US1866603A - Acoustic device - Google Patents

Description

July 12, 1932;

v. A. SCHL'ENKER ACOUSTIC DEVICE Filed May 11, 1921 I 4Sheets-Sheet 1 BYW July 12, 1932. v. A. SCHLENKER ACOUSTIC DEVICE Filed May 11, 1927 4 Sheets-Sheet 2 m m w m VESPEI? 4. 60/

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' ACOUSTIC DEVICE Filed May 11, 192"v I 4 Sheets-Sheet 5 fieli.

Arron/ Er 12, 1932- I v. A. SCHLENKER' 1,866,603

ACOUSTIC DEVICE Filed May 11, 1927 4 sheets-sheet 4 Had .10 aluminum, and of large size. In a Specific A Patented July 12,1992

UNITED STATES PATENT OFFICE VESPEB, A. SOHLENKER, OF ORANGE, NEW JERSEY, ASSIGNOR 'IO BELL TELEPHONE LABORATORIES, INCORPORATED, OF NEW YORK, N. Y., A CORPORATION OF NEW YORK Application 'filed Kay 11,

This invention relates to. acoustic devices and particularly to a sound radiator having a high and substantially uniform eiiiciency over at least a large portion of the frequency.

,6 rangeemployed in speech and music.

The invention has its preferred embodlmentin a sound radiator comprising a thin,

highly tensioned diaphragm of light Ina-- terial, such as an alloy consisting largely of form of the invention which has been chosen for the purpose of illustration, the diaphragm consists of a circular sheet of aluminummanganese alloy 0.002 inch thick and 29' 15 inches in diameter clamped at its periphery to a metal ring which maintains it undera tension of 40 lbs. per inch. The diaphragm ,is driven by an electrodynamic unit of the type disclosed in Patent'No. 1,707 ,544 to A.

39 L. Thuras, issued April 2, 1929. The coil of this unit is made very light and is. preferably secured to the diaphragm at a position slightly ofi' center in order to reduce distortion due" to reflected waves reaching the driving unit. It has been found that a sound radiator of this type has substantially uniform efiiciency over at least a large portion of the frequency range essential to speech and music. This appears to be due, in part at least, to the action of the large mass of air w 'ch is in effect coupled to the diaphragm at low frequencies as compared withv the effect of the air at high frequencies. The diaphragm may be placed in a cabinet in a manner to-increase this effect. It is found also that veryhigh efliciency is obtained in accordance with this invention.

In accordance with another feature ofthe invention, which is applicable both to large and small diaphragms', those'which'are tensioned and those which are not, there'is provided a diaphragm so shaped, and driven at such a tion'sdntersect within one or more small areas on the diaphragm remote from the driven portion. 'andmeans are provided at these areas for V dissipating the reflected wavefl energy. -"A desirableform of diaphragm in a stretched diaphragm supported at its periphwhich this feature is embodied is. a circularaconsrrc DEVICE 1927. Serial No. 190,484.-

tween the periphery and the center, and provided with a damping unit located symmetrically with respect to the driving means and on the opposite side of the center of the diaphragm. I 7

The invention may be readily understood by referring to the following description and the accompanying drawings in which:

Fig. 1 is a perspective view of a sound radiator constructed in accordance with this invention V Fig. 2 is a rear view of the sound radiator removed from the cabinet in which it is shown mounted in Fig. 1;

- Fig. 3 is a sectional view taken along the line 3-3 of Fig. 2; v

Fig. 4 is an enlarged perspective view showing the construction of the coil which is employed for driving the diaphragm; Fig. 5 is an enlarged view partially in section taken along the line 55 of Fig. 2

Fig. 6 is a rear view of a sound radiator which is a modification of the sound-radiator shown in Fig. 2;

liqgF-"l', ofFig. 6;

gs. 8, 9 and 10 are vlews partly in section showing various'modifications of the dam ing uni employed in conjunction with the sound rgdiator shownin Figs. 6 and 7; and Fig. 11 is a diagrammatic view of a modified form of diaphragm constructed in accordance with the present invention. Referring now particularly ta Figs. 1 to 5 of the drawings, the diaphragm 10 is-main- Y tained under hightension uniformly in all directions by. the supporting frame 11 and I the clamping ring 12 which is secured to the frame by the screws 27. This frame and ring are preferably of a material having the same 6 coeflicient of expansion as the diaphragm ma- .is not affected appreciably by changes'in temperature. A diaphragm material of an alloy composed of approximately 98.5% aluminum and 1.5% manganese and rolled down to a 1 thickness 1cm than 0.005 inch and'preferably 0.001 inch or 0.002 inch has been found satisfactory .although' other materials having 't'erial so that thetension of the diaphragm V i coupled thereto a suflicient amount of the airproperties similar to those possessed by this and about 2 X 10 lbs. per square inch for the 0.002 inch thick material. The inherent stiffness of the material is very low, that is,'it is very flexible before being tensioned. The inherent stiffness of the material has been tested by clamping a strip of the 0.005 inch thick material 1 inch in width so that a piece 1 inch long is free. A uniformly distributed force was applied to the edge of this strip opposite the clamped edge and the deflection noted. For forces within'the elastic limit of the material the stiffness was found to be about 0.3;

lbs. per inch deflection. Because of the low internal resistance possessed by this material (which is also characteristic of other metals which are not tensioned beyond their elastic limit) practically no energy is dissipated within the diaphragm itself.

The purpose of lowmass and high tension is to enable the radiator to respond effectively at the higher portion of the speech and music frequency range. Attempts have heretofore been made to design radiators to employ highly tensioned diaphragms, but so far as known, all such attempts fall short of producing a practical'radiator having a substantially uniform frequency-response characteristics in the lower part of the frequency range as compared with the higher. The present invention is based on the discovery that by sufiiciently increasing the size of the highly ten; sioned diaphragm the benefits of high tension are retained and at the same time a substantially uniform response is obtained throughout a very large part of the range of desired frequencies due to a broadening of the range at the lower frequency side to avery lowpoint. This appears to be due to the fact that the diaphragm of increased size has effectively adjacent thereto to produce this extension of the band of radiated frequencies without deleteriously affecting the characteristic in the upper portion of the frequency range. This broadening of the band at the low frequency side may perhaps be looked upon as being due in large part to lowering the natural fundamental frequency of the diaphragm in air by virtue of loading it with the air adjacent thereto.

Satisfactory results have been obtained by applying'a tension of approximately 40 lbs. per inch to the diaphragm (this giving it a diaphragm of the kind herein disclosed may be substantially increased bypartially confining the air in proximity to the diaphragm. This may be done by placing the diaphragm in a container so that the .walls of the con-' tainer' project forwardly or backwardly or both from the plane "of. the diaphragm a short distance. Thus in Fig. '1 the diaphragm 10 is placed in the cabinet 13 in a Wooden partition 29, which is inclined somewhat to the vertical. Protective screens 28 which may comprise a silk covering mounted on a suitable frame are positioned in the openings in the front and ,backof the cabinet 13." This covering has very little effect, if

any, on the frequency-response characteristic of the complete apparatus. The partial enclosure for the air'atthe front or back of the diaphragm or bothappears to act somewhat as a horn and somewhat as a bafile in the plane of the diaphragm. If desired, spacemay be provided in the cabinet for phonograph or radio apparatus for use in conjunction with the sound radiator.

The lower cut-off frequency of a sound radiator employing a diaphragm 29 inches in diameter tensioned to 40 lbs. per inch has been found to be about 100 cycles per second when employed without a bafile'or sound chamber of any kind. This cut-off frequency may be lowered about 10% by employing a baflle in the plane of the diaphragm or by positionin the diaphragm in a relatively shallow ca inet. A sound radiator employing a diaphragm of this type positioned in a deeper cabinet, such asshown in Fig. 1, measuring 22inches between its open sides, has been found to have a lower cut-off frequency at about 75 cycles' per second. A sound radiator-having a lower cut-off frequencyat approximately 250 cycles per second may be obtained by employing in free air a diaphragm having an area of about 45 square inches and tensioned-to have a propa-.

gation velocity of one-fourth the velocitv of sound in air, i. e.. to about 15 lbs. per inch.

The diaphragm 10 is driven by an electr'odynamic unit 16 which is supported by the cross members of the frame 11', preferably in the form of a unitary casting. The coil 19 of the electrodynamic unit 16 is secured to the diaphragm so as to drive it along a circular line and is positioned between the annular I ole faces 20 and 21 of its magnetic structure.

of cobalt steel shaped similarly to the electroductor may be insulated from each other by ,magnet shown and preferably of laminated construction.- It? should be noted that the shape of the magnetic structure is such that the cross-sectional area of the space between it and the adjacent portion of the diaphragm tapers from the coil outwardly. The central opening in the magnetic structure likewise has a tapered cross-sectional area. arrangement serves to avoid the vefi'ect of an enclosed or resonant body of air.

As best shown in Fig. 4, the coil 19 is made of a thin ribbon conductor preferably of copper and of the order of 0.002 inch thick and 0.014 inch wide, wound so that the plane of the ribbon is perpendicular to the axis of the coil. The adjacent turns of the ribbon conapplying a thin coating of a solution of bakelite dissolved in acetone, or other suitable insulating material, to the conductor both before and after winding the coil. This coating not only serves as an insulating medium but also to give rigidity to the coil when-dried. Such an insulating material, moreover, takes up so little space that when the coil is positioned between the pole faces 20 and 21 of the magnetic structureof the electromagnetic unit 16, the air gap between the pole faces'is substantially-filled with the conducting material of the "coil. A strip of oiled silk or; other thin, light fabric is cemented along one edge to the inner surface of the coil. The opposite edge of the strip is cemented to the diaphragm as shown in Fig. 5 and a coating of the bakelite solution is apinvar steel, or other material having a.

plied to the strip to increaseits rigidity.

For a diaphragm "tensioned to a certain degree, the mass and size of the driving coil are important factors in determining the upper cut-off frequency of the sound radiator, that is, the frequency at which the ra' diation efficiency decreases sharply from a high to a relatively low'value. Between'this upper cut-off frequency and the lower cut- 0 frequency of the sound radiator, referred to above, the radiation efficiency is high and substantially uniform. Satisfactory results have beenobtained by employing a coil 2 inches in diameter having a total mass less than 0.011 lb. and preferably about 0.003 lb.

or, in other words, one having a mass per unit length of coil periphery less than 0.0 02 lb. per inch and preferably about 0.0005 11). per inch, respectively. It has been found that to a certain extent the mass of the coil employed for driving a diaphragm which This a coil 2 inches in diameter and wei hing about 0.003 lb. are preferably employe and this sound radiator has its upper cut-ofi' frequency at aboiit 5000 cycles per second.

The ends 25 and 26 of the coil winding are brou ht out to the terminal block 24 in anysuita le manner. The coil 19 is secured to the diaphragm. preferably ata position slightly off center so that vibrational waves reflected from the edge of the dia hragm will not reach the driving unit. ood results have been obtained bymaking the distance between the center of the diaphragm and the center of the coil about 1 or 2 tenths of the diameter of the diaphragm. For this condition, the number of peaks and valleys in the frequency response curve is small. As the distance between the center of the diaphragm and the coil is increased, the numer of peaks and valleys in this response characteristic becomes larger, but their amplitude is somewhat decreased.

While various methods may be employed .for mounting the diaphragm material 10 on the frame 11, one method which has been found satisfactory is as follows: A sheet of aluminum alloy is placed between clamping rings which have aninside diameter slightly larger than the outside diameter of the frame 11 and ring 12 and which are made of iron,

phragm are now placed in an oven and raised to a high temperature at which they are maintained while screws are secured through the clamping-rings and the diaphragm ma terial. The temperature is then lowered, thus subjecting the diaphragm to tension due to the fact that the diaphragm'material contracts at& higher rate than'the iron or invar 'steel'clamping rings. When the diaphragm 10 is cool the frame 11 and the clampin ring 12,-of a material havin the same coe cient of expansion as that o the diaphragm material, are placed inside of the iron or invar steel rings and secured to the diaphra and to each other by means of the screws 2 The iron or invar steel rings are now removed and the diaphragm is ready to be mounted in the partition 29 of cabinet 13. j

In Figs. Guild 7 is shown asound radiator Y which differs from the one shown in Figs. 1

to 5 respectively, and described above, in that I pled to the leads 51 and 52 of the coil 42 means are provided for dissipating vibration- :11 energy reflected from the peripheral support for the diaphra m. Two similar electrodynamic units 40 an 41 are employed respectively, for driving and for. damping reflected waves in the diaphragm 47. These units are supported by the angle bars 34 and 35 and the cross bars 36 to 39 inclusive, which are secured' to the angle bars 34 and 35. The'movable coils 42 and 43 of these units are secured to the diaphragm 47 so that the centers of the coils lie substantially on a diameter of the diaphragm andare approximately equldistant from the center. These coils are positioned between the annular polefaces of the magnetic structures of units and 41 and the battery 44 supplies current to the polarizing windings 45 and 46, respectively, of these units. A source of electrical energy suchas an electromagnetic phonograph reproducer, or radio receiving apparatus is conthrough the amplifying equipment 56. The leads 53 and 54 of the coil 43 are connected to the externalresistance 57 which is preferably made of such a value that the impedance of the electrodynamic damping unit 41 is substantially a pure resistance, and is approximately equal to the impedance of the diaphragm measured at the position where the coil 43 is attached. It is possible that the coil 43 employed in the damping unit 41 may be made as a closed solid metallic ring, in which case the external resistance 57 is not required. The resistance of this metallic ring together with the density of the linkage flux will determine the aniount of damping. Instead of employing electrodynamic units for driving and damping the diaphragm other units such as those of the electromagnetic type may be used. I

- While a damping means similar to the unit employed for driving the diaphragm is in some cases preferable, in many cases other damping means such as those shown in Figs. r 8, 9 and 10, may be employed if desired. In Fig. 8. opposite surfaces of a porous material 59 such as felt are secured to a diaphragm 60 and a supporting structure 61, respectively.

As the diaphragm .vibrates, air is alternately I forcedinto and out of the pores of the material 59 and this'action serves to dampen the.

vibrations reaching this portion ofthe diaph 4 3 for damping'vibrations in a diaphragm. An

annular member 62 is secured'td the diaphragm- 63 and fits into a recess 66 in themem v ber 64 which is secured to a support 65. As

fthe diaphragm 63 vibrates, air is forced in and out of the chamber through the recess '66 thereby. damping vibrations in this por- In Fig. is shown a dash-pot arrangement;

cured to a support 68 so that one of the plane 4 surfaces of the cylinder 67 lies substantially parallel and in close proximity to the surface of diaphragm 69: -With this arrangement, as

with that shown in Fig. 9, vibrations of the diaphragm are damped due to the air being forced in and out of the space bet een the d1aphragm 69 and the cylinder 67 In the case of the-sound radiator employing a circular diaphragm driven and damped as described above, it is seen that waves set up in the diaphragm by the driving means travel in all directions therefrom towards the supported edge of the diaphragm and are then reflected.

These reflected waves come to a focus within a small area on the diaphragm at which a damping means is applied for dissilpating the reflected waves. Instead of emp oymg a diaphragm which is circular in shape, diaphragms of other shapes may be employed, as for instance, an elliptical shaped d1aphragm which is driven at one focal point and in which the reflected waves come to a focus-substantially at the conugate focal point where they are dissipated.

The dia hragin 70, shown dia rammatically in Fig. 11 has a periphery ormed by portions of four ellipses having a common focal point- 71 at which the diaphragm is driven and four conjugate foci 72, 73, 74 and 75 at which means are supplied for damping reflected vibrations in the diaphra The dottedlines '76 indicate that the s ape of the diaphragm is equivalent to four superposed ellipses having one common I focus. The dash. lines 77 represent various radii of a circular vibrational wave initiated tionof transverse waves is the same throu hout the diaphragm. In the case of a circu ar 'diaphragm driveh ofi center such as shown in Fig, 6 the waves reflectedfrom various portions along its periphery, instead of coming to a focus at'a point as is the case when an elliptical shaped diaphragm is employed, come to afocus within a small area to which the damping means is applied formula and its mass is very low compared with the mass of the air which the, diaphragm sets into motion at low frequencies. At low freqlllencies of vibrationthe efl'ective mass of t e air coupled to a diaphragm which is exposed on both sides thereto 1s given by the inch thick) is equal to mass per unit area I multiplied by thickness= 0.098 X 0.000196 lbs. per sq. in. Hence it will be noted that at low frequencies of vibration the mass per. unit area of the diaphragm is only approximately 17% of the mass of the air per unit area of diaphragm-coupled to it.

The coil preferably used for driving the diaphragm is about 2 inches in diameter, weighs about 0.003 lbs. and is a negligible factor in determining the radiation'efliciency of the sound radiator at low frequenciesA However, at higher frequencies the mass of the air coupled to the diaphragm is relatively small. Therefore, the mass of the coil be comes an im rtant factor in determining the upper cut-o frequency of the sound radiator. The cut-off frequency of a vibrational system is a function of the ratio of its stiffness to itsmass and the stiffness of this sound radiator is proportional to its tension. The

. mass of the sound radiator at high frequen-' cies is substantially equal to the mass of the coil aloneand when the diaphragm is ten sioned to have a pro agation velocity of a proximately. one-hal the velocity of sound 1n air and the 2 inch diameter coil weighing about 0.003 lbs. is used the upper cut-off frequency is about 5,000 cycles. per second. Since at low frequencies the stiffness factor is again proportional to the tension applied the diaphragm, and since the mass at low frequencies is substantially equal to the mass of the air coupled to the diaphragm, the low frequency cut-01f pointdepends on the area of the diaphragm. For a diaphragm tensioned to have a propagation velocity 'onehalf'tha't ofsoundin .air and having a diameter-of approximately 29 inches this low frequency cut-oil pointjs approximately 100 cycles'per second. When the tensioning ofthe diaphragm is decreased so that it has a propagation velocity approximately one-'- fourth the velocity of sound inair and, the mass of the driving coil is increased to .011

lbs., the high frequency cut-ofi' point will be approximately 1050cycles per second. The .I

radiation efliciency of the sound radiator de creases rapidly at the cut-off frequency, perdecreases slowly with increasing frequency.

obtained by driving the diaphragm at the center it has been found that the radiation efficiency of the sound radiator is made more uniform over a wide range of frequencies by displacing the driving unit slightly from the center. It is believed that this is due to the fact that vibrational waves reflected from the edge of the diaphragm do notcome to a focus at the driving unit but are concentrated at a small area remote therefrom. Additional improvement in the frequency response char- 7 acteristic of the sound radiator was obtained by applying a damping means to this small area, at which the reflected waves come'to a focus for dissipating the reflected wave energy.

' As pointed out heretofore the impedance of the means employed, for damping reflected vibrations should be substantially a pure resistance equal in magnitude to theimpedanc'e of the sound radiator measured at the port ionof the diaphragm to which the damping means is applied. When this is the case an optimum damping action takes place. If the. resistance of the damping element is too high or too low, undesirable wave-reflection will take place from this damping element as well as from the supported edge of the diaphragm. In some cases it may be desirable to employ a plurality of driving units working in phase in which case each of the driving units serves to dampen reflected vibrations initiated by the remaining units.

. This invention has b n described aboveas a sound radiator, but it may also-be used I versevibrati'onal wavesftherein isgreater than one-fourth the velocity of sound in air,

said diaphragm having a sufiiciently large ab haps about 10 transmission. units, and then.

. surface area asto havesa low fundamental natural frequency.

2. A diaphragm for radiating sound con sisting of thin material having low internal resistance and tensioned to such a degree that the velocity of propagation .of transverse vibrational waves therein approaches the velocity of sound in air, said diaphragm being suf- I thick having ficiently large so that when tensioned it has a fundamental natural frequency in the lower partof the audible range.-

3. An acoustic diaphragm of thin metallic material stretched uniformly in all directions and of sufficient size to have a natural fundamental frequency in air lying in the lower being tensioned to such a degree that the velocity of transverse vibrations therein is hi h, and means secured to said diaphragm for driving it. y

6. A stretched acoustic diaphragm of a material consisting almost wholly of aluminum alllld a material to increase the tensile stren 7. stretched acoustic diaphragm of a material consisting almost wholly of aluminum and manganese.

8. An acoustic diaphragm tensioned to such a degree that the velocity of propagation of transverse vibrational waves therein is greater than 275 feet per second and having its natural fundamentalfrequency in air below 250 cycles per second. 7

9. A metallic acoustic diaphragm, the mass per unit area of whichis less than 5 X 10- lbs. per sq. in. and the area of which is greater than 45 sq. in., said diaphragm being tensioned to such a degree that the propagation velocity of transverse vibrations therein is greater than one-fourth the velocity of sound in air. L

10. An acoustic device comprisin a ,tensioned diaphragm and means secure to said diaphragm for drivingit along a line, the ratio of the tensioning of said diaphragm in lbs. per in. to the mass of said driving means per unit length along said line in lbs. per in.

ing eater than 8,500.- Q

11 acoustic device comprisin a tensioned diaphragm and means secure along a line on said diaphragm and within the boundary edge thereof for driving it. T

12.' An acoustic device com rising a ten sioned diaphragm and means ecured to said diaphragm for driving it along a line, said means having amass per unit length of said line less .than .002 lbs. .per inch.

r 13; An acoustic device comprising a diaphragm tensioned to such a degree that the propagation velocity of transverse vibrations therein is greater than 275 feet er second, and means secured to said diap ragm for driving it, said means having a mass per unit length less than .002 lbs. per inch.

14. An acoustic device comprisin a tensioned diaphragm and means secure to said diaphragm for driving it, the mass of said means being less than .011 lbs.

15. A large diaphragm for radiating sound consisting of sheet material having a mass per unit area less than 5x10 lbs. per square inch and tensioned to such a degree that the velocity of transverse vibrations therein is greater than 27 5 feet per second.

16. An acoustic diaphragm having a natural fundamental frequency of vibration in air below 250 cycles per second, consisting of a material which has an inherent stiffness such that for a piece of the diaphragm material one inch square rigidly supported at one edge the force a plied to theopposite edge required to pro uce a given deflection thereof is less than 0.3 lbs. per inch.

17. An acoustic device comprising a substantially circular diaphragm supported at its periphery and tensioned substantially uniformly in all directions and means secured to the diaphragm for driving it at aposition between its center and its periphery.

phragm of metal containing principally aluminum and having a thickness of less than .005 inch, means havin a temperature coefficient of expansion 0 the same order as that of said diaphragm material for supporting said diaphragm at its peri hery and maintaining it under tension, an means se- 18. An acoustic device comprising a diacured to said diaphragm at a position remote from its periphery for driving it. p

19. A sound radiator comprising a metallic diaphra m having a thicknessof the order of .002 in stretched tohave'a tension of the order 0 40 lbs. per inch, means for drivin said diaphragm, said means having a mass 0 approximately .003 lbs., and means for fastening said first mentioned means to said diaphragm at a position remote from its periphery. 4

20. In combination, a diaphragm, drivin 'means therefor, and means "associatedwit said diaphragm for dissipating reflected vibratory energy therein.

21. In combination, a diaphragm, driving means therefor, and means positioned between said driving means and the peri buy of said diaphragm for dissipating re ected vibratory energy therein.

22. In combination, a diaphragm, means for maintaining-said diaphra tions in said diaphragm and dampin means associated with -a-small area on said diaunder tension, actuatin means for setting up vlbra phragm for dissi ating reflected vibrational waves traveling t erein.

23. An acoustic device comprising a diaphragmof thin flat material and means for tensioning said diaphragm to a degree below the elastic limit and to an extent such as to allow vibrations to be propagated therein at a rate greater than 275 feet per second, said diaphragm being sufiiciently large so that tensioned it has a fundamental natural frequency below 250 cycles per second.

24. In a radio apparatus including a cabinet, a baflie-board mounted in said cabinet at an oblique angle with the walls thereof, and a sound board.

25. In combination, a sound producer, a cabinet adapted to provide a common housing for a radio receiving apparatus and said sound producer, a baflle-board for and associated with the sound producer, said baflle- .board being mounted in the cabinet and secured along two of its edges tothe walls thereof at an oblique angle to said walls whereby compartment resonance and the transmlssion of sound vibrations to such receiving apparatus are prevented.

In witness whereof, I hereunto subscribe my name this 10th day of May, A. D., 1927.

VESPER A. S'CHLENKER.

producer mounted on said bafile--

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