US1869178A

US1869178A - Sound translating device

Info

Publication number: US1869178A
Application number: US475468A
Authority: US
Inventors: Albert L Thuras
Original assignee: Bell Telephone Laboratories Inc
Current assignee: AT&T Corp
Priority date: 1930-08-15
Filing date: 1930-08-15
Publication date: 1932-07-26
Anticipated expiration: 1949-07-26
Also published as: US2104433A; FR722108A; GB382268A; FR46410E; DE590736C

Description

July V26, 1932.

A. L. THURAS SOUND TRANSLATING DEVICE Fil'ed Aug. 15, 19:50 3 sheets-Sheet 1 FIG.

70 80 90 |00 FREQUENCY-N PER SECOND INVENTOR A. L; THURAS A TTORNEV July 2 6, 1932. A. L. THURAS SOUND TRANSLATING DEVICE 3 Sheets-Sheet 2 Filed Aug. 15, 1950 Y F/G. 6

/NvE/vron A. THURAS Arrow/vir- .muy4 26, 1932.

A. THuRAs 1,869,178

SOUND TRANSLATING DEVICE 3 Sheets-Sheet 3 Filed Aug. l5, 1950 0mm M ATTORNEY Patented July 26, 1932 UNITED STATES PATENT NoFl-lcE ALBERT L. Taurus, or' NEW Yoan, N. Y.,

i TORIES, INCOIREORAZI.El),.OFr NEW YORK, N. Y., A. CORPORATION OF NEW YORK 'soUNnrnANsLATING nEvIcE Application ledAugust 15, 1930. Serial No. 475,468.

This invention relates to sound translating devices and more particularly to a method and means for increasing the response of such devicesi at the lower frequencies.

translated and in a more proportionate dei0 gree to the' higher notes than hasv been possible hitherto.

One feature of this invention comprises utilizing the 'sound vibrations translated by the two surfaces of a diaphragm to eEectively'reenforce each other by causing the sounds translated by one surface to.be changed in magnitude and phase with reference to those translated by the other surface.

Another feature of this invention resides in its application to a loud speaking receiver whereby the radiation resistance per unit area of the diaphragm thereof at the lower frequencies is effectively increased by providing an acoustic path from the rear surface to the front surface of said diaphragm which at its outlet, in close proximity to said diaphragm, is substantially the same area as that of the diaphragm, said acoustic path defining an acoustic lter.

Another feature of this invention Iis its application to a device for translating acoustic vibrations into electrical vibrations which consists in increasing the amplitude of vibration of the vibratory element of said device by causing a portion of theacoustic vibrations to be translated to be directed against the rear surface thereof after having passed through an acoustic filter so proportioned as to stiffness and mass that such acoustic vibrations act to reenforce the effect of the vibrations impressed on the front surface of' mass element, respectively, of an acoustic filter, so proportioned that when the diaphragm'is actuated to translate acoustic vibrations, the vibrations particularl those of the lower frequencies", translated y each surface of said diaphragm, are caused to reenforce each other to increase the response of said sound translating device.

A more complete understanding of this invention will be obtained by reference to the appended drawings wherein:

Fig. 1 is a front view of a sound translating device embodying features of this invention;

Fig. 2 is a sectional view taken along the line 2-2 of the device 'shown in Fig. 1;

Fig. 3 shows a simplified schematic sketch of the device shown-in

Fi

1 and 2;

Fig. 4 illustrates the equivalent impedance circuit of the device shown in Figs. 1 and 2; Fig. 5 shows graphically the operating .characteristics of a" sound translating device such as shown in Fig. 1;

Fig. 6 shows a view partly in section of another embodiment of this invention;

Fig. 7 shows a front view of still another embodiment of this invention;

Fig. 8- is a sectional view of the embodiment shown in Fig. 7 taken along the line 8-8; 4.

Fig. 9 shows al sectional view of a telephone transmitter incorporating features of this invention;

Fig. 10 is an electric circuit analogy of the acoustic elements involved in the operation of the device shown in Fig. 9;

Fig. 11 is an electric circuit analogy of the acoustic elements involved in the operation of a device similar to that shown in Fig. 9 without features of this invention;

Fig. 12 shows graphically the operating characteristics in the low portion of the frequency range of the device shown in Fig. 9 and of the same device without features of this invention; and

Fig. 13 shows typical overall response characteristics of the device shown on Fig. 9 vincorporating and not incorporating the features of this' invention.- V

Referring now to the drawings, there is AssIeNoB 'ro BELL TELEPHONE momi- Vso showin in Figs. 1 and 2, a sound translating I device of the loud speaking receiver type embodying the features of this invention consisting of enclosure member l wh1ch 1s here shown as of rectangular and box-like shape whose wall elements are held together by'screws 3 and which provides a fluid chamber 5 behind a vibratory element or diaphragm 7.

The diaphragm 7 may be of a l1ght material such las an alloy of aluminum or other suitable material. It comprises a curved central portion with a surrounding reversely curved portion terminating in a flat rmi. The diaphragm isV shown supported 1n an aperture in the front wall 15 of the enclosure member 1 and at its periphery 1s secured to said wall between mounting members 8 in any suitable manner, for example, by screws 14. Current coil 9 is attached to the inner or rear surface of the diaphragm and is dis osed in the magnetic air gap 16 provided y the magnet 10 which may be either of the permanent magnet or of the electromagnetic type. Central pole portion 17 of the magnet is preferably hollow thereby providing a passage-way y11 whereby sound disturbances radiated by the rear surface portion of the diaphragm inside the driving coil 9 may readily pass into the chamber 5. The magnet 10 is supported on the cross piece 13 which is, in turn, attached to a plurality ofvertical supports 12 secured to the inner surface of the front wall 15.

passage from the chamber 5 to the atmosphere adjacent the front surface of the diaphragm. These tubular members are pref-4 erably situated as closely as possible adjacent the periphery of the diaphragm. As

shown, each tubular member is at one extremity substantially Hush with the front wall 15 of the enclosure member and has its other extremity projecting into the chamber 5. When the diaphragm 7 is caused to vibrate by the movements of coil 9 in response to the passage therethrou h of currents which produce a variable geld interacting with the fixed strength field of the magnet 10, both the front and the rear surfaces of the diaphragm will cause acoustic disturbances to be propagated through the fluid thereadjacent. The sound waves generated by the rear surface of the diaphragm, particularly the low frequencies, are caused to be conducted through said chamber and outlet passal e to combine with the sound waves generate b the front surface of the diaphra m' an low requencies.

In order that this invention maybe more readily understood it is believed that a dis- 'cussion ofthe theoretical considerations involved in its operation and application to the loud speaker receiver of Figs. 1 and 2 should be outlined.

Theory and measurements show that generally speaking, the direct radiating type of loud speaker of moderate size, has a relatively small response and inadequate power output capacity at low frequencies. By the method comprising one feature of this invention and with a structure as described above for increasing the response of loud speakers at low frequencies, both sides of the diaphragm 7 are utilized and the radiating resistance per unit area of the diaphragm is thereby increased. By means of an acoustic stiffness and mass, the air displacement from the back of the diaphragm is shifted approximately 180 in phase and led out at the front of the diaphragm. The acoustic stiffness is obtained from the volume of airin the chamber 5 at the back of the diaphragm and the acoustic mass from the air in the outlet passage comprising the paths 6 in the tubular members 4 leading from the chamber to the front of the diaphragm. The phase relation of the volume velocity of the diaphragm and the air to reenforce the same at the in the passage opening is somewhat similar to that in an electrical low-pass filter in which the currents in the two adjoining series arms (containing small resistances) are nearly 180 out of phase above the cutoff frequencyV of the filter. rIhe acoustic system as regards Vpower output differs considerably from the electrical filter in that the radiation resistance and air mass of the diaphragm and the corresponding radiation resistance and air mass of the passage have a mutual reaction on each other and furthermore their acoustic impedances vary with frequency. A solution of this problem is rather complicated, but by making a few simplifying assumptions a general idea of the gain in sound power output can be obtained. Ifthe area of the passage is taken equal to the effective area of the diaphragm and a frequency such that the vibratory velocity of the diaphragm is equal to the vibratory velocity of the air in the passage, then the effective radiating area 1s doubled and the radiation resistance per' Passage ingly over that which would obtain at ,the same frequency with the passage closed. This will give an increasingly higher gain down to the v cutoi of the acoustic system. Above the fre-4 jphase shifting acoustic system has been Worked out and approximate equations -formulated for the case in which the area of the passage is equal to the effective area of the diaphragm and the mass of the mechanical moving element (diaphragm and coil) is equal to the mass ofair in the passage.

The response of a loud speaker connected to the output of a vacuum tube may be expressed in terms of the acoustic power radiated when a constant voltage is impressed in series with the electrical impedance of the driving coil and the impedance of the source. The acoustic power is obtained by (l) deriving an expression for the velocity of the diaphragm and of the air in the passage in terms of the acoustic impedance and constant impressed voltage in the electrical circuit; (2) deriving an expression for the acoustic impedance of the air and the mutual impedance caused by the velocity of the diaphragm and velocity of the air in the opening. The

` power output is then the sum of the products of the square of these velocities and corresponding resistances.

Refer now to Figures Sand 4 which show a simplified schematic sketch of the loud larger resistance load on the diaphragm sidered` are long compared with the dimenptxiis of the acoustical system.

E=constaut voltage in the electrical circuit.

ma' F=mcchanical iorce-----IOR VZ=impedance in the mechanical circuit caused by the electrical circuit.

which is equal to Z =II%0:-If the impedance iu the electrical circuit is assumed to be pure resistance equal to R.

H==e1d intensity l=length of wire in the moving coil` J.'lfcdge stiffness rcactance of the diaphragm S j-u stiness reactancc of volume of air in the enclosure.

jwm=mass roactance of the diaphragmL-:mass reactance of the air in the opening.

V,V|, Vz=velocities in the various parts of tne acousticalmechanical system.

V 2V AZ and -Vz AZ=air impedances imposed on the diaphragm and on opening respectively.

A=eective area of the diaphragm=areaof the outlet passage.

Z=ar impedance per unit area ot an area twice the area of the diaphragm.

In the impedances and W2 Kirchhoiis laws to the circuit in Fig. 4:

speaker7 and its equivalent impedance circuit. F: V1 V1 jwm V1 +A .ZV+ V Fig. 3 shows a single opening 18 only lead .70 .Tw ing from the chamber behind the diaphra AZ V S i but the theoretical considerations are t e 0=3mv2+ 2 +52, V same regardless of whether one or more open- V: V .V ings are employed if the area relations here 1 2 considered are not departed from. If Z=T+Jx To simplify the calculations the following assumptions have been made-(1) the area Then the aCOUSlC pOWer Output 1s I real V 2 real V i? Ar 2,

www 2V1AZ i V1 Lpm 2V2AZ)V2 2 (l) of the outlet assage from the .chamber is equal to the e ective area of the diaphragm5 the .eii'ective mass of the moving element, and (3) the wavelengths of the frequencies con- V: S., 2. 5' Z.,AZ Q AZS0 S S .2

-1(2) the mass4 of air in the opening is equal to Substituting in equation (1), expressing F in terms of E and dividing by 10', gives the acoustic power output in watts.

.low frequencies.

I- Tlie following'electrical,*mechanical and acoustical constants were used in an experiimentalloud speakerincorporating the features of this invention:-

Z -pCS(X IY) where p=density of uuid C =velocity ot sound S=area of diaphragm considered as pstoa X== resistance factor of diaphragm considered as piston Y=reactance factor of diaphragm considered as a piston.

spaced 3 in. holes were provided in which were fitted tubular members of phenol fibre 6 in. in length. These tubes were used because they were foundeasier to construct than the single opening for the loud speaker box. In this particular embodiment ofthe lnvention, the volume of the air in the chamber and in the tubular members 4 gave an acoustic stiffness and mass respectively which had a cutoztl:l frequency of 40 cycles. v l

The full line and the dotted line curves in Fig. 5 picture the sound output to be eX- pected when aloud speaking device, such as described above, is operated with passageways connecting front and rear surfaces of the diaphragm closed and opened respectively, the source of impedance being equal to twice the impedance of the coil attached t0 the diaphragm of the speaker. These curves were plotted from power output calculations based on Formula `(2) above. The broken line curve is drawn through values of response at particular frequencies obtained by taking the difference between measured values of response for the loud speaker with the passage-ways opened and closed and using the calculated response for the loud speaker with the chamber outlet passage-way closed as a base line. It will be apparent from these curves that the actual response ofa loud speaker embodying the features of this invention agrees closely with the theoretical response to be expected. If it is desired to avoid any pronounced peak in the response near the cutoff acoustic damping material may be placed in the outlet passage.

It is apparent that this invention not only increases the relative response, but also increases the sound power output capacity 'of direct acting types of loud speakers in the The air displacement from aloud speaker with this acoustic system is and the ratio of the air displacement to the diaphragm displacement increases as the freh quency decreases thus compensating to a large extent for the decrease 1n radlatlon resistance Since direct acting loud speakers are limited in their power output calpacity by the amplitude of motion of the diap ragm at'lowfrequencies, the importance of this acoustic system is evident when the large acoustic powers developed by the long pipes ofthe organ and Ithe low frequency instru- Iments of the orchestra are considered. This lgreater than the 'diaphragm displacement if' invention will permit the reproduction of the low frequencies of speech and vmusic in more proportionate degree to the high frequencies than has been hitherto possible.

It is to be understood that the descri tion of the experimental loud speaking device uilt in accordance with this invention is of an illustrative nature only and is not intended as a limitation on the scope of this invention nor is it .intended that the recitation of what has been found to be a desirable relation between the diaphragm area and that of an outlet passage connecting the Huid adjacent each surface of the dia hragm and that between vthe mass of the iuld in said passage and that of the diaphragm and coil should be construed as a limitation. It will be apparent to those skilled in the art that variations in structure and proportions may be made without departing from the essence of the invention.

Fig. 6 shows al side view partly in section of. another embodiment of this invention. Instead of a rectangular enclosure behind the rear surface of the diaphragm, a cylindrical member 22, which may be of metal, is at tached to an annular member 24 which supports a diaphragmv 7 similar to that of Fig.

2, and a plurality of tubular members 4 pro- /viding an outlet passage from within the chamber 28, provided by said member 22 and its curved base 27, in a manner similar to that described with reference to Figs. 1 and 2. Magnet 10 is supported by a cross piece 19 which, at its extremities, is attached to angle pieces 29 in turn riveted or otherwise secured to the inner wall of the member 22. The base 27 of the member 22 is preferably slightly curved to avoid the possibility of a drum head effect when the loud speaker is in operation and the member 22 is preferably cylindrical `in order'to substantially eliminate the eiect which is provided also with a. hollow central pole as shown in Fig. 2.

Cylindrical tubes

33 and 34 held in spaced relation by the separating members `38 provide an annular passage vconnecting the chamber 32 with the atmosphere adjacent the front surface of the diaphragm 7. The magnet 10 is supported b the cross piece 42 which at its extremities 1s attached to angle pieces 43, which are in 'turn secured to the inner surface of the tube 33. The diaphragm, its actuating mechanism and the spaced

tubes

33 and 34 may all be supported by the front wall 30 of the chamber 32,

as shown, being secured thereto by screws 37.Y

The air in the chamber 32 and that enclosed between the

tubes

33 and 34 have such stiffness and mass respectivel that they form elements of an acoustic lter whereby the sound vibrations translated by each surface of the diaphragm at the low frequencies act to reenforce each other, thereby `increasing the relative response and power output of the device in the low frequenc region.

This invention is not limited in its application to sound translating devices for reproducing sound, but may be applied to sound translating devices for changing acoustic vibrations into electrical vibrations. In Fig. 9 there is shown a transmitter ofthe moving coil type comprising' a ma et structure which is preferably of t e ermanent magnet type having a central po e portion 62 to which a curved pole piece 51 is secured by means of the screw 63. The other pole piece consists of an annular plate member 56 secured t the magnet by screws 57 and having a central aperture, the bounding wall of which is in spaced relation to the pole piece 5l thereby providing a magnetic air i gap in which a current coil 61 attached to the diaphragm is disposed. The diaphragm 60 has a curved central portion surrounded by a flat portion and is preferably of a light metal, suchk as alloy of aluminum, although any other suitable material may be used therefore. Surrounding the upper portion of the central pole and the magnet immedi- 'ately under the pole piece 51 is an annular member 52 which provides a very shallow recess 65 under the moving coil to insure suiiicient clearance for. the coil when vibrating and a narrow passage-way 66 therefrom leading into the hollowed portion or chamber 53 of the magnet. The diaphragm 60 is supported at its periphery and clamped between

ring members

55 and 59 by screws 58 which thread into plate member 56. The ring 55 is provided with a tapered inner surface 67 to substantially eliminate the increase in pressure on the diaphragm caused by the resonance of the chamber above the diaphragm. Extending through an opening provided in the

ring members

55 and 59, the diaphragm 60 and the plate member 56, is a tube 54 which projects into the chamber 53 of the magnet thereby providing a passageway 64 connecting the interior of said chamber with the atmosphere adjacent the front surface of the diaphragm.

The operation of this device as a sound translating instrument for changing acoustic vibrations to electrical vibrations is as follows: v

' The acoustic vibrations are impressed upon the front -surface of the diaphragm such that a portion thereof acts thereagainst and another portion passes through the tube 54 into the chamber 53 and acts upon the rear surface of the diaphragm. Those vibrations however whichA act upon the rear surface o the diaphragm are so changed in phase and magnitude that they act to reenforce the effect of the vibrations impressed on the front surface ofthe diaphragm whereby the amplitude of vibration of the latter is increased.` .This reenforcing action is particularly prominent at low frequencies, a great increase in the response of the device over its response when not incorporatin this feature of the invention being obtaine It will be understood, of course, that when acoustic vibrations are impressed upon the diaphragm currents varying in accordance with said acoustic vibrations will be produced in the coil 61 because of its movement in the ield' provided by the magnet 50.

It will be apparent that the method out- A lined in the previous paragraph `for increasing the response of the transmitter at the low frequencies is similar tothat for increasing the response of a loud speaking receiver such as that described with reference to Figs. 1 and 2. It is believed, however, that a clearer understanding of this invention as applied to a transmitter of the moving coil type will be obtained from the consideration which follows ofthe factors involved in its operation.

The voltage generated by a moving coil transmitter is proportional to the veloclty of the diaphragm. This fact makes the problem of getting a uniform response down to low fre uencies diicult in a transmitter of practica sensitivity. It means that either the diaphragm lmust have a very low stiffness or 'some wa must be provided to amplify the action o the sound wa'ves on the diaphragm relative to that at higher frequencies. f The method here described increases the force on the diaphragm at low frequencies sufficiently to compensate for the reactance of the edge stiffness down to relatively low fre uencies. It is possible by this method to malntain a uniform response two octaves below the frequency at which the edge stiil'ness of the diaphragm begins to show a decrease in the response characteristic of the transmitter.

'The force on the diaphragm at low frequencies is increased by means of the connecting tube 54 between the front of the dialli) lili

phragm 60 and the ehemher 53 beck ore the diaphragm whereby the pressure et the front of the diaphragm' ufterv heilig chenged both in magnitude and phase is impressed on the hack of the diephragm. The magnitude end phase or the pressure on the'huck of the diuphragm ere controlled hy the ecoustic constants of the air in the tube and chamber. Fig. 9 shows the transmitter with the open end oi the connecting tube close to the die.- phragm so thet the phase of the pressure is `the same ever the diaphregm und tube opening. Fig. i@ is u circuit dlagrem with pressure applied to the mechanical end ecoustic constants et the trensmitter. Fig. Ill is a circuit diagram or the trensmitter shown in Fig. 9 without the tube 54.-. A. comparison p J,

er these twe .figures readily shows the change wrought therein hy the insertion oi the tube. rlhe smell air chamber `between the diaphragm and ecoustic resistance, R0, has been omitted in the diegrerns of Figs. l() and ll heceuse of its negligible eect et .low frequencies. For convenience the pressure end the constants of the transmitter have `been referred te the eective urea. ef the diaphragm; for examplea the force llis equei te the open eir pressure at the diaphragm multiplied by the eiective eres. or the diaphragm.

Fiilie response or veituge generated by the moving coil of the transmitter is proportlonel te the velocity the diaphragm dr vlded by the force on the diaphragm. Es'.- pressing this retire in terms of the mechenicel and ecoustlc constents of the transmitter for large tubes can be used. The tube im A edunee coecient was therefore calculated rom the general expression 600 ohms at about 50 cycles. llt will be suiciently accurate to use these average velues of impedence in the response Equation (3).

The measured end calculated constants of etrensrnitter constructed in accordance with this invention were Substituting these values in Eqnetion (3) end solving for absolute velues gives the upper full line curve A., shown in Fig. l2, plotted in decibels.

lf the tub-e 54s is closed Equation (3) rethere is obtained: duces to 7Jc' v R lf- Q S l S Sit SR /3 L L1,

s.m+sm.s2m.m+sm 33 *new (nems---wmul Y is c: e: o;

in which v'" l T ,ma :erlective muse of.' the diaphragm, mass of the l R +37 wm gf- (4:)

coil and the mass of air in the resistance gap a 9 reexgfd to tlegaggirahgm. T h =edge s ness o e .ap ragni. Jj" =resistance of the air gap' referre to the da# -n i t. e abgve Vaules are ubstluted DOW Hl phrsgm. equation (ai) the lower rull llne curve B., S :stiffness of the air in the transmitter chamber referre to the diaphragm. B. and m=impedance of the sir in the compensating tube referred te 'the diaphragm.

ln calculating the impedance eithe in the tuhe it was found that the radius (ro) of the tube gave e, veine er 'the discriminent fm, 112m/ lL-.pw

between l and l in which neither Posseuilles coecient for narrow tulle nor the simplified expression shown in'lig. 12, is obtained. |lihese calculated response curves are indlcatlons of the greuter response at the lower frequency to he expected of a given device when the method bodied therein.

rlllie dotted curves C and D show the eX- perimentally measured response of the transmitter in decibels when the tube is closed and opened respectively. These measurements it will be noted e ree closely with the theoretic-al calculations; t e variation in the two sets of curves below 50 cycles can be accounted for by e, prohable error of 10% in the measurement of the edge sti'ness of the diaphragm S0.

Fig. 13 shows the overall frequency respense of av moving coil transmitter and assov ciated transformer between and 10,000 with the tube 54 opened and closed. It will end means' comprising this invention are em be noted that above 200 cycles the responseof the particular transmitter considered above appears to be unaffected by the presence of the passage-way between the front and rear of the diaphragm but that below that frequency the increase in response is very marked.

It will be understood, of course, that althou h specific dimensions and proportions have een given in describing the application of this invention to sound translation devices, the embodiments` described are illustrative only of this invention which is to be considered as limited only by the scope of the appended claims.. v Y

What is claimed is: n i

1. A sound translating device comprisin a diaphragm, means definin an acoustic sti ness adjacent one surface o said diaphragm,

the other surface of said diaphragm being open to the atmosphere, and means defining an acoustic mass providing an acoustic path between the two surfaces of said diaphragm, said acoustic mass and stiffness proportioned so that over a portion of the fre uency range the sound vibrations translated y the o posite surfaces of said diaphragm have t eir magnitude and phase so related that they act to effectively reenforce each other whereby the amplitude of the translated vibrations 'is increased.

2. A sound translating device comprising a diaphragm, one surface of which is open to the atmosphere, means defining a Huid chamber enclosing the other surface of said diaphragm, and means closely adjacent the periphery of said dia hragm and defining a fluid passage-way lea ing rom said chamber to the atmosphere and through which a movement of fluid may occur when said diaphragm is actuated whereby over a portion of the frequency range the vibrations translated by each surface thereof reenforce each other thereby increasing the amplitude of the translated vibration.

3. A soundtranslating device comprising a diaphragm one surface of which is open to the atmosphere, means defining a fluid chamber enclosing the other surface of said diaphragm, and means defining a fluid passage-A way leading from said chamber to the atmosphere and through which a movement of fiuid into and out of said chamber may take place, the Huid in said chamber and said passageway having such stiffness and mass, respectively, that when the diaphragm is actuated the low frequency vibrations translated by each surface are caused to have such magnitude and phase relationship that they effectively reenfoi'ce each other to increase the amplitude ofthe translated sound.

4. The method of increasing `the response of a loudspeaking device at the llow frequencies which comprises utilizing the radiation from each surface of the diaphragm whereby the radiation resistance per unit area ofthediaphragm is e'ectively increased and caus`ng the sound waves from one surface of the diaphragm to act in phase with those emanating lfrom the other surface by providing an acoustic 'stiffness and. mass through which the sound emanations from the one surface must act before combining with those from the other surface. y i 5. A loudspeaking device comprising a daphragm, one surface of which is o en tothe atmos here, means defining a flui chamber enclosing the other surface of said diaphragm, and means defining a fluid passageway leading from -said chamber to the atmospliere closely adjacent the surface of said diaphragm open to the atmosphere, sad passage-way having an area substantially e ual to that of the diaphragm and when said iaphragm is actuated enabling fluid to be pumped into and out of said chamber wherey over a portion of the frequency range the acoustic disturbances emanating from the two surfaces of said diaphragm combfne to reenforce each other.

6. A loudspeaking receiver comprisin a diaphragm, means for driving said iaphragm, and an enclosure for said driving means and the rear surface of said dia-cy phragm, said enclosure having an aperture therein of substantially the same area as the diaphragm and adjacent thereto, the chamber behind said diaphragm and the volume enclosed by said aperture providing the stiff- 'ness and `mass elements respectively of an acoustic filter through which sound disturbances over a portion of the frequency range emanating from the rear surface of said diaphragin pass and are shifted through approximately 180o before combining with the sound disturbances emanating from the front of the diaphragm.

7. A sound translating `device comprising a diaphragm, means for utilizing the front and rear surfaces of said diaphragm as effective sound translating surfaces, said means comprising a fluid enclosure for the rear surface of said diaphragm defining an acoustic stiffness, and a tubular member leading into said fiuid enclosure connecting the saine with the outside atmosphere adjacent the front surface of the diaphragm andv definin an acoustic mass, said acoustic mass and sti ness acting as elements in an acoustic filter to change the magnitude and phase of the acoustic "disturbances over a portion of the frequency range which are transmitted to the rear surface of the diaphragm whereby said d"sturbanccs` act to reenforce tlieeffect of the sound disturbances impressed on the front surface of said diaphragm to increase the amplitude of vibration thereof.

8. A sound translating device comprising a hollow magnet structure dening a magnetic air-gap, a diaphragm, means for supporting said diaphragm, driving means atbetween the atmosphere and said hollowed portion, the duid in the latter and that enclosed by said-tube having such stiffness and mass, respectively, that the front surface is subjected to substantially all of the impressed acoustic disturbances and the rear surface of said diaphragm is subjected to a portion only of the impressed acoustic disturbances, said mass and stidness acting to change the magnitude and phase of a portion of said sound disturbances before the latter act upon the rear surface of said diaphragm whereby the disturbances impressed on opposite' sides of said diaphragm reenforce one auf other to increase the amplitude of vibration of the diaphragm.

9. A sound translating device comprising a sound translating diaphragm having a driving means secured thereto and an enclosure for one surface of said diaphragm, theother surface being open to the atmosphere, said' enclosure .providing a shunt stiffness into which said one surface of said diaphragm works, and means providing an acoustic passage from said enclosure to the atmosphere adjacent the other surface of said diaphragm,

the area of said acoustic passage being substantially equal to the edective area of the diaphragm and the mass of the fluid therein being substantially equal to the ed'ective mass of the diaphragm andsaid driving means.

10. A sound translating device of the loudspeaking receiver type comprising a sound translating diaphragm having a drivingl means secured thereto, an enclosure for one surface of said diaphragm defining a body of uid there adjacent, said fluid providing an acoustic stidness into which said one surface of said diaphragm works, and means providing an. acoustic passage between the enclosed surface and the other surface of said diaphragm, the duid in said passage having a mass substantially equal to the edective mass of said diaphragm and said driving means and the duid in said enclosure an acoustic stidness such that the low frequency sounds translated by the enclosed surface of said diaphragm act through said stiffness and mass to reenforce the sounds translated by' the other surface of said diaphragm.

ll. A sound translating device comprising a diaphragm and means defining an acoustic filter of the lowpass type between the two surfaces of said diaphragm, the stiffness and mass element of said 4filter proportioned with reference to said diaphragm so that sound vibrations translated by the two surfaces thereof are eectively utilized to increase the response of said device at the low frequenciesl .over that of a device without said means.

-ilter of the low pass type providing an acoustic path between the two surfaces of said diaphragm, said filter so proportioned that the low frequency sounds translated by said surface of each diaphragm are so related in phase and magnitude as to e'ectively reenforce each other.

13. A sound translating device comprising a diaphragm and means defining anacoustic filter of the low pass type providing an acoustic path between the two surfaces of said diaphragm, the mass and stid'ncss elements of said filter proportioned so that the phase of the low frequency sounds translated by the surface of said diaphragm is substantially reversed with reference to that of the sounds translatedv by the other surface of the diaphragm whereby the response of said device is increased over that of a device without said means.`

le. A sound translating device comprising a diaphragm and means dening an acoustic filter providing an acoustic path between the two surfaces of said diaphragm whereby the sounds translated by one surface of said diaphragm are caused to so act with reference to those translated by the other surface of said diaphragm so that-the response of thel device is increased at the lower .frequencies over that of a device not having said means, said filter acting to substantially reverse the sound vibrations propagated therethrough and translated by said one surface.

15. A sound translating device comprising a diaphragm and means defining an acoustic filter providing an acoustic path between the opposite surfaces of said diaphragm, the stiffness 'and mass of said lter proportioned so that the cutod frequency is in the region of the lowest frequency to be translated.

i6. A sound translating device comprising a diaphragm and means defining an acoustic lter providing anacoustic path between the opposite surfaces thereof, the sti'ness and mass elements of said filter proportioned so that the acoustic vibrations translated by each surface of said diaphragm are caused to reenforce each other at low frequency.

17. In an acoustic device, a diaphragm;

means substantially closing one surface of said diaphragm from the atmosphere and defining an air chamber there-adjacent, and means for acoustically coupling the opposite surfaces of said diaphragm, said means comprising a tubular member having one end open. to the atmosphere and the other end within and opening into said air chamber.

18. In an acoustic device, a diaphragm, means substantially closing one surface of said diaphragm from the atmosphere and delining an air chamber there-adjacent, and

genaue f means for coupling the opposite surfaces of said diaphragm through said air chamber ,so that sound waves originating on the exposed Vsurface of the diaphragm act on opposite surfaces of the diaphragm to eiectively increase the response thereof over the response with sound waves acting on one surface only, said means comprising a tubular member havin an open end in the vicinity of the expose surface of said diaphra and another open end within the air chamildi. i

19. An acoustic device. comprising a diaphragm, drivingY means attached thereto, an ,enclosure for one surface of said diaphragm, the other surface being exposed to the atmospiere, said enclosure providing an air chamr into which said one surface of the diaphragm works, and means providing an acoustic passage from said'enclosure to the atmosphere, the larea of said, passage being substantially equal to the effective area of the diaphragm and the mass of the fluid therein being substantially equal to the effective mass of the diaphragm and driving means, said means comprismg` aplurality of tubular members.

20. Anacoustic `device comprising a diaphragm, driving means attached thereto, an enclosure for one surface of said diaphragm, the other surface being exposed to the atmosphere, said enclosure providing an air chamer into which said one surface of the diaphragm works, and means providing an acoustic passage -from said enclosure to the atmosphere, the `area of said assage being substantially e ual to the effective area of the diaphra an the mass of the uid therein bein su stantially equal to the eil'ective mass of t e diaphragm and driving means, said means comprising a plurality of concentric spaced anuli.

21. Anacoustic device comprising a diaphragm, driving means attached thereto, an

the .diaphragm and the mass of "the fluid enclosure for one surface of said diaphragm, the other surface being exposed to the atmoshere, said enclosure providing an air cham- Ber into which said one surface of the dia- .phragmworks, and means providing an acoustic` passage from .said enclosure to the atmosphere, the area of said passage bein substantially equal to the effective area ol therein being substantially equal to the eiective mass, of the diaphragm and driving means, said means comprising a plurality of individual tubes each defining of the acoustic passage. y

22. An acoustic device comprising a diaphragm, an air chamber on the'back side of the diaphragm, a second and larger air chamber on the back side of the diaphragm, said chambers being connected by a narrow passageway, and hollow'means providinga passage for sound waves of a portion of the nu audio frequency range between the front .impedance to waves of a frequency a portionchamber.

23. An acoustic device comprising a casing, said casing comprising a plurality of wall members deiining an air chamber, one

of said wall members having an opening 'side o`f the diaphragm and said larger air 24. A n acoustic device comprising a da l phragm, an air chamber on the back side of the diaphragm, a second and larger air chamber on vthe back side of the diaphragm,

chambers being connected by a narrow passageway, and means providinga passage for sound waves from the front side of the diaphragm to said larger air chamber, said passage having an 'impedance characteristic such that substantially no waves having a frequency `greater than approximately two hundred cycles per second pass therethro h. .25. An acoustic device comprisin a aphragm, an enclosure providing an air cha'rnber on the back side of said diaphragm, and means providing a. passage from the front side of said diaphragmto said air chamber, said means comprising a hollow member o en at its ends, said passage oifering such a lugh eater than approximately two hundred cyc es r second that substantially no waves above t at frequency pass therethrough.

26. An acoustic device comprising a diaphragm, aV casing, said casing providing a substantially closed air chamber on one side of said diaphragm, said casing having an4 and a tubular member open at each end a cent said casing and providing a continuation of said opening, the casing Vchamber and said opening therein. adjacent said diaphragi,

tubular member being proportioned so that -sound waves in -the region below approxiing an air chamber on the other surface of thediaphragm, and means for fixing the lower cut-off of the device, said means comprising a hollow air enclosing member open at each end, extending into said chamber, and connecting the latter with the atmosphere adjacent `yIthe exposed surface of the diaphragm. Y

In witness whereofl I hereunto subscribe my name this 13th day of August, 1930.

ALBERT L. THURAS.