising screw unit axially securing said housing, said cartridge means and said magnetic structure together in position and axially disposed in said transducer. 3. The electroacoustic transducer, as set forth in claim 2 further comprising a spacer member secured axially to the rear of said cartridge unit, said screw means passing through said spacer member, said spacer member abutting the rear of said cartridge unit, thereby spacing the latter from the rear of said housing. 4. The electroacoustic transducer, as set forth in claim 1, further comprising a plurality of strips peripherally spaced and disposed between said housing and said cartridge unit in said air duct. I 5. The electroacoustic transducer, as set forth in claim 1, wherein said cartridge unit is positioned behind said magnetic structure.
6. The electroacoustic transducer, as set forth in claim 1 wherein said cartridge unit is annularly positioned about said magnetic structure. t 7. The electroacoustic transducer, as set forth in claim 1, wherein said cartridge unit including wall means includes,
a cylindrical wall, a front disc sealingly enclosing the end of said cylindrical wall facing said diaphragm supporting basket, and a rear perforated disc covering the other end of said cylin- 5 drical wall.
8. The electroacoustic transducer, as set forth in claim 7 wherein said cylindrical wall includes a rear inwardly flanged annular portion, and said rear perforated disc disposed within said cylindrical wall abutting said flanged annular portion. 9. The electroacoustic transducer, as set forth in claim 7, wherein said discs are circular members. 10. The electroacoustic transducer, as set forth in claim 7, wherein said diaphragm supporting basket and said magnetic structure constitute clamping surfaces, and said front disc is annular and sealingly pressed at its inner periphery against said clamping surfaces. 11. The electroacoustic transducer, as set forth in claim l0, wherein said front disc is made of wood or fiber board, said cylindrical wall made of metal or plastic, said cylindrical wall formed with circumferentially spaced notches at said end facing said diaphragm and tongues bent inwardly onto said front disc securing said front disc thereto, and sealing means for airtight sealing said front disc and said cylindrical wall together. 12. The electroacoustic transducer, as set forth in claim l0 wherein said damping means comprises fiber glass rings compressed between said front disc and said rear perforated disc.
14. An electroacoustic transducer, comprising a housing including walls,
an acoustic diaphragm means for generating acoustic pressure variations to a fluid medium enclosed in said housing at the back of said acoustic diaphragm constituting a first chamber,
a dead end low frequency filter and accumulator disposed in said housing for response to low-frequency pressures applied thereto, comprising a cartridge unit, and compacted compressed fibrous material disposed within said cartridge unit constituting a second chamber for presenting a high acoustic impedance to said applied low-frequency pressures,
said cartridge unit spaced from said walls of said housing defining a narrow duct therebetween,
said cartridge unit perforated at one end and permitting passage of pressure variations to and from said second chamber in said vessel from and to said fluid medium enclosed in said housing, and
said duct communicating with said second chamber through said perforated one end thereof and with said first chamber adjacent the back of said acoustic diaphragm.
15. in an electroacoustic transducer a dead end lowfrequency filter and accumulator adapted to be disposed in a transducer housing for response to low-frequency pressures applied thereto, comprising a cartridge unit and compacted compressed fibrous material disposed within said cartridge unit constituting a chamber presenting a high acoustic impedance to said applied low-frequency pressures,
said cartridge unit adapted to be spaced from the housing walls to constitute a narrow duct therebetween,
said cartridge unit perforated at one end and permitting passage of pressure variations to and from said chamber