US2587684A - Directional microphone - Google Patents

Description

March 4, 1952 B a BAUER 2,587,684

DIRECTIONAL MICROPHONE Filed OCT.. 15, 1948 2 SHEETS--SHEET 1 B. B. BAUER DIRECTIONAL MICROPHONE March 4, 1952 2 SHEETS-SHEET 2 Filed Oct. l5, 1948 j; /qooo INVENTOR fLz/amzfz E .Bauen persecond Such crystals are Patented Mar. 4, 1,952

DIRECTIONAL MICROPHONE Benjamin B. Bauer, Oak Park, Ill., assigner to Shure Brothers, -Incorp corporation-of Illinois orated, Chicago, Ill., a

Applicationv Cctoher 13, 1948, SerialNo. 54,221

23 Claims. 1

This invention 'relates `to directional sound translating devices, more fparticularly to directional microphones `having displacement responsive -sensitive elements, and `it is an object ofthe invention vto provide improved Amicrophones of this character.

Directional microphones, that is, microphones which have lgreater -response for sound lreceived from one direction than 'from another and which utilize displacementresponsive elements such Vas piezoelectric crystals are well known. One 'form of such microphones wherein va single transducer or piezoelectric crystal is used and produces its output through sound Veiects from two points in space is described and claimed in the Patent No. 2,237,298 to Benjamin Baumzwieger, now by judicial change 'of name Benjamin B. Bauer, assigned to the same assignee as the present invention. -Microphones of the character described in the patent utilizing a single sound translating element, such as a diaphragm vwhich is acted upon by sound from two points in space, are essentially Vpressure difference type microphones and when the diaphragm thereof drives a displacement responsive element, "such as a piezoelectric'crystal, an output or response is obtained which increases with increasing frequency. 'This occurs because the pressure difference between the two points in 'space increases with frequency and `the dominant factor in controlling 'movements of the crystal is its stiffness.

'Crystalshave natural frequencies of vibration or free resonances at which the amplitude of vibration is large. In `prior constructions (such as described in thepatent referred to) the crystal dimensions have been chosen so that the natural frequency of vibration :of the crystal .and diaphragm is in the upper range of thefrequencies to vbe translated. Thus, for the normal frequency spectrum of about 100 cyclesiper second to 10,000 cycles per second, the natural frequency of vibration may be in the vicinity of 4,500 to 6,000cycles quite'stiff, thereby requiring a relativelylarge ldriving force per-unit output voltage with consequent relatively low output or sensitivity.

The rise in response at resonance (natural frequency) in the prior devicesis compensated for by providing a relatively small amount of damping such vaslafcloth placed incloseassociation with the diaphragm. Flat `or uniform response over substantially the whole -useful Afrequency range ydespite the rising response withfincreasing frequency is produced by vutilizing anelectrical compensating 'circuit or network which introy duces-attenuation'increasing in proportion .to vthe frequency increase. Whilesuch a circuit maintains nat response, yit .produces a Yloss in lover-.al1 output .and keeps the output at the low level determined by the circuit despite the higher out- :put available. At the higher frequencies the c'ircuit loses its eirectbut the :output vremains substantially the same due todiffraction effects.

The dampingorresistance used inconnection with crystals and diaphragms of the prior microphones .has .been small and .only prevented the rise in .output 4at resonance, Vhigher .damping `to lObtain iat lresponse vover a greater frequency range being avoided becauseof theiresultant low microphone output obtained and the diiculty of making physical-structureshaving the necessary acoustical properties.

Accordingly, V=it .is a further object Aof 'the invention to provide an improved microphone .of the character indicated having improved response yor output level.

It is a further object of .the invention to provide an improved v:microphone of the Vcharacter indicated wherein vflat or uniform response at .a higher output level is obtained vand the compensating network is eliminated.

Microphones of Vthe character described have been VlAmade directional through .the use 'in each `unit `of -a phase Vshifting ynetwork which4 applied ka force 4to one side 'of the fmicrophone diaphragm. While such a phase shift network has resistance ordamping associated therewith, it does` not have fany substantial effect in Acontrolling the ampli- .element of the phase shift network.

VIt is a `further object vo1" the invention to provide anlimproved microphone of the character described having high output level wherein the 'dominant elementin controlling the diaphragm movements is the .damping component of the Vphase shift network.

Accordingto one form of the invention,a directional microphone for a range of frequencies is providedcomprising, a diaphragm adapted to be .substantially exposed-on one side to sound waves in a medium, aphaseshifting networkiincluding resistance and .a cavity or compliance between the other side of the diaphragmfand sound waves in the medium, piezoelectric means external to the cavity for producing an output, lever means coupling the diaphragm to the piezoelectric means, the natural frequency of the system including the piezoelectric means, and the diaphragm being in the upper range of the frequencies to be translated, the lever means producing a natural frequency of the moving system including the piezoelectric means, the diaphragm and the lever approximately equal to the geometric mean frequency of the frequency range to be translated, and the resistance or damping of the phase shift network being approximately equal to the reactance of the moving system at tne lower range of frequencies to be translated.

For a better understanding of the invention, reference should be had to the accompanying drawings in which:

Figure 1 is a front elevational view of the outer casing of a microphone embodying the invention;

Fig. 2 is a sectional view taken substantially along lines 2-2 of Fig. 1;

Fig. 3 is a perspective view of the microphone unit of Fig. 2 on a larger scale embodying the inventon;

Y Fig. 4 is a rear elevational view partly broken away of the unit shown in Fig. 3;

Fig. 5 is a sectional View on a larger scale taken substantially in the direction of arrows 5-5 of Fig. 3;

Fig. 6 is a front elevational view of a modified microphone unit embodying the invention;

Fig. 'l is a sectional elevational view of a modified microphone unit embodying the invention;

Fig. 8 is a series of response curves for microphone units useful in explaining the invention;

Fig. 9 is a sectional elevational view of a modifled microphone unit embodying the invention, and

Fig. 10 is a rear elevational view partially broken away taken substantially in the direction ofY arrows Iii- I0 of Fig. 9.

Referring to the drawings, the invention is shown embodied in a microphone unit I2 held in a grillwork casing I3 which comprises front and rear grillV sections or halves I4 and I5', respectively, held together by screws for example, not shown, and a swivel I6 pivoted to the rear grill section. A stand (not shown) may be attached to the swivel and connecting leads may pass therethrough to the active part of the microphone unit. The casing and the grillwork may have any desired configuration, and attached to the inside thereof are relatively porous cloth pieces I1 and I8 to provide wind shielding.

Microphone unit I2 is held to casing I3 by means of a strap 2I and a screw 26, the strap being attached at its nrespective ends to transverse members 22 and 23 which have pieces of sponge rubber 24 cemented or otherwise attached at their ends and in turn attached to brackets of base 25.

The microphone unit comprises a base 25 including a rearwardly formed (such as by punching) portionv 21, a generally conical curved diaphragm 28, a piezoelectric crystal 3I cemented to the base, and a lever 32 connecting the crystal and the apex of the diaphragm, leads 33 and 34 extending from the crystal for bringing out the electrical output to terminals 35 and 36 (Fig. 4) respectively.

Diaphragm 28 is sealed around its edges to base member 25 thereby forming, together with depressed portion 21, a cavity 31, the diaphragm being made of thin material such as aluminum, stiff paper or the like. The edge of the diaphragm is made quite resilient or flexible so that it vibrates easily, but the diaphragm in general, including its apex, is quite rigid so that the diaphragm tends to vibrate as a whole and has sumcient rigidity to actuate (deflect) the crystal. The diaphragm rigidity, while the diaphragm is light, obtains mainly because of the curved conical form.

Extending through depressed portion 21 is a series of holes 38 whereby sound waves at the rear of the base 25'may enter intovcavity 31 and thus act on the rear side of the diaphragm. Cloth 4I or other sound permeable material is placed over each of the holes for a purpose to be described subsequently in this specification. place of cloth which may be, for example, silk with very ne interstices, other material having acoustic resistance and inertance such as slits, diaphragms and the like may be used.

Crystal 3I is of the torsion type wherein an output is produced by twisting thereof, one end of the crystal being relatively rigidly supported across its full width on a. block of material 62 cemented to the base and crystal. The other end of the crystal is relatively free so that torsional movements may take place, but this end is lightly supported on a second member- 43 which may be a resilient material introducing no substantial resistance to crystal movement. Torsion of the crystal is produced by lever 32 in transmitting movements of the diaphragm apex.

Lever 32 may be of any suitable shape and of any light material such as metal (magnesium, for example) or plastic, and is shown in the form of a truss to lend the necessary rigidity. One end of the lever is coupled to the diaphragm apex by any suitable means such as cementing and the other is attached to both sides of the crystal such as by having the crystal received in notches, as seen best in Fig. 5. Hence as diaphragm 2B vibrates back and forth the right end of the crystal is twisted back and forth, the twisting taking place about the center line of the crystal.

Crystal 3| may have a natural frequency when vibrating by itself above the highest frequency to be translated, but when coupled to a diaphragm, the natural frequency of the moving system including the crystal, diaphragm, and coupling structure may be in the upper range of sound frequencies to be translated. For example, in a well known microphone, such as described in the patent referred to, the natural frequency of the crystal alone has been in the vicinity of 15,000 to 18,000 cycles per second and the natural frequency of the crystal, diaphragm, etc. has been in the Vicinity of 4,500 to 6,000 cycles per second. When such a microphone operates on the principle of pressure difference, it has a relatively low output since the forces available for actuation are small.

In Fig. 8 there is drawn a curve 43 showing the response or output of a pressure difference type crystal microphone such as where the natural frequency is inthe vicinity of 4,500 cycles per second, the frequency being shown on a logarithmic scale and the response being given in decibels (db.) with reference to an arbitrary output level. It will be seen that the response rises uniformly as the frequency increases until in the vicinity of f2 where diffraction effects cause the pressure difference to become constant the response levels olf, the response rises sharply in the vicinity of f0=4,500 cycles per second due to the natural frequency, and the response falls olf rapidly above resonance. The frequency fz depends on the :ass-7,1684

transverse dimension v(outside diameter) of the unitfandin the vicinityof fav/here the transverse dimension `'is equalto vone-quarter wavelengthof sound the pressure tends to become constant. The sharp rise at resonance vmay be .prevented by a relatively smallamcunt .of damping or resistance and results in the response vcurve following the dotted portion 44. The curve 43, .4.4 is onesuch as might be obtained with the microphone shown in Fig. 6 of the rPatent No.v2,237,298 where the stiff crystal is coupled directly to the diaphragm. With such-a'microphone a ilatresponse may be obtaineolby using anV electrical-'attenuating circuit or network which begins functioning at some low frequency such as 100 -cycles per second. Usingsuch a circuit, a-response folu lowing curve 45 may 'be obtained which, whe

See Fig. 8 where fo is the natural frequency of the .moving system without the lever, and f1 is the natural frequency of the moving system with the lever. L1 and L2 represent the lengths of the lever arms, as may be seen from Fig. 5. It is a relatively simple matter to choose a lever ratio such that the natural frequency of the moving system is reduced from 4,500 cycles per second to 1,000 cycles per .second or from and to other values. The moving system includes in each unit the crystal, the diaphragm and the lever or other coupling structure.

Changing the resonant frequency by changing the stiffness reflected on to the diaphragm results in a response curve for an undamped crystal, diaphragm and coupling structure which follows curve 4l of Fig. 8. Utilizing a lever as shown produces a greater deilection of the crystal for the same pressure exerted on the diaphragm and thus the output or response is increased below resonance over the case Where no lever is used. The response also increases with increasing fren quency. At resonance there is a sharp increase in response, and above resonance the response falls at one rate since the pressure difference available is still increasing. This continues up vto f2 where diffraction effects cause the pressure `difference to become constant and the response falls of at a more rapid rate.

By providing the resistance material 4I having acoustic resistance equal to the reactance of the moving system at a low frequency, for example 100 cycles per second, the response curve following curve 48 may be obtained which is substantially fiat over the range from 100 cycles to 10,003 cycles per second, i. e; substantially the 'useful frequency range. The response or output'shown by curve 48 is considerably above that of 46 and is accomplished through damping alone and without an attenuating network. It has been found that an average increase in response cf db. or more over the whole frequency range may be obtained in this fashion:

It has been found that the preferred results are "obtained when the natural or resonant frequency of Athe moving system is placed approximately .at

the geometric mean .frequency of `the k*fret'ruency' range to be translated, the geometric mean :frequencybeing the square-rootofthe .productief the lowest and.highestlfrequenciesto be translated. Thus if the lowestand highest frequencies to be translated are respectively `and .10,000 cycles per second, the geometric .mean frequency .is equal to v\/l00 v10,000or 1,000 cycles per second. It has likewise been found that good results fare obtained to a `considerable range oneachside of the geometric mean frequency such as down to one-half vthereof and up to twice thereof. It is also to `vbe understood that the frequencies :of 100 and 10,000 chosen for the lower and yupper limits may be varied'o-ver considerable `range as desired.

In the-Patent No. 2,237,298, it-has been shown that a microphone havinga-diaphragmcoupled to a crystal wherein a phase shifting vnetwork transmits a force to the rear side of the diaphragm, acardioid directional response pattern is obtained when the volume of the cavity and the resistance and inertance comprising the network are-chosen so that where The phase shifting network comprises the cavity 3l and the resistance and inertance of cloth 4|, and couples the rear side of the diaphragm ato ithesound pressure waves existing at the Yperforations 38. The delay or phase shift introduced by the network combined with the force at the `front .surface of the diaphragm produces the directional pattern. When the phase shift lproduced bythe network is equal to that'experienced `bysound travelling .the effective distance from the front of the diaphragm Yto -the network inlet for .normalfront incidence, cardioid response is obtained.

In the instant application, referring to Fig. 5, d is the effective distance in centimeters from the front of diaphragm 23 to the openings 3B, .and is closely approximated by the diameter of the-diaphragm plus the thickness of case 25; Cv is as dened; C is the compliance of cavity 31; R, and Lare the resistance and inertance of ma- -terial 4| in units as specified. While material 4i fhasinertance and resistance, it is principally resstancefto produce kthe proper Yphase shift.

kWhen the constants are chosen in accordance with .the'formulas given, a cardioid directional response pattern is obtained in the subject invention similar to that of the patent.

In the structure of the ypatent the resistance of'thenetwork entered mainly into shifting the lphase of .the force available at the rear network, the diaphragm being damped at its natural frequency by resistance material placed at the front thereof. Furthermore, the crystal was mounted inside of the cavity making it advisable to have the cavity large. Accordingly, the product RC was chosen equal to but with R small and C large relatively. Then because of the rising response with increasing frequency, the attenuating network was used to obtain a fiat response over the frequency range.

It has been found that when the natural frequency of the moving system is approximately at the geometric mean frequency of the frequency range to be translated, the resistance R of the phase shift network may be so chosen relative to the volume of the cavity (which determines the compliance C) that the phase shift for directional operation and for damping the diaphragm at its natural frequency are obtained from the same element. That is, the resistance R of material 4l may be chosen to accomplish both damping and, in combination with the cavity and inertance of material Bl, phase shifting.

This in accomplished by choosing the product RC equal to choosing R in acoustic ohms equal to the reactance of the moving system at the low frequency to be translated and then choosing C to complete the relationship. The reactance of the moving system depends on its mass and stiffness and is relatively large at low frequencies. Hence the resistance component R is relatively large and in effect it becomes the dominant element in controlling the diaphragm movements. rIhat is, the moving system may be said to be resistance controlled. Consequently from the frequency at Which R equals the reactance of the moving system the response of the microphone is nat over the frequency range selected.

The quotient is readily found for any construction. Cv, the velocity of sound in centimeters per second, is known, and d in units consistent with Cv, for example centimeters, can 4be measured from the physical dimensions of the structure.

To obtain resistances and reactances of the proper values, various samples of these elements are tested until elements of the proper Values are found according to generally known formulas and procedures.

Acoustical impedance in ohms may be dened as the complex quotient of the alternating pressure applied to the system and the resulting volume current. See for example Elements of Acoustical Engineering, by Olson, page '73, second edition, published 1947 by D. Van Nostrand Company, Inc. rvlhe real part of this quotient is the resistance, and the imaginary part is the reactance. The definition of acoustical impedance is sometimes stated as a quotient of sound pressure and particle velocity. See for example an article entitled An acoustic transmission line for impedance measurement published in The Journal of the Acoustical Society of America, volume 11, July 1939, page 142.

In the case of openings such as slits or the interstices of cloth, the resistance R is a function of the dimensions of the openings or interstices. In slits which may have precise dimensions, the resistance may b fairly well calculated by the expression Bauer, entitled Conversion of Wave Motion into Electrical Energy and assigned to the same assignee as the present application. In this expression:

R is the acoustic resistance in ohms,

u is the viscosity coefficient of the medium,

Z is the passage length in centimeters in the direction of flow,

t is the passage thickness in centimeters, and

L is the peripheral length yof the passage.

See also The Journal of the Acoustical Society, July 1931, volume 3, page 49.

When the resistance is cloth, the same formula applies, but there is a practical difliculty in determining the dimensions of the cloth interstices. For this reason the resistance may be measured by forcing a stream of air, for example, through a sample of the material and measuring the pressure drop thereacross and the volume current therethrough. The resistance R in acoustic ohms may be given by the expression Where p is the pressure drop in dynes per square centimeter, and o is the flow in cubic centimeters per second.

belge i Where A is the area of sample in square centimeters, p 1s the pressure drop in dynes per square centimeters V 1s the volume velocity m cubic centimeters per second.

In the article referred t0 as published in volume 11 of The Journal of the Acoustical Society of America, the impedance of an element may be determined by utilizing the element to terminate an already calibrated tube and supplying the tube with sound pressure of a particular frequency. The impedance Z may be determined by the expression Pmax 2TI'L r3.1) HQWTH The real part of the formula is the resistance Where V and the imaginary part is .the reactance. The

method described may also-bensed.forfdetermining the impedance of cloth, butsince itis known thatcloth is primarily resistance, 4the method described in the O. S. R. D. publicationmay be used also.

Acoustical reactance of the mechanical components at anyvfrequency of the. crystal 3l, the lever 32 or other coupling structure, andthe diaphragm-28 may be obtained by constructing such a combination and testing it according to the procedure outlined in volume 11 of The Journal of the Acoustical Society. of America.

Acoustical reactance is afunction ofthestiffnessA or rigidity (or the. inverse thereof, compliance) of the diaphragm, crystal and coupling, the mass of these elements and the frequency of the sound. If the combined elements are very stiff or rigid, then little sound will be transmitted thereby and theV impedance is high. Likewise, if the mass of these elements vis great, little sound willbe transmitted thereby and theimpedance is high. Conversely, if the combined elements are very iiexible or compliant and of a very small mass, substantially all of the sound incident will be transmitted and the reactanceis low.

Experience has shown in general what dimensions of crystal to use in order to obtain sufficient electrical output and the physical dimensions and characteristics of a diaphragm to drive the crystal. It is not believed necessary to set out these criteria here. A particular selection-of crystal, diaphragm and coupling may be made and the impedance thereof determined according to the procedure set out inthe article in The C. and RC for other directional patterns is disclosed. It has equal application here and. thus is not given in detail.

Journal of the Acoustical Society of America,

volume 11. While the procedure there outlined will give both resistance and reactance, it is known that the resistance of vsuch amechanical system is very largely negligible compared to its reactance. The reactance. having been determined at the lower frequency toA be translated, a cloth having the requisite resistance may be selected having this same value..

The resonant or natural frequency-of the combined crystal, coupling and diaphragm may `be obtained by exposing the structure to sound of varying frequency and noting the frequency at which maximum vibration amplitude occurs.

In Fig. 8, curve 48 represents the case described, it having been assumed that the resistance is equal to the reactance at. about 10G cycles per second. Havingv chosen a resistance of this value, the resonant rise at 1,000 cycles per second is eliminated, the response is fiatover substantially the whole Ifrequency range, and the directional pattern is that of a cardioid up to where the transverse dimensions of the casing are equal to one-quarter wavelength of the frequency being translated. Thereafter diffraction effects maintain the directivity. Comparing curves 48 and d5, the output obtained by shifting the natural frequency of the system andv using the resistance of the phase shift circuit for damping is much higher and fiat response is obtained without the use of an attenuating network.

While in the case described. a cardioid directional pattern was assumed, it is understood that any other directional pattern from pressure response to bi-directionalu or cosine law response may be obtained by lchoosing the product RC in known fashion relativeA to and thereafter choosing R. to obtain the neces-V As seen best in Fig. 2, a second layer ofcloth. 5|, which may be silk having relatively fine interstices, may be placed across the front of the unit to improve the shielding out of windsound, etc. Thev support blocks 2li of sponge rubber or. the like prevent substantially the transmissionF of mechanical vibrations tothe unit.

Obtaining a moving systemhaving a natural frequency at the relatively low frequency of. the geometric mean. of. the frequency range to.. be translated may be .obtained by using avery thin or flexible crystal and applying stresses to it di.- rectly without. the useof a lever. In Fig. 6. there is shown a microphonev unit 52 having a crystal 53 of this character. In .other respects,.that is, the diaphragm 54, the base 55,.the cavity'and the cloth material (not shown), unit52 may be' substantially identical to unit i2. Crystal. 53 is thin and is placed under bending stress rather than torsion, as may also be seen from .the construction of Fig. 7 showing a modification havinga Isimilar crystal but differing in other respects; The crystal, being thin, has aV low natural frequency of vibrationin combination with the diaphragm, but isquite fragile compared to the stiffer crystals already. described..

The resistance component for damping the diaphragm asv well as for phaseshifting may be obtainedby sound permeable means otherA than cloth. In Fig. 7 there is shown a structure wherein the resistance components R is. obtained-,by the resistance to flow of air, the microphone unit havingr a case. 55, a diaphragm 51,. and a crystal 58.. The cavity 6| and.` diaphragm 51 maybe the same as. described for theother embodiments and crystal 58. may be the` same as described for the embodiment ofV Fig; 6'. The back of casing 5B' includes a series of openings or perforations. 60 between which` there are elements. of solid material 62.' Snaced rearwardly from.` elements 62-is a thin membrane (-534 held so spaced by, any suitable means suchv as by cement:- ing to a-circular ridge, as shown.

Membrane 63 is a thin iiexible member through whichsound is transmitted without significant decrease in pressure over the frequency range desired. and may be madeY of foils of metals, plasticV materials, rubber, and the like. As the membrane. moves. back and forth under sound waves,.air or other gasbetween the back surface ofmembers 52 andthefront of the membrane is forced. to. iiow in and` out. Due to the viscosity of suchv air or. gas, its movement introduces resistance andinertance as is under.- stood.. in this art. and provides the phase shift and damping..

In. this embodiment, the., cavity, .the resistance produced as described, together with. the equivalent inertance introduced by the. membrane, forniv the phase shift network.. The...membrane is` an inlet to. sound waves. since. with. proper choice: of.r characteristics. it. will, transmit. sound pressureY over. a. substantial range., of.. frequencies without any significant reduction in the pres'- sure magnitude.

v'tion 86 provided with a series of perforations 61. Closing the front end of depressed portion 66 is a diaphragm 68, and placed across the inside of perforations 61 is porous material 1I having acoustic resistance and inertance. Diaphragm 68 and depressed portion 66 dei-lne a cavity 12 which, together with the resistance and inertance of porous material 1l, provides a network for shifting the phase of pressure before it acts on the rear surface of the diaphragm in like manner to the embodiments already described.

Coupled to the apex of diaphragm 68 is one end of a lever 13 having its other end connected to a stationarily arranged crystal 14, also as already described. Spaced away from the front surface of diaphragm 68 is a perforated metal 'supporting member 15 having porous material 18 dening acoustic resistance and inertance attached to the inside surface thereof, the space between the porous material and the diaphragm defining a 'cavity 11.

Associated with the rear surface of depressed portion 66 is a shutter 18 shaped, for example, as shown in Fig. 10, the perforations B1 being arranged in a similar pattern. Shutter 18 is connected to a tube 8l which in turn is connected to a knob 82 by means of which shutter 18 may be rotated. A stud 83 is firmly attached to the back of base 65 to form a support for member 8l and a. spring 84 is arranged as shown to hold shutter 18 firmly against the back of the casing.

Assuming in one instance that the front support member 15 and acoustic material 18 are not present, the microphone unit is similar to that shown in Fig. except that the number of perforations forming an inlet into the microphone unit at the rear may be varied. This construction may be such that when all of the perforations are open, the microphone has a cardioid directional pattern, and when shutter 18 completely closesall of the perforations the microphone is pressure responsive. By vsuitably choosing the number and size of perforations 61, a

virtually continuous variation between these two patterns is obtainable.

When acoustic material 16 is being held adjacent the front surface of diaphragm B8, a second phase shifting network is added to the microphone unit, this phase shifting network including the cavity 11 and the resistance and inertance of material 18'. The constants of this network may be so chosen that they are equal to the constants of the network at the rear of the diaphragm whenever shutter 18 is completely open. Under these conditions the same phase vshift is produced by the two networks and a cosine law or bi-directional response microphone results. As the lshutter 18 isclosed, the phase shift Yproduced by the rear network increases 'thereby producing a different directional pattern until when shutter 18 is completely closed a pressure response microphone is had. This variation may be virtually continuous or in discreet steps, depending on the size and number ofthe perforations chosen.

In accordance with the teachings of this invention, the volume of cavities 12 and 11 and the resistance of porous materials 1| and 18 may be so chosen that the dominant factors in the control of the diaphragm movements are the resistance components. By utilizing lever 13 the natural frequency of the moving system may oe substantially at the geometric mean frequency of the frequency rangeV to be translated.

Assuming, as previously, that the geometric mean frequency of the frequency range is 1000 cycles per second and the lower frequency is cycles per second, the resistance of materials 1| and 16 may be chosen to be substantially equal to the reactance oi" the moving system at the lower frequency. Approximately one-half of the resistance so obtained may be placed in each of the resistance materials. The-reactance of the moving system being relatively high at this low frequency, the resistance values will be high and thus the volumes of cavities 12 and 11 may be made quite small. Consequently, the resistance of both materials becomes the dominating element in controlling the movements of the diaphragm, the rise in response at the resonant frequency of 1,000 cycles per second is eliminated, and a substantially fiat response over the whole frequency range is obtained.

When the shutter 18 completely closes all of perforations B1 and the microphone is pressure responsive, the natural frequency of the moving system is consideraly increased over that when the shutter is open. This follows from the fact that when shutter 18 is closed the air in cavity 12 acts as a greater stiffness (due to no escape through the perforations) which, in association with the diaphragm, the crystal and the lever, results in the moving system having a frequency approximately 4,000 to 6,000 cycles per second, whereas when shutter 18 is open the natural frequency of the system may be 1,000 cycles per second. When the microphone is operating as a pressure microphone, the resistance of material 16 acts to damp out the rise in response at resonance and tends to produce a flat response over the frequency range, the response being about at the same level as that for the bi-directional case. It has been found that the same general level of output is obtained with other directional patterns, that is, for positions of shutter 18 between fully open and fully closed.

Throughout thisv specication and claims the following definitions and concepts obtain.

Where constants of the microphone are referred to, and relationships between the constants governing the operation are given, it is understood that the values thereof are in a consistent system of units, such for example as acoustic units.

A displacement responsive element is one producing an output proportional to its displace ment or deflection from a normal or equilibrium position. Thus the voltage of crystal which is one form of displacement responsive element is proportional to its deformation.

Where phase shift networks and the transmission o sound pressures are referred to, it will be understood that the pressure does not change significantly in magnitude.

An acoustic phase shift network is one which shifts the phase of acoustic pressure without regard to the character of the elements producing the phase shift. A network utilizing acoustic components is one where the components are gaseous elements such as cavities and physical passages.

While particular embodiments of the inventionhavefbeen shown, it will'be understood, ci course, that the` invention is not limited thereto since manyVV modifications may be made, and it is, therefore, contemplated by the appended claims to cover'any such modifications as fall within theitrue spirit and scope of the invention.

Having thus described the invention, what is claimedv and desired to be secured by Letters Patent is:

l. A directional sound translating device for a range of frequencies comprising, a translating unitadapted to-be exposed to sound pressure along one portion thereof for producing an output proportional to displacement, a sound pressure phase shifting network including resistance and compliance forcoupling another portion of said translating unit to sound pressure, said translating unit having a natural frequency of approximately the geometric means frequency of the'V range to be translated, and the resistance of said network being approximately equal to-the reactance of said translating unit atthe lower end of the frequency range to be translated.

2. A directional sound translating device for a range-of frequencies comprising, a translating elementadapted' to be substantially exposed to sound Waves: in a medium on one side thereof, a sound pressure phase shifting network including resistance and compliance-between the other side of said translating element and sound waves in said medium, transducing means producing an output proportional to displacement thereof, means coupling said transducing means to said translating element, said transducing means, said translating element and the associated coupling structure having a natural frequency approximately equal to the geometric mean frequency of the frequency range to betranslated, and the re sistance of said network being approximately equal tothe reactance of the translating element, thel transducing means coupled thereto, and the associated coupling'structure at'the lower end of the frequency range to be translated, thereby to effect substantially constant output from said transducing means over said frequency irange.

3. A directional sound translating device for a rangev of frequencies comprising, a diaphragm adapted to be substantially exposed to sound waves in a medium on one side thereof, a sound pressure phase shifting network including resistance and compliance between the other side of said diaphragm and sound waves in said medium, transducingmeans producing an output proportional to displacement thereof, means coupling f said transducing means to said diaphragm, said transducing means, said diaphragm and the associated coupling structure having a natural frequency approximately equal' to the geometric meanl frequency of the frequency range to be translated, the resistance of said network being approximately equal to the reactance of the diaphragln, the transducer coupled thereto, and the associated coupling structure at thelower endof the frequency range to be translated.

Ll. A directional sound translating device for a range of frequencies comprising, a diaphragm adapted to bev substantially exposed to sound waves in a medium on one side thereof, a sound pressure phase shifting network having acoustic compliance and acoustic resistance between the otherfside ofsaid diaphragm and sound waves in said medium, transducingmeans producing an output.4 proportional; to displacement. thereof, means couplingv said.l transducing means toi said coupled thereto, and the associated coupling structure at theV lower end of the frequency range to betranslated, thereby to effect substantially constant output from said transducing means over said frequency range.

5. A. directional sound translating device for a range of frequencies comprising, a diaphragm adapted to be substantially exposed to sound waves in a medium on lone side thereof, a sound pressure phase'shifting network having acoustic compliance and acoustic resistance between the other side of said diaphragm and sound waves in said medium, piezoelectric means for producing an output, means for coupling said piezoelectric means to said diaphragm, said piezoelectric means', said diaphragm and the associated coupling structure having a natural frequency approximately equal to the geometric mean frequency of the frequency range to be translated, and the resistance of said network being approximately equal to the reactance of said piezoelectric means, said diaphragm and said associated coupling structure at the lower end of the frequency range to be translated.

6. A directional sound translating device for a range of frequencies comprising, a diaphragm including an apex adapted to be substantially exposed to sound waves in a medium on one side thereof, a sound pressure phase shifting network including acousticV resistance and acoustic compliance between the other side of said diaphragm and sound waves in said medium, transducing means for producing an output proportional to the displacement thereof, means for connecting said transducing means substantially directly to the apex of said diaphragm, said transducing means, said connecting means and said diaphragm havingA a natural frequency approximately equal to the geometric mean frequency of the frequencyrange to be translated, and the resistance of said network being approximately equal to the' reactance of the transducing means and the diaphragm at the lower end of the frequency range to be translated.

'7. A directional sound translating device for a range of frequencies comprising, a diaphragm including an apex adapted to be substantially eX- posed to sound waves in a medium on one side thereof, a. sound pressure phase shifting network including resistance and compliance between the other side of said diaphragmand soundv waves in saidmedium, transducing means for producing an output proportional to the displacement thereof,

said transducing means and diaphragm having a natural frequency within the higher range of frequencies to be translated, lever means coupling the apexof said diaphragm to said transducing means, said lever means producing a natural frequency of rthemoving system including said transducing means, said diaphragm and said lever means approximately equal to the geometric mean frequency of the frequency range to be translated, and the resistance of said network being approximately equal to the reactance of said moving syse tem atthe lower end. of the frequency range to be translated.

andirection'allsound translating device; for a range/of. frequencies .comprisinga diaphragm inl cluding an apex adapted to be substantially exposed to sound waves in a medium on one side thereof, a sound pressure phase shifting network including acoustic resistance and acoustic compliance between the other side of said diaphragm and sound waves in said medium, transducing means for producing an output proportional to the displacement thereof, said transducing means and diaphragm having a natural frequency within the upper range of frequencies to be translated, lever means coupling the apex of said diaphragm to 'said transducing means, said lever means producing a natural frequency of the moving system including said transducing means, said diaphragm and said lever means approximately equal to the geometric mean frequency of the frequency range to be translated, and the resistance of said network beingr approximately equal to the reactance of said moving system at the lower end of the frequency range to be trans- L lated.

9. A directional sound translating device for a range of frequencies comprising, a diaphragm including an apex adapted to be substantially exposed to sound waves in a medium on one side thereof, a sound pressure phase shifting network including acoustic resistance and acoustic cornpliance between the other side of said diaphragm and sound waves in said medium, piezoelectric means for producing an output, said piezoelectric means and diaphragm having a natural frequency within the upper range of frequencies to be translated, lever means coupling the apex of said diaphragm to said piezoelectric means, said lever means producing a natural frequency of the moving system including said piezoelectric means, said diaphragm and said lever means approximately equal to the geometric mean frequency of the frequency range to be translated, and the resistance of said network being approximately equal to the reactance of said moving system at the lower end of the frequency range to be translated.

10. A directional sound translating device for a -range of frequencies comprising, a diaphragm including an apex adapted to be substantially exposed to sound waves in a medium on one side thereof, a sound pressure phase shifting network including acoustic resistance and acoustic' compliance between the other side of said diaphragm and sound waves in said medium, piezoelectric Vmeans for producing an output, said piezoelectric means having a natural frequency within the higher range of frequencies to be translated, lever means coupling the apex of said diaphragm to said piezoelectric means, said lever means producing a natural frequency of the moving system including said piezoelectric means, said diaphragm and said lever means approximately between one half of and twice the geometric mean frequency of the frequency range to be translated, and the resistance of said network being approximately equal to the reactance of said moving system at the lower end of the frequency range to be translated.

.11. A directional sound translating device for a range of frequencies comprising, a diaphragm including an apex adapted to be substantially exposed to sound waves in a medium on one side thereof, structure at the other side of said diaphragm forming a cavity, a sound inlet to said cavity, acoustic resistance means associated with said inlet, said cavity and said resistance means comprising a phase shift network for sound pressure.,transducing means for producing an output having a natural frequency within the upper frequency range to be translated, lever means coupling the apex of said diaphragm to said transducingr means, said lever means producing a natural frequency of the moving system including said transducing means, said diaphragm and said lever means approximately equal to the geometric mean frequency of the frequency range to be translated, and the resistance of said resistance means being approximately equal to the reactance of the moving system at the lower end of the frequency range to be translated.

l2. A directional sound translating device for a range of frequencies comprising, a diaphragm including an apex adapted to be substantially eX- posed to sound waves in a medium on one side thereof, structure at the other side of said diaphragm forming a cavity, a sound inlet to said cavity, acoustic resistance means associated with said inlet, said cavity and said resistance means comprising a phase shift network for sound pressure, transducing means for producing an output having a natural frequency within the upper frequency range to be translated, lever means coupling the apex of said diaphragm to said transducing means, said lever means producing a natural frequency of the moving system including said transducing means, said diaphragm and said lever means approximately between one half of and twice the geometric mean frequency of the frequency range to bue translated, and the resistance of said resistance means being approximately equal to the reactance of said diaphragm, said transducing means, and said lever means at the lower end of the frequency range to be translated thereby to effect substantially constant output from saidtransducing means over said frequency range.

13. A directional sound translating device for a range of frequencies comprising, a diaphragm including an apex adapted to be substantially exposed to sound waves in a medium on one side thereof, structure at the other side of said diaphragm forming a cavity, a sound inlet to said cavity, acoustic resistance means associated with said inlet, said cavity and said resistance means comprising a phase shift network for Sound pressure, transducing means external to said cavity for producing an output having a natural frequency within the upper frequency range to be translated, lever means coupling the apex of said diaphragm to said transducing means, said lever means producing a natural frequency of the moving system including said transducing means, said diaphragm and said lever means approximately equal to the geometric mean frequency of the frequency range to be translated, and the resistance of said resistance means being approximately equal to the reactance of said diaphragm, said transducing means, and said lever means at the lower end of the frequency range to be translated thereby to effect substantially constant output from said transducing means over said frequency range.

14. A directional sound translating device for a range of frequencies comprising, a diaphragm including an apex adapted to be substantially exposed to sound waves in a medium on one side thereof, structure at the other side of said diaphragm forming a cavity, a sound inlet to said cavity, acoustic resistance means associated with said inlet, said cavity and said resistance means comprising a phase shift network for sound pressure, piezoelectric means external to said cavity for producing an output having a natural fretranslated, lever means coupling the vapex of said diaphragm to said piezoelectric means, said lever means producing a natural frequency of the moving system including said piezoelectric means., 'said diaphragm and said lever means approximately vequal to the geometric mean frequency of the frequency range to be translated, and the resistance of said resistance means being approximatelyv equal to the reactance vof said diaphragm, said piezoelectric means, and said lever means at the lower end of the frequency range to be translated thereby to effect substantially constant output from said piezoelectric means over said frequency range.

A directional sound translating device for a range of frequencies comprising, a diaphragm including an apex adapted to be substantially exposed to soundwaves in a medium on one side thereof, structure at the other side of said diaphragm forming a cavity, a sound inlet to said cavity, acoustic resistance means Aassociated with said inlet, said cavity and said resistance means comprising a phase shift network for sound pressure, piezoelectric means for producing an output having a natural frequency within the upper frequency range to be translated, lever means coupling the apex of said diaphragm to said piezoelectric means, said lever means producing a natural frequency of the moving system including said piezoelectric means, said diaphragm and said lever means approximately between one half of and twice the geometric mean frequency ofthe frequency range to be translated, and the resistance vof said resistance means being approximately equal to the reactance of said diaphragm, saidv piezoelectric means, and said lever means. at the lower'end of the frequency range to be translated thereby to effect substantially constant outputfrom said piezoelectric means over said frequency range.

16. A directional sound translating device for a range of frequencies comprising, a diaphragm including an apex adapted to be substantially exposed to sound waves in a medium on one side thereof, structure at the other side of said diaphragm forming a cavity, sound permeable means forming an inlet to said cavity and defining acoustic resistance and inertance, the compliance of said cavity and the resistance and inertance of said permeable means forming a phase shifting network for sound pressure, piezoelectric means external to said cavity for producing an output, said piezoelectric means having a natural frequency within the upper frequency range to be translated, lever means coupling the apex of said diaphragm to said piezoelectric means, said lever means producing a natural frequency of the moving system including said piezoelectric means, said diaphragm and said lever means approximately equal to the geometric mean frequency of the frequency range to be translated, and the re -sistance of said sound permeable means being approximately equal to the reactance of said diaphragm, said piezoelectric means, and said lever' means at the lower end of the frequency range to be translated thereby to eiect substantially constant output from said piezoelectric means over said frequency range.

17. A directional sound translating device for a range of frequencies comprising, a diaphragm including an apex adapted to be substantially exposed to sound waves in a medium on one side thereof, structure at the other side of said diaphragm forming a cavity, passage means into 18 said cavity, porous material defining principally acoustic resistance covering said passage means, piezoelectric means external to said cavity for producing an output, said piezoelectric means having a natural frequency within the upper frequency range to be translated, lever means coupling the apex of said diaphragm to said piezoelectric means, said -lever means producing a natural frequency of the moving system including said piezoelectric means, said diaphragm and said lever means approximately equal to the 'geometric Vmean frequency of the frequency range to `be translated, and the' resistance of said porous material being approximately equal to the reactance of said diaphragm, said piezoelectric means, and said lever means at the lower end of the frequency range to be translated thereby to effect substantially constant output from said piez^electric noeans over said frequency range.

18. A directional sound translating device for a'range of frequencies comprising, a diaphragm including an apex adapted to be substantially exposed to sound waves in a medium on one side thereof, structure at the other side of said diaphragm forming a cavity, sound permeable means forming an inlet to said cavity and defining acoustic resistance and inertance, the compliance ofsaid cavity and the resistance and inertance of said permeable means forming a phase shifting network for sound pressure, the resistance and inertance of said sound permeable means and the compliance of said cavity being chosen so that the phase shift produced'thereby is substantially equal to the phase change produced in sound pressurebetween said one side of said diaphragm and said inlet for normal front sound incidence, piezoelectric means external to said cavity for producing an output, levermeans coupling the apex ofv said diaphragm to said piezoelectric means, said lever means producing a natural frequency ofthe moving system including said piezoelectric means, said diaphragm and said lever means approximately equal to the geometric mean frequency of the frequency range to be translated, and the resistance of said sound permeable means further being approximately equal to the reactance of said moving system at the lower end of the frequency range to be translated thereby to eifect substantially constant output from said piezoelectric means over said frequency range.

19. A directional sound translating device for a range of frequencies comprising, a diaphragm including an apex adapted to be substantially exposed to sound waves in a medium on one side thereof, structure at the other side of said diaphragm forming a cavity, sound permeable means forming an inlet to said cavity and dening acoustic resistance and inertance, the compliance of said cavity and the resistance and inertance of said permeable means forming a phase shifting network for sound pressure, piezoelectric means external to said cavity for producing an output, said piezoelectric means having a natural frequency within the upper range to be translated, lever means coupling the apex of said diaphragm to said piezoelectric means, said lever means producing a natural frequency of the moving system including said piezoelectric means, said diaphragm and said lever means approximately equal to the geometric mean frequency ofthe frequency range to be translated, means for varying said sound permeable means thereby to eiect different output directional patterns, and the resistance of said sound permeable means beadsense I9 ing approximatelyequal to the-reactance of sai diaphragm, said piezoelectric means, and said lever vmeans at the lower endof4 the frequency range to be translated thereby to effect substantially constant output from "said piezoelectric means over said frequency range. f

20. A directional sound translating device for a range of frequencies comprising, a diaphragm including an apex adapted to be substantially exposed to sound waves in a medium on one side thereof, structure at the other side of said diaphragm forming acavity, a thin diaphragm forming sound inlet to said cavity,` a perforated plate closely spaced to said thin diaphragm, said thin diaphragm having equivalent inertance and air movement between said thin diaphragm and perforated plate dening acoustic resistance, the compliance of said cavity and said resistance and inertance forming a phase shifting network for sound pressure, piezoelectric means external to said cavity for producing an output, said piezo.- electric means having a natural frequency within the upper range to be translated, lever means cou;

pling the apex of saiddiaphragm to said piezoelectric means, said lever means producing a natural frequency of themoving systeml including said piezoelectric means, said diaphragm and said lever means approximately equal to the geometric mean frequency of the frequency range vto be translated, and said resistance being approximately equal to the reactance of said diaphragm, said piezoelectric means, and said lever means at the lower end of the frequency range to be translated thereby to effect substantially constant output from said piezoelectric means over said frequencyrange. .q

21. A directional soundtranslating device -fora. range of frequenciescomprising, a diaphragm including an apex, first porous material having acoustic resistance and inertance spaced from one side of said diaphragm, second` porousmaterial having acousticresistance and inertance spaced from the other side of said diaphragm, saidfirst and second porous materials defining first and second cavities and with said respective .porous materials defining first and secondv sound pressure phase shifting networks having approxi;

means external to said cavities for producing an output, said piezoelectric means having av natural frequency within the upper frequency rangev to be translated, lever-means coupling the apex of said diaphragm'to said piezoelectric means, said lever means producing a natural frequency of the moving system including said piezoelectric means, said diaphragm and said lever means approximately equal to'the geometric mean frequency of the frequency range to be translated, the total resistance of both said porous materials being approximately equal to the reactance of said moving system at the lower end of the frequency 'range to be translated, and means for varying said second network from phase shift equality with 'said first network to closing oi the other side of said diaphragm. l 22. The invention as defined in claim 21 wherein thephase shift of said second network may be varied substantially continuously.

23. The invention as defined in claim 21 wherein the phase shift of said second network may be variedin discrete steps.l

, BENJAMIN B. BAUER.

, REFERENCES CITED The'follo'wing references are of record in the file of this patent:

UNITED STATES PATENTS Great Britain Nov. 1'1, 1947`

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