US1812389A - Acoustic device - Google Patents

Description

June 30, 1931. E. c. WENTE ACOUSTIC DEVICE Original Filed April l, 1925 996 :2: \5 SQ GE 20:

2 umb aha 2-1 5 radius freq F 1 mum"? Patented June 30, 1931 UNITED STATES v PATENT OFFICE EDWARD C. WENTE, OF NEW YORTK, N. Y., ASSIGNOR, IBY MESNE ASSIGNMENTS, TO WESTERN ELECTRIC COMPANY, INCORPORATED, A CORPORATION OF NEW YORK .ecous'rrc DEVICE Application filed April 1, 1925, Serial No. 19,793. Renewed September 11, 1930.

aims to provide means capable of faithfully reproducing complex waves, and especially to provide a loud speaking telephone receiver capable of faithfully reproducingelectrical "ariations' as sound such as speech-or music.

The invention will be described hereinafter with especial reference to such a receiver. In

accordance with such application of the invention, an inflexible diaphragm is so mounted and so actuated, as set forth hereinafter, that the ratio of the power output in the form of sound to the electrical power input due to the current corresponding to the sound to be reproduced, is substantially the same for all frequencies in the range of frequencies to be reproduced.

Fig. 1 Of the accompanying drawings is a diagram, and Fig. 2 a graph, for facilitating explanation of the invention; Fig. 3 is a sectional elevation of an embodiment of the invention; Fig. 4 is a top plan of a detail of Fig. 3, partly broken away; and Fig. 5 is a fragmentary sectional perspective view of the device of Fig. 3.

Before describing the structure of Figs. 3 to 5 in detail, it will be well to set forth certain theoretical considerations regarding an horn.

When a body vibrates in an elastic medium such as air, the power that is developed in the form ofsound waves may be expressed by P =p '0 where P is the power,

v is the velocity with which the body vibrates,

' plane 8, as shown in Fig. 1, vibrates as a unit perpendicularly to its plane, p will vary after the manner shown in the curve A of Fig. 2, as may be seen if values of the radiation resistance per unit area. of the diaphragm, for values of the product of diaphragm radius times frequency which lie between and 10 are calculated (all in c. g. s. units) from Equation 10, page 164, section 302 volume 2 of Rayleighs Theory of Sound (published by MacMillan and Company, London, 1894),

and plotted as in Fig. Here R is the radiusof the disc, f is the frequency, and

the ordinate. is the radiation resistance per unit area of the disc. For values of Rf up to 10" c. g. s. units, the quantity value of Rf for a disc of given radius the radiation resistance would vary as the square 'of the frequency, e. g. if we made R=4 cm.,

we would have from 0 to 2500 p. p. s. or p K and P=Kf o up to 2500 p. p. s. electromechanical loud speaker without a In any practical case such a disc would have to have some .form of elastic support. This would mean that the disc, which has a certain mass, would have a resonant frequency of some finite value. Above this resonant frequency the velocity of the disc is given, to a close approximation by This equation shows that, P. the sound developed by the disc would be independent of frequency for a constant applied force. If

now" we apply the force F by electrical means in such a way that F is proportional to the current, I, we shall have the ratio of sound output to electrical power input (which is roportional to P) thesame for all freuencies between resonance and 2500 p. p s.

Above 2500 pp. s p varies comparatively little withfrequency. At 5000 p. sfthe velocity of motion of the disc under t e assumed condition would have one half the value it has at 2500 p. p. s. T From 2500 to 50003). p. s.

the sound output would, therefore, rop 6 .T. U. and up to 10,000 p. p.'s. another 6 T. U.

(The abbreviationT. U. is. for the transmission unit discussed in the following articles:

The Transmission Unit, by R. V. L. Hart ley, in Electrical Communication, July, 1924,

published by the International Western Elec-' tric 00., Inez, New York; and The Transmission Unit and Telephone Transmission Systems, by Martin, and Practical Application of the Transmission Unit by C. W. Smith, both in the .Bell System Technical Journal, July, 1924, published by the 5 American Telephone and Telegraph Co., 's variationis not excessive New York.)- T for this large expanse of frequency,

- In practice it might not be advisable to use the disc in a plane of very largeextent as so was assumed in the preceding discusion. If

the back of the disc is closed oil so as to prevent radiation from the side, the sound outut will be reduced by 6 TAU. at the lower equencies by the removal of the infinite s5 plane. The omission of the infinite "plane thus in part offsets the drop at the higher frequencies'because of the fact that there p approaches a constant value. A receiver con- ,structed as described below renders it pos- 40 sible to transmit a band of frequencies extending from, say 100 to 7000 cycles per sec-' ond, with the transmimionloss varyin less than 6 T. U. throughout such frequency and,

-. w, and at the same time, to maintam the diam- 5 eter of the diaphragm large enough to economically obtaln satisfactory value of sound output.

g0omtruction of the receiver To operate on the principles just set forth, a receiver should-preferably satis the following conditions: The disc or diaphragm should be constructed so that it will move 'as,

co the disc should-be comp etely shut oil from the outside air; the force on the diaphragm should bepr'oportional to the current input; the impedance of the receiver winding should not va greatly with frequency; and the disc shoul be mounted so that it is .free

to execute large amplitudes of motion, in

lar e'volume.

Y ne'construction. adapted tomeet these conditions is shown. in Figs. 3 to 5. In that construction, the diaphragm 10 comprises a circular central portion 11 bulging in one direction, and an annular rim 12 flarmg from the periphery-of the central portion in that direction, the diaphragm thus having a W-s'haped cross section, as indicated in Fig. 3. The central portion 11 is preferably dome-shaped, as shown, since a structure of this shape is speciall adapted to move as a unit up to very high requencies when driven from its base as described hereinafter. However, the portion 11 may be of any shape, for instance conical, suitable for obtaining the requisite stifiness, and may, if desired, be annularly or radially corrugated to increase its stiffness. For the sake of rigidity, the rim or outer portion 12 of the diaphragm 10 order to obtain a sound output of sufliciently is preferably frusto-conical, asindicated in Figs. 3 and 5. However the rim 12 may be of any suitable shape, for instance, downwardl concave and upwardly convex, suitable or (obtaining the stiffness requisite to enable. the rim to e vibrated as a unit up to 'veryjhigh frequencies when driven from its inner periphery as described hereinafter. 'If desired, the rigidity ofthe rim 12 may be increased by annular or radial corrugations, not shown. Preferably, the diaphragm 10 is of thin light metal, for instance duralumin -or aluminum. Its diameter should be sufiiciently small to insure the desired degree ofv uniformity oftransmission efliciency over the fre uency range to be transmitted. The mass 0 the diaphragm should be sufficiently small to enable the receiver-to have the desired sensitivity. Preferably the diameter should be as small as 5 inches, and the mass as small as 5 gr. v

v The diaphragm is so mounted as to close an opening in a wall 15 of a casing 16 which closes off one side of the diaphragm from the surrounding air. The magnet 18, which is shown as an electromagnet but ma if desired, be a permanent magnet, is enc osed in the casing 16 and rigidly connected to the casing by an suitable means such, for instance, as ro s 19. As shown, the magnet comprises a winding orcoil '20 surrounded by a. paramagnetic fi-ux 'path21 havin an annular air-gap 22 therein at the end o the hollow core 23adjacent to, the diaphragm 10.

Terminals 24, only one of which is shown,-

are provided forcoil 20, so that the coil may be energized by direct current.

The r.'

1V1I1g system for the diaphragm is shown as of the movable coil type, the coil 25 for receiving the telephonic current being situated in the radial magnetic field in the aingap 22. This type of drivin unit has a orcefactor, and the impe ance oi large posts 26, only one of which is shown, form terminals for the coil 25. This coil is rigidly connected to the diaphragm at the junction of the'central portion 11 and the rim 12,

as shown in Figs. and 3, so that the elements 11, 12, and form a rigid unit. A ring preferably of very thin metal, for instanceduralumin or aluminum extends outwardly from the periphery of the rim 12, in the plane of that periphery. Bin 30 is supported from the wall 15. Prefera ly the outer periphery of the ring engages the outer cylindrical wall 31' of'an annular'recess 31 in the inner edge of the wall, and above and below the ring 30 the recess is filled with paper rings 32, such as are used for so-called book damping. If desired, the pa hold ring 30 tig ltly near its outer periphery and loosely near its inner edge, for instance by tapering (not shown) the recess so that it is narrower near wall 31' than near the periphery of the,- diaphragm. The paper rings offer only slight opposition to movement of the diaphragm 10 in the direction perpendicular to the plane ofthe ring 30, and thus permit substantially free vibration of the diaphragm as a unit in that direction, with lar e amplitude, for instance a double amplitu e of two millimeters, while at the same time maintaining the coil 25in proper position for vibration in the air-gap 22. A member 34 preferably in the form of an annular spider comprising a hub portion 35, a rim 36, and radial spokes 37 connecting the hub and the rim, is rigidly connected to and extends outwardly from'the junction of the central portion 11 and the rim 12 of the diaphragm 10. Preferably, the hub of the spider 34 is integral with the central portion 11 of the diaphra m, as indicated in'Fig. 5, and is of thin ralumin or aluminum, with the rim annularly corrugated as at 38, the hub annularly corrugated as at 39, and the spokes radially corrugated as at 40 (see Fig. 4). All of these corrugations open into the plane of the spokes.

- air-gap. The

The rim of the spider is supported from the structure of magnet 18 and casin 16, preferably by being clamped in the inner edge of a ring rigid with magnet 18. The

spider 34, as well as the ring 30, supports the diaphragm and moving coil for substantial- 1y free vibration of the spider, with large amplitude, while maintaining the coil in proper position in the annular corrugations 38 and 39 allow the width of the spider 34 to increase when the spider is flexed from its normal position and the radial-corrugations 40 tend to prevent undesirable'vibrations in the spider.

In order to keep the resonant frequency of er rings may be caused to light metal, for instance duperpendicularly to the plane the diaphragm low, refdrabl as low as 100 cycles 'per second, t e cham r formed by casing 16 is of such size that the volume of 'air in the chamber is large compared to the maximum volumetric displacement executed by the diaphragm during its normal 0 era-- tion. For instance, with a diaphra ing a diameter of the order of 3% inc es, and a magnet having a diameter of the order of 6 inches and 9. Ion h of 4 inches, the casing 16 may be a box-1i e casing having dimensions of the order of 12"by 12-b 8' in depth. To reduce standing waves wit in the casing, preferably the hole through the magnet core is flared and the portion of the chamber not occupied by the magnet is substantially filled with some dampin material such as loose wool or down 51 as indicated in the drawing.

To reduce eddy currents, slits 50, parallel to the axis of core 23, are provided in the end of the core adjacent coil 25'.

The aper rings 32 effectively prevent passage 0 air or sound waves around the edge of the diaphragm 10, from the outside to the inside, or vice verse, of the chamber formed by casing 16, and yet, as noted above, permit substantially free vibration of the diaphragm, as a whole, through large amplitudes, at the frequencies to be reproduced. The paper rings also prevent sound originating at one face of the diaphragm from impinging on one face of the ring 30 and being transmitted through that rin to the air on the other side of that ring. The paper rings also prevent resonant vibrations of the ring 30 itself.

In the operation of the device as a telephone receiver, coil 20 is energized by direct current, to create a steady, radial electromagnetic flux across ga 22; and the binding posts 26 (only one 0 which is shown) of coil 25 are connected to the source (not shown) of telephonic current, the variations of which are to be reproduced as sound. The diaphra m and the coil 25 vibrate as a whole, in accor ance with the variations in the latter current, the diaphragm giving off the sound corresponding to those current variations. The device is of course reversible. That is, sound waves impinging on the diaphragm will cause the device to function as a transmitter, generatin spondingly varying E. F.

By properly fixing the extent of the wall 15 beyond the periphery of the diaphragm 10, the effect of the wall in increasing the sound output of the receiver at low frequencies more than at high frequencies, can be made such in coil 25 a corre- I quency range upto 3500 cycles per second or 5500 cycles per second in these two cases, respectively. The specific fre- -quency and radius values here mentioned are merely illustrative. As a further example, if the frequency range which it be desired to transmit with substantially uniform efiiciency extends from 5000 cycles down to quite low frequencies, and the radius of the diaphragm be 4 cm., then it may be desirable to have the extent of the wall negligibly small, in accordance with the principles hereinbefore explained.

Distortion due foam-linear response Not only may the receiver of this invention be made to have practically a uniform response over the Whole frequency range to be reproduced, as for instance the range of frequencies of importance in music, but it is, moreover, free from another type of distortion introduced by most loud speakers. This latter type of distortion'is producedif at a given frequency the displacement is not proportional to force. Since this receiver operates above the resonant frequency of the diaphragm, the amplitude,

' if the diaphragm moves as a unit m, the effective mass of the diaphragm, is constant and a and F are proportional.

The broad features involving the general principles disclosed herein may be embodied in many organizations widely different from those specifically shown and described, Without departing from the spirit of the invention defined in the appended claims.

What is claimed is:

1. An acoustic device comprising 9. diaphragm, an air chamber for which said diaphragm forms a closure member, the air inclosed in said chamber forming a cushion for said diaphragm, and means mounting said diaphragm on said air chamber, the volume of said inclosed air being sufficiently large and the stiffness of said mounting means suffici ently small in magnitude to maintain the resonance frequency of the diaphragm below the frequency range of importancein music.

2. An acoustic device comprising a direct actin diaphragm, an air chamber for which said 'aphragm forms a closrre member, the air inclosed in said chamber forming a cash ion for said diaphragm, and means mounting said diaphragm on said air chamber, the volume of said inclosed air bein sufficiently large and the stiffness of said mounting means sufficiently small in magnitude to maintain the resonance frequency of the diaphragm below the frequency range of importance in music. g

3. An acoustic device comprising a plunger type diaphragm having a W-shaped cross sectionand an air chamber for which said diaphragm forms'a closure member, said chamher being too large to cause resonance of the diaphragm above cycles per second.

4, An acoustic device comprisinga plunger type diaphragm havin its natural frequency below the frequency range of importance in speech, an air chamber for which said diaphragm forms a closure member, and a driving coil rigid with said diaphragm, said chamber being too large, in comparison to the maximum volumetric displacement executed by said diaphragm in normal operation, to cause resonance of the diaphragm in said frequency range.

5. A device comprising a diaphragm having a natural frequency of vibration below the range of frequencies of importance in speech and having a radiation resistance varying substantially as the square of the frequency of vibration of the diaphragm over said frequency range,'and means responsive to current of speech frequencies for driving said diaphragm with a force proportional to said current.

6. An acoustic device comprising a diaphragm having a natural frequency of vibration below the range of frequencies of importance in music and having a radiation resistance varying substantially as the square of the frequency of vibration of the diaphragm over said frequency range, and means responsive to current of frequencies of said range for driving said diaphragm With a forceproportional to said current.

7. A device comprising a diaphragm having a natural frequency of vibration below the range of frequencies of importance in speech and having a radiation resistance varying substantially as the square of the frequency of vibration of the, diaphragm over said frequency range, and means responsive to power variations representing sound waves for causing said diaphragm to generate sound wave power proportional in magnitude to said first mentioned power variations.

8. A device comprising means responsive to electromotive force, of the frequencies of the range important in music, for generating mechanical force substantially proportional to the electrical current delivered to said I phragm having a natural frequency of vibration below the range of frequencies of importance in speech and having a radiation resistance varying substantially as the square of the frequency of vibration of said diaphragm over said frequency range, means responsive to current of speech frequencies for driving said diaphragm with a force proportional to said current, and a wall extending outwardly from, and substantially in the plane of, the periphery of said diaphragm, and increasing the sound power produced by said diaphragm.

' 10. A device comprising a piston type, direct acting diaphragm having a natural frequenc of vibration below the range of frequencies of importance in music, means responsive to current of the frequencies of said range for driving said diaphragm with a force proportional to said current, and a wall extending outwardly from, and substantial- 1y in the plane of, the periphery of said diaphragm, a distance of such magnitude, with regard to the diameter of the diaphragm, as

to substantially equalize the sound power output, per unit of said force, over said range.

11. An acoustic device comprising a diaphragm, an air chamber for which said diaphragm forms a closure member, the air 1nclosed in said chamber forming a cushion for said diaphragm, and means mounting said diaphragm on said air chamber, the volume of said inclosed air being sufiiciently large and the stiffness of said mounting means sufficiently small in magnitude to maintain the resonance frequency of the diaphragm below the frequency range of importance in music, said first mentioned means comprising book damping means for said diaphragm.

12. A device comprising a diaphragm having a W-shaped cross section, book damping means for said diaphragm, and driving means for said diaphragm attached to the symmetrical cusps of said cross section.

13. A device comprising a diaphragm provided with book damping means and having a natural frequency of vibration below the range of frequencies of importance in speech and having a radiation resistance varying substantially as the square of the frequency of vibration of the diaphragm over said frequency range, and means responsive to current of speech frequencies for driving said diaphragm with a force proportional to said current.

14. An acoustic device comprising an inflexible diaphragm having a natural frequency below a range of frequencies to be reproduced, means closing ofi one side of said diaphragm from the surrounding soundpropagating medium, means supporting said diaphragm for substantially free vibration,as a unit with large amplitude, at thefrequencies of said range, and means responsive to current of the frequencies of said range for driving said diaphragm with a force proportional to said current. I 15. A loud speaking telephone receiver comprising a diaphragm having a forwardly bulging central portion and a rim flaring backwardly from the bulging portion, means closing one side of said diaphragm off from the surrounding air and mounting said diaphragm for vibration as a whole, at the frequencies of importance in speech, and means attached to said diaphragm at the junction of said bulging portion and said rim and re sponsive to current of said frequencies for drivin said diaphragm. 16. device comprising a paramagnetic body forming a flux path with an air gap therein, an inflexible diaphragm for vibration as a whole, a coil, means rigidly connecting said coil to said diaphragm, and means supporting said diaphragm for vibration as a whole, at the frequencies of importance in speech, with said coil centered in said gap, said means including annularly and radially corrugated means connected to said diaphragm and extending outwardly from said diaphragm, with respect to the axis of said diaphragm in the direction of said vibration. 1 A loud speaking receiver comprising a paramagnetic body forming a flux path with an air ap therein, a diaphragm having a forwar lybulging central portion and a rim flaring backwardly from said bulging portion, a coil, means rigidly connecting said coil to the junction of said central portion and said rim, an annular web extending outwardly fromthe periphery of said diaphragm, an annular spider extending outwardly from the periphery of said central portion, and means, including said web and said spider for supporting said diaphragm and said coil for vibration with said coil centered in said air gap.

18. An acoustic device comprising a diaphragm, an air chamber for which said diaphragm forms a closure member, the air in closed in said chamber forming a cushion for said diaphragm, loose fibrous material occupying the major portion of said air chamber, an means mounting said diaphragm on said air chamber, the volume of said inclosed air being sufiiciently large and the stiffness of said mounting means sufficiently small in magnitude to maintain the resonance frequency of the diaphragm below the frequency range of importance in music.

19. A loud speaking receiver comprising a piston type, direct acting diaphragm having a natural frequency of vibration below of said current, as high as of the maximum value of said ratio for said frequency range. g y 20. An acoustic device comprising a direct acting, piston typediaphragm, an air chamher for which said diaphragm forms a 010-- sure member, 'fibrous material in said air chamber for reducing standing waves therein, the air enclosed in said chamber forming a cushion for said diaphragm,-and means mounting said diaphragm on said air chamber 'for vibration of said diaphragm as a whole, the volume of said enclosed air being sufiiciently large and the stifiness of said mounting means sutliciently smallin magniture to maintain the resonant frequency of the diaphragm below one hundred cycles per second, said diaphragm having a diameter sufliciently small to render said device capable of transmitting a band of frequencies ex-' tending from one hundred cycles'to seven thousand cycles per second with a maximum variation in transmission loss, with frequency, of six transmission units.

21. A loud speaking receiver comprising a piston type, direct acting diaphragm having a natural frequency of vibration below one hundred cycles per second; means closing one side of said diaphragm oil from the surrounding air, and means responsive to current of 40 the frequencies of importance in music for driving said diaphragm, said diaphragm having a diameter sufliciently small to render said receiver capable of transmitting a band of frequencies'having a width of approximately seven thousand cycles per second with a maximum variation of transmission loss as low as six transmission units.

22. Aloud speaking receiver comprising a' piston type, direct acting diaphragm, means closing one side of said diaphragm ofi from the. surrounding air, and means responsive to current of frequencies of the range of'imn the order of one hundred cycles per second to a frequency of the orderof six thousand cycles per second. a 23; An acoustic device comprising a diaphragm, a coil secured thereto for driving the diaphragmfa pair ofwencentric lepieces. separated by a gap in which sai coil center mam ' for centering said coil in said gap, said means a pair of concentric pole-pieces rtion being'expansible when'said coil is moved from its normal position to enable the coil to move axially. I

25. An acoustic device comprising a rigid diaphragm, means at ther'iphery of the diaphragm for supporting it so that it is free to move dil phragm comprising pole-pieces separatedby a narrow ga and an actuatin to said diap agm, and a spi er-like-memher for preventing lateral movement of said coil in said gap, said member being rovided. with annular corru tions to allow t e width means for actuating said diaP coil secured of the member to increase when it is flexed form its normal 'podtion and with radial corrugationsto prevent undesirable vibrations in said member when the coil is v1- brated.

26. An acoustic device rectly to theisurrounding air, means attached to said diaphragm at points removed from the center thereof for driving it, means to prevent the sound waves radiated by the exposed face of the diaphragm from interfercomprising a diaphragm having one face thereof exposed diing with those radiated by the other face of said diaphragm, and means for damp 27. An acoustic device comprising a casing havin a front wall with an aperture therevin, si e and back walls, a magnet mounted within said casing having annular pole pieces spaced toform an annular air gap, a diaphragm comprising a curvedportlon and a surrounding annular portion, a coil in said air gap secured to the diaphragm at the edge of the curved portion thereof, means for an porting the outer rim of said diaphragm 1n the waves given off by said other face of the diathe aperture in said casing so that it is to move substantially as a whole, and means within said casing and surrounding said magnet for damping sound waves "given of b one side of the diaphragm 28. A loud speaking device comprising 3,' dished diaphragm having one me thereof. exposed directly to the surrounding air and of sufficient size to radiate sound directly into the surrounding air without the emplo ent of a horn, means including a coil a e'd'to said di hragm at dgqiuts removed from the thereof for s eems' 1m an aperture into which said dia hragm fits, and means with-in said casing or dampin the sound waves given off by the unexpose face of said diaphragm.

29. The combination of a diaphragm for radiating sound waves directly into the surrounding air without the employment of a horn, means including a coil attached to said diaphragm and a magnet structure for driving it, a casing having an aperture in one wall thereof in which said diaphragm is disposed and a filling of sound deadening material in said casing and surrounding said magnet structure.

In witness whereof, I hereunto subscribe my name this th da of March A. D. 1925. ED ARD c. wENTE.

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