US2401328A

US2401328A - Sound translating device

Info

Publication number: US2401328A
Application number: US472575A
Authority: US
Inventors: Jr Robert Black
Original assignee: Bell Telephone Laboratories Inc
Current assignee: AT&T Corp
Priority date: 1943-01-16
Filing date: 1943-01-16
Publication date: 1946-06-04
Anticipated expiration: 1963-06-04
Also published as: BE467509A; FR939369A; NL72895C; GB574611A

Description

June 4, 1946. R. BLACK, JR V 1,

v SOUND TRANSLA'IING DEVICE Filed Jan. 16, 1943 2 Sheets-Sheet 1 FIG.

IN [/5 N TOR RBLACK JR. BY

WM 5. 7M

' A T TORNEY RESPONSE IN D 8 June 4, 1946. R. BLACK, JR 2,401,328

SOUND TRANSLATING DEVICE 'Filed Jan. 16, 1945 2 Sheets-Sheet 2 7 W Z Z H M R M R M R o L 2 2 4 Z 4 is, z,

- 0 i fl o FIGS 3 A III 2 E 45 I x l 1 1 l l l l I;

456789! 2 a 4567B9l FREQUENCY IN CYCLES PER SECOND -IIS i 2 a 4 5 e '7 a 9 2 a 10o I000 |o.o0o

FREQUENCY m CYCLES PER SECOND INVENTOR R. BLACKJR.

A TTORNE Y Patented June 4 1946 ics SOUND TRANSLATING DEVICE Robert Black, Jr., South Orange, N. 3., assiznor to Bell Telephone Laboratories, incorporated, New York, N. Y., a corporation oi New Yorlz Application January 16,, 1943, Serial No. 472,575

18 Claims. 1

This invention relates to sound translating devices and more particularly to microphones having a marked directional characteristic.

One object of this invention is to. assure sub stantially uniform discrimination by a directional microphone throughout a wide frequency range, for ex mple throughout the range oi frequencies from iout 50 to 10,000 cycles per second.

Another object of this invention is to obtain a substantially uniform response throughout a wide rant of frequencies for a directional microe phone.

A further object of this invention is to obtain a high amplitude directional discrimination icy a single unit microphone.

Still another object of this invention is to simplify the construction of a unidirectional mioro pnone.

In one illustrative embodiment oi this inven tion, a microphone comprises a single sound translating unit, for example of the moving coil type, mounted within a casing, both surfaces oi the diaphragm oi the unit being in communion tion with the atmosphere, one substantially iiirectly and the other by was of an acoustic net work defined in part by a chamber hounded the casing and opening" into the atinosniiere.

in accordance with one feature of this invention, the communication between the chamber and the atmosphere is established through mnltiplicityoi restricted apertures arranged in groups spaced difierent distances from the die nhragm to provide a plurality oi sound paths of different lengths betweenthe two surfaces of the diaphragm, the enertures having acoustic resistance material extending thereacr ss. This enables realisation of both directional discrimi nation throughout a wide frecuencyrange and substantially uniform response throughout this range. This construction enables also attaii'iment of a terminating impedance for the acoustic network associated with the diaphragm, which is composed essentially of only a resistance and mass and havins sufficient resistance to assure a large magnitude of directional discrimination.

In accordance with a further feature of this invention. the resistance and mass components of the terminating impedance are related in such manner that the directional discrimination due to the phase shifting effect of the acoustic network is substantially uniform throughout the range of frequencies below that at which the discrimination due to difiraction effects of the easing becomes substantial.

In accordance with another feature of this Eli a once vvitl'i-cne feature oi invention, an acoustic impedance is associated with the diaphragm, this impedance being predominantly mass in character at low frequencies and predominantly resistive in character at high frequencies whereby the response at the low ire quency end of the range to be translated is materially enhanced without substantial reduction in discrimination at low frequencies. and substantially uniform response throughout a wide band of frequencies is obtained.

The invention and the'aliove-noted and other features thereoi will he understood more clearly and fully from the following detailed description with reference to the accompanying drawings in which:

Fig. l is a perspective view of a sound translatins device illustrative of one embodiment of this invention;

Fig. 2 is a side view mainly in section of the device shown in El is 1;

3 is a perspective exploded view of the transmitter unit included in the device shown in i, a portion of the unit losing: broken away to illustrate details of construction more clearly;

Fig. is circuit diagram illustrating else analog oi the acoustic oi the device shown in l 2i;

5c is graph iliustratins the enhancement oi the low frequency response Gl steel in accord sic invention: and

Fig. s is a graph illustrat in, the .tesnonse and the directional character of the device shown in Referring now to the draw illustrated in Fig tially ovate casin which may he composed of two nor s itfor example of a "phenol condensation product and metal respectively, telescopically together, the casing being fitted in an external hand it? secures. to a suitable coupler member it for mountain; the microphone on a support. The inner wall of the casing portion ill is formed with an annular seat is in whlch'a microphone unit it is fitted. the microphone unit being locltecl in place by a resilient strip it bearing thereagainst and sprung into locking engagement with projections H on a metallic insert it in the casing portion iii. The base is of the casing portion I0 is apertured to provide direct communication between the atmosphere and the diaphragm of the microphone unit. The casing portion II also is provided with apertures 29 covered with acoustic silk 8%, the arrangement and function of which will be described hereinafter.

s, microphone suhstan The microphone unit I! may be of any of the well-known typ s or forms. In one particularly satisfactory structure it is of the moving coil or electrodynamic type and of the construction illustrated in Fig. 3. As shown in this figure, the microphone unit comprises a one piece permanent'magnet having a central pole 2i and an outer pole 22 comprising four arcuate arms mounted in cylindrical array and encompassing and coaxial with the central pole, the two poles being of opposite polarity. Seated upon the poles 2i and 22 and held thereto by magnetic attraction is a unitary structure including concentric pole-pieces 23 and 24 defining an annular gap 25, and a non-magnetic ring 26 provided with a plurality of restricted apertures 21, all but one of the apertures being covered by acoustic resistance material 34, such as silk. The inner polepiece 23, which abuts the central pole 2|, is provided with a domed surface 28.

A diaphragm is mounted on the magnet structure and comprises a central, bodily vibratile dome portion 29 conforming substantially to the domed surface 28 of the inner pole-piece 21, a marginal portion 30 secured to the outer polepiece 24, and an intermediate, flexible tangentially corrugated portion 3|. Secured to the diaphragm adjacent the periphery of the dome portion 29 thereof is an annular signal coil 32 which is positioned in the gap 25 between the pole-pieces 23 and 24. Vibration of the diaphragm in response to sound waves impinging thereon results in the generation of a corresponding electrometive force in the coil 32.

A tube 35, for example of rubber, the function of which will be pointed out hereinafter, is secured to the non-magnetic ring 25 in alignment with the one of the apertures 21 which does not have the acoustic resistance material 34 extending thereacross.

The device illustrated in Figs. 1 to 3 constitutes an acoustic transducer which may be represented by the multimesh, electrical analog r1etwork shown in Fig. 4. In this figure, Z is the diaphragm impedance composed of the mass Mo, resistance R0 and stiffness So, Z1 is the impedance, substantially entirely stiffness, of the chamber between the diaphragm and the pole-pieces 23 and 24, Z2 is the impedance of the passage defined by the gap 25, which impedance is composed of a mass M: and a resistance R2, Z3 is the impedance, substantially entirely stiffness, of the chamber adjacent the diaphra m side of the rin 26, Z4 is the impedance, composed of a mass M4 and resistance R4. of the apertures 21 and the acoustic silk 34, Z5 is the impedance, substantially entirely stiffness, of the chamber bounded by the housing or casing I0, I I, and Z6 is the impedance of the apertures 20 and the acoustic silk 3! thereacross. The tube 35 constitutes an impedance Z1, having a mass M1 and resistance R1, in shunt with the impedance Z4.

The diaphragm of the microphone unit, it will be noted, has both surfaces thereof in communication with the atmosphere, the front surface being exposed directly to the atmosphere through the apertured base H of the casing portion l0 and the rear surface being in communication with the atmosphere through a three-section network terminating in the impedance Zs of the silk covered apertures 20. Hence, the diaphragm is acted upon by two forces, F1 and F2 due to sound waves arriving at the microphone, the force F2, in general, being in opposition to the force F1. The magnitude of the resultant force (Fl-F2) is dependent upon the angle of incidence of the sound waves upon the microphone, as is apparent, and is dependent also upon the length of the air path between the two surfaces of the diaphragm, the length of the path being determinative in part of the phase relation between the two forces F1 and F2. Thus, the response of the microphone is dependent upon the angle of incidence of the sound waves and the device has a directional characteristic.

In a device of the character disclosed, directional discrimination is due to two effects, namely, the phase shifting effect of the network behind the diaphragm and the diffraction effect of the casing. In general, the former effect predominates throughout the lower portion, for example, up to of the order of 3,000 or 4,000 cycles, of the frequency range to be translated and the latter effect is predominant at the higher frequencies. The lowest frequency at which the diffraction effect becomes of substantial magnitude, for example of the order of seven or eight decibels, can be determined in ways known in the art from the dimensions and contour of the casing.

The phase shifting effect of the network behind the diaphragm is dependent in the main upon the character of the impedance Z6, the portions of the network in Fig. 4 to the left of the impedance Z5 being essentially only response determining impedances. The impedance Z6, it will be apparent, has both a resistive and a reactive component, the reactive component being essentially a mass. It ha been found that an optimum relationship exists between these two components for the realization of large amplitude directional discrimination, substantially uniform throughout a wide frequency range, 1. e. up to the frequency at which the diffraction effect becomes of substantial magnitude as noted l1e reinabove.

Both the resistive and reactive components of the impedance Z6 vary with frequency, the resistive component, in general, decreasing with frequency from a maximum at zero cycles per second and the reactive component increasing with frequency from a minimum, substantially zero at zero cycles per second, throughout the major portion of the range of frequencies in which the phase shifting effect predominates.

The impedance Zs can be expressed in the form a-l-y'b where a and b are the resistive and reactive components respectively, the reactive component being essentially a mass as pointed out heretofore. It can be shown readily from an analysis of the network illustrated in Fig. 4 that maximum discrimination between incidence angles of 0 degrees and degrees obtains when 8 a sin 0) and ' quencies.

substantially the middle frequency in this band. Thus, for example. if the band extends from zero to of the order of 3,000 cycles per second, the quantities a and b as given by Equations in and 1b should be matched at substantially 1,500 cycles.

At low frequencies in this band, the reactive component b is small as is apparent from Equation lb, and the quantity a is substantially equal to as is apparent from Equation in, bearing in mind that for small angles, the angle is substantially equal to the sine thereof. The electromotive force generated in the coil 32. which force is a measure Fl(ZQ cos 0) where Z is a function of the mechanical impedance of the device.

2 As pointed out above, at the low frequencies, the

quantity b is substantially zero and the impedance Z6, therefore, is essentially resistive and substantially equal to Hence, the numerator of Equation 2 can be writ-= ten in the form C g from which it is seen that the directive pattern of i the device is cardioidal in form at the low fre- It has been found that this pattern is cardioidal also at the higher frequencies. From what has been said above, it will be seen that the range of frequencies throughout which directional discrimination, in the manner indi cated is obtainable is dependent upon the path difference D. This in turn is dependent upon the distance between the apertures in the easing and the diaphragm. In general, the greater this distance the narrower is the frequency range throughout which the discrimination is realized,

and the upper frequency of this band is that for m which the wave-length is comparable with the path difference D. On the other hand, the magnitude of the discrimination is dependent upon the magnitude of the impedance Z6. Furthermore, the magnitude of the response at the lower frequencies has been found to be dependent upon the path length D and, in general, the greater this length the greater is the response at the lower frequencies.

Thus, it will be seen that the attainment of directional discrimination has two aspects. namely frequency range and magnitude, and that a common factor involved in these two aspects is the path length D. It will be appreciated further that this factor enters into the determination of the magnitude and the frequency range of directional discrimination in different. and in a sense contradictory, ways in that, generally speaking, a wide frequency range dictates the attainment of a short path length whereas high magnitude dictates the use of a long path length. In devices constructed in accordance with a feature of this invention both the desired wide frequency range and high magnitude of directional discrimination are realized. ll

(l-l-cos 0) 35 once Ze and the'impedances Z4 and Z1.

As shown in Figs. 1 to 3, the housing p rtion I i is provided with a multiplicity of silk covered apertures 20. These apertures are arranged in a number. for example four, circular rows, the apertures in the rows being equally or unequally spaced, the rows being equally spaced, and the apertures in adjacent rows being in staggered relation. There are thus provided a plurality of paths of different lengths between the two surfaces of the diaphragm.

For example, the row of apertures farthest from the diaphragm may be spaced therefrom of ffthe order of 10 centimeters and the row of apertures 20 nearest the diaphragm may be spaced therefrom a distance of the order of 6 centimeters. These distances correspond re- .spectively to half wave-lengths of approximately ertures together constitute the impedance Ze.

Furthermore, it has been found that the arrangement of apertures in the device illustrated in Figs. 1 to 3 constructed to provide the resistance and mass relation for Zn as described heretoi'ore results in substantially uniform directional discrimination throughout the frequency range up to about 3,000 cycles per second. This is shown clearly in Fig. 6 where curve 0 indicates the response of a device. of the construction at shown in Figs. 1 to 3 for zero degree incidence angle and curve D indicates the response of the same device for incidence angle of degrees.

It may be noted that the construction of the housing member ii and the arrangement of the apertures 2b therein facilitates the attainment of an impedance Z6 composed essentially of re-' sistance and mass. The resistance of an aperturecovered with acoustic silk is a function of the area of the aperture.

impinging thereon and thus act as an impedance having a material stiffness component as well as mass and resistance. The use of a large number of small apertures 20 assures not only the attainment of the requisite resistance but also the provision of an impedance 26 which is essentially without a stiffness component.

Above about 3,000 cycles, the magnitude of the directional discrimination due to the network behind the diaphragm decreases markedly. However, above this frequency directional discrimination due to diffraction effects becomes pronounced so that, as shown by the curves of Fig. 6, substantially uniform discrimination is obtained throughout the range from about 50 cycles to about 10,000 cycles.

The amplitude of the response of the device shown in Figs. 1 to 3 is dependent largely upon the impedance Z5, which as' previously noted is essentially a stiffness, the terminating imped- With- Although the requi--' site resistance might be obtained by the use of aromas out the impedance Zr, it has been found that the response of the device falls off rather rapidly at the low frequency end, i. e. at frequencies below about 300 cycles, of the range-to be translated.

In general, the response at the low frequency end of the range is directly proportional to the volume of the chamber within the casing i0, ii and inversely proportional to the resistance Ra. The construction shown in Figs. 1 to 3 allows the use of a fairly large volume chamber so that some enhancement of the low frequency response due to the correspondingly low chamber stiffness is realized. However, practical design considerations and also the requirements for attainment of directional discrimination throughout a wide range of frequencies limit the enhancement of the low frequency response which can be realized by increasing the volume of the chamber. Also. as noted heretofore, realization of maximum directional discrimination entails use of a resistance in the impedance Ze.

However, the impedance Z4 can be operated upon to obtain an increase in the response at the low frequency end of the range to be translated without reducing the directional characteristics of the microphone. Theoretically. en-- hancement of this response would result if the impedance Z4 were primarily a mass. However, inherently because of the required damping of the diaphragm to reduce the peak due to diaphragm resonance, the impedance Z4 must have a substantial resistance component. Further, a mass adequate to effect the desired enhancement would result in substantial attenuation of the high frequency response.

The desired enhancement at the low frequency end without substantial attenuation of the high frequencies and without substantial reduction of the directional discrimination is realized by the use of the tube 35. This tube constitutes an impedance element having mass M: and resistance R1. as pointed out heretofore, which is in shunt with the impedance Z4. The tube is made of relatively large diameter so that the mass M1 thereof is very large in comparison to the resistance R1. The resistance R4, it will be apparent, is large in comparison to the mass Mi. Hence, the impedances Z4 and Z: are essentially a mass M1 and a resistance R4 in parallel and the combination of Z4 and Z1 is an impedance element which acts primarily as a mass at the low frequencies and primarily as a resistance at high frequencies, and is effective to increase the response at low frequencies without attenuation of the high frequencies.

The effect upon the response in the low frequency range, of the impedance introduced by the tube. II is illustrated in Fig. 5, wherein curve A indicates the resp nse for a device without the tube 35 and curve B indicates the response of a device with the tube. It is seen .from a. comparison of curves A and B that the I tube It results in substantial enhancement of the response at frequencies below about 300 cycles per second so that a substantially uniform response is obtained between about and 300 cycles per second.

An increase in the response below 100 cycles per second to extend the frequency range, wherein a uniform response is realized, can be obtained by appropriate design of the electrical circuit associated with the microphone. Specifically, if, as is usually the case, the microphone is coupled to an amplifier through a transformer, a condenser may be connected between the coil 12 and the primary of the transformer and in series therewith, the condenser having such capacitance that the input circuit to the amplifier is resonant at about 50 or 60 cycles per second.

In some cases standing waves may be produced within the housing ill, ii in the chamber behind the diaphragm. The production of such waves can be prevented by providing a filling of a material such as lambs wool in this cavity. Such a filling, from the impedance standpoint, has ,the character of a distributed resistance and does not affect deleteriously the directional pattern and response of the device.

Although a specific embodiment of this invention has been shown and described, it will be understood that it but illustrative and that various modifications may be made therein without departing from the scope and spirit of this invention as defined in the appended claims.

What is claimed is:

l. A directional microphone comprising 0. diaphragm one surface of which is exposed substantially directly to the atmosphere, signal translating means cooperatively associated with said diaphragm, and means defining an acoustic phase shifting network coupling the other surface of said diaphragm to the atmosphere and including resistance means and a chamber having communication with the atmosphere through a plurality of apertures spaced different distances from said diaphragm.

2. A directional microphone comprising a diaphragm having one surface exposed substantially directly to the atmosphere, signal translating means cooperatively associated with said diaphragm, means defining a chamber substantially enclosing the other surface of said diaphragm, said chamber having a plurality of groups of apertures spaced different distances from said diaphragm, and acoustic resistance means extending across said apertures, said acoustic resistance means being constructed in such relation to the stiffness of said chamber that the directional pattern of the microphone is cardioidal.

3. A directional microphone comprising a casing, a diaphragm, within said casing and having one surface communicating substantially directly with the atmosphere through an opening in said casing, signal translating means cooperatively associated with said diaphragm, and an acoustic phase shifting network coupling the other surfacev of said diaphragm to the atmosphere, said network including a resistive termination and a chamber bounded by said casing, said casing having in the chamber bounding portions thereof a plurality of restricted apertures arranged in groups and each group being spaced from said diaphragm a distance different than each of the other groups.

4. A directional microphone comprising a diaphragm having one surface in substantially direct communication with the atmosphere, signal translating means cooperatively associated with said diaphragm, and phase shifting means cou pling the other surface of said diaphragm to the atmosphere, said phase shifting means including a chamberproviding a plurality of paths of different lengths coupling said other surface of said diaphragm to the atmosphere and acoustic resistance means in said paths.

5. A directional microphone comprising a casing having an apertured end portion, a diaphragm within said casing and having one surface adjacent said end portion, means associated with said diaphragm to translate vibrations thereof into electrical variations, said casing having therein a pluralityof groups of apertures remote from said end portion, the apertures in each group being spaced a different distance from said end portion than the other groups, and air pervious acoustic resistance material overlying said apertures.

8. A directional microphone comprising an elongated casing having an apertured end portion, a diaphragm within said casing, extending transversely thereof and having one surface ad- Jacent said end portion, said casing bounding a chamber coupled to the other surface of said diaphragm and having therein a plurality of apertures spaced from said endportion, said apertures being arrayed in circular groups each of which is disposed about the longitudinal axis of said casing, each group being spaced from said end portion a distance different than the other groups, acoustic resistance means adjacent said apertures and defining with said chamber a phase shifting acoustic network such that the directional pattern of the microphone is cardioidal, and signal translating means cooperatively associated with said diaphragm.

'7. A directional microphone comprising a dia phragm having one surface in substantially direct communication with the atmosphere, means including a chamber and acoustic resistance means defining an acoustic network coupling the other surface of said diaphragm to the atmosphere, said chamber having communication with the atmosphere through a first group of apertures spaced of the order of 6 centimeters from said other surface, a second group of apertures i0 tures therein spaced from said apertured end, said apertures being arranged in circular rows symmetrical about the longitudinal axis of said casing and spaced different distances from said apertured end of said casing, and acoustic resistance silk extending across said apertures.

11. A directional microphone comprising a casing having a predetermined directional diffraction characteristic, a diaphragm within said casing and having one surface in substantially direct communication with the atmosphere through an aperture in said casing, said casing bounding a chamber in communication with the other surface of said diaphragm and having an apertured portion spaced from said other surface, and an acoustic network including said chamber and apertured portion, said network including also an acoustic impedance composed essentially of a resistance and a mass-and the resistive and reactive components of which are substantially equal at the frequency substantially midway in the frequency range below the frequency at which the directional diffraction effect of said casing becomes substantial.

12. A directional microphone comprising a casing having an apertured end portion and a predetermined directional diffraction charac-' teristic, a diaphragm within said casing and having one surface adjacent said apertured end portion, said casing defining a chamber opposite the other surface of said diaphragm and having communication with the atmosphere through restricted apertures spaced from said other surface,

spaced of the order of 10 centimeters from said other surface and apertures between said first and second groups, each of the apertures establishing communication between said chamber and the atmosphere independently of the other apertures, and means for translating vibrations of said diaphragm into electrical variations.

8. A directional microphone comprising an elongated casing having an apertured end portion, a diaphragm within, coaxial with and having one surface adjacent said end portion of said casing, said casing having a plurality of groups of apertures spaced from said end portion, the apertures in each group being in circular array about the longitudinal axis of said casin said groups being spaced different distances from the other surface of said diaphragm and adjacent groups being substantially equally spaced, the group of apertures nearest said end portion being spaced from the other surface of said diaphragm a distance substantially equal to one-half wavelength of apreassigned frequency in the operating range of the microphone, acoustic resistance means adjacent said apertures, and means for translating vibrations of said diaphragm into electrical variations.

9. A directional microphone in accordance with.

claim 8 wherein the group of apertures nearest said end portion is spaced of the order of 6 centimeters from the other surface of said diaphragm and wherein the roup of apertures farthest from said end portion is spaced of the order of 10 centimeters from said other surface.

10. A directional microphone comprising a substantially ovate casing the larger end of which is apertured, a diaphragm within said casing. ex-

tending transversely thereof and having one surface adjacent said apertured end, means for translating vibrations of said diaphragm into electrical variations, said casing having a plurality of aper- 75 and acoustic resistance means between said other surface and the atmosphere, said chamber, apertures and resistance means defining an acoustic network the mass reactance and resistive components of which are substantially equal at the frequency substantially midway betwen zero and the frequency at which the diffraction effect of said casing becomes substantial.

13. A directional microphone comprising a diaphragm having one surface in substantially direct communication with the atmosphere, and means defining a phase shifting network coupling the other surface of said diaphragm to the atmosphere, said means including a stiffness defined by a chamber bounded by a casing having walls extending from adjacent said diaphragm and having also a predetermined directional diffraction effect upon the response of said dia phragm, said network including also a terminating impedance composed essentially of a mass and a resistance, and the resistive and reactive components of said impedance being substantially equal at substantially the'midfrequency in the band below the frequency at which said diffraction effect is substantial.

14. A directional microphone in accordance with claim 13 wherein said casing has a directional diffraction effect of the order of eight decibels at of they order of 3,000 cycles per second and wherein said resistive and reactive components are substantially equal at 1,500 cycles per second.

15. A microphone comprising a diaphragm, means adjacent one surface of said diaphragm defining a chamber therewith, said means including a member having a plurality of apertures therein, acoustic resistance means extending across all but at least one of said apertures, and a tube having a high mass to resistance ratio communicating with said chamber through an aperture not covered by said acoustic resistance means.

16. A microphone comprising a diaphragm, ii cans for translating vibrations of said diaphragm into electrical variations, means adiacent one surface of said diaphragm defining therewith an acoustic network including a chamber and an acoustic resistance, and means in communication with said chamber defining a predominantly mass impedance in shunt with said acoustic resistance.

17. A microphone comprising a diaphragm, means for translating vibrations 01' said diaphragms into electrical variations, means including a multiapertured annular member ad- Jacent and coaxial with one surface 01' said diaphragm defining a chamber therewith, one of the apertures in said member constituting an open passage communicating with said chamber, an elongated tube having a high mass to resistance ratio, one end of said tube being secured to said member in communication with said one aperture, and acoustic resistance material extending across other of the apertures in said member.

18. A directional microphone comprising a diaphragm having one surface in substantially direct communication with the atmosphere, means defining an acoustic network including a chamber coupling the other surface or said diaphragm with the atmosphere, said chamber having communication with the atmosphere through a plurality of groups of restricted apertures spaced different distances from the other surface of said diaphragm, acoustic resistance means included in said network and so constructed and arranged in relation to said chamber that the directional pattern of the microphone is a cardioid, means adjacent said other surface oi said diaphragm defining an acoustic impedance having a high resistance component in series with the diaphragm impedance, and means defining a predominantly mass acoustic impedance in shunt with said resistance component.

ROBERT BLACK, JR.