US 3007012 A
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H. F. OLSON DIRECTIONAL ELECTROSTATIC MICROPHONE Oct. 31, 1961 Filed March 14, 1958 4 Sheets-Sheet 1 INVENTOR. 17014011 ATTORNEY Oct. 31, 1961 H. F. OLSON DIRECTIONAL ELECTROSTATIC MICROPHONE 4 She ets-Sheet 2 Filed March 14, 1958 IN VEN TOR. Harry E 00011 BY A Z'TORNEY Oct. 31, 1961 H. F. OLSON 3,007,012
DIRECTIONAL ELECTROSTATIC MICROPHONE 4 Sheets-Sheet 3 Filed March 14, 1958 Z/NE INVENTOR.
dizzy F0/J011 ATTORNEY Oct. 31, 1961 H. F. OLSON 3,007,012
DIRECTIONAL ELECTROSTATIC MICROPHONE Filed March 14, 1958 4 Sheets-Sheet 4 ZZZ 0077 07" IN V EN TOR.
ATTORNEY United States Patent M 3,tlh7,d12 DWECTIONAL ELECTROSTATIC MICRGFI-IGNE Harry F. Olson, Princeton, NI, assignor to Radio Corporation of America, a corporation of Delaware Filed l'l Iar. 14, 1953, Ser. No. 721,447 17 Claims. (Cl. ll'79--illll) This invention relates to microphones, and more particularly to directional microphones incorporating electrostatic transducers.
In motion picture and television productions, it is desirable to use microphones which are small in size. Such microphones are unobtrusive and may be handled easily on small, light weight microphone booms. The complexity of productions in television and sound motion pictures gives rise of a great deal of noise behind the scenes. Microphones having directional characteristics which discriminate against unwanted noise are, therefore, preferable. Small, light weight essentially non-direc tional electrostatic microphones are presently available which are capable of satisfying the size and weight requirements of television and motion picture work. While it is desirable to combine the features residing in the small size and light weight of the electrostatic microphone with the feature of directional sensitivity, it has been difficult to provide a directional electrostatic microphone which is uniformly sensitive over the entire audio frequency range. A microphone should have a uniform frequency response so as to be compatible with high fidelity recording systems used in sound motion pictures and with the sound systems used in television. Available directional electrostatic microphoneshave involved either structures requiring accurately made parts which require delicate assembly techniques, or complex electrical circuitry.
Briefly described, an electrostatic directional microphone provided in accordance with the present invention includes a vibratile diaphragm which cooperates with an electrode spaced adjacent thereto. The diaphragm is subjected to two different sound pressures acting on the opposite surfaces thereof; namely, the surface facing the electrode and the surface facing the ambient. Means are provided for introducing phase shift between these two sound pressures acting on the diaphragm. This phase shift means, in accordance with the invention, includes an acoustical network for controlling the amount of phase shift introduced between the two sound pressures with respect to the frequency of the actuating sound waves. This network has a substantially resistive impedance characteristic. The controlling acoustical impedance may be rovided, in accordance with an embodiment of the invention, by an acoustical resistance element disposed behind the diaphragm in the path of the sound waves which produce the sound pressure at the surface of the diaphragm facing the electrode. Thus, the microphone will be predominately sensitive along the axis through the diaphragm extending into the ambient.
It is an object of the present invention to provide an improved electrostatic microphone.
It is a further object of the present invention to provide an electrostatic microphone having higher directional efficiency.
It is a still further object of the present invention to provide an electrostatic microphone having a uniangular directional characteristic with the maximum response along an axis of the microphone extending into the ambient.
It is a still further object of the present invention to provide an improved directional electrostatic microphone having a uniform amplitude response characteristic substantially independent of the frequency of the incident sound waves.
3,007,l2 Patented Oct. 31, i
Other objects and advantages of the present invention will, of course, become apparent and immediately suggest themselves to those skilled in the art to which the invention is directed from a reading of the following description in connection with the accompanying drawings in which:
FIGURE 1 is a sectional view of a directional electro static microphone constructed in accordance with the present invention;
FIGURE 2 is a sectional view of the microphone shown in FIGURE 1, the section being taken along the line 2--2 of FIGURE 1 when viewed in the direction of the arrows,
FIGURE 3 is another sectional view of the microphone shown in FIGURE 1, the section being taken along the line 3-3 of FIGURE 1 when viewed in the direction of the arrows,
FIGURE 4 is still another sectional view of the microphone shown in FIGURE 1, the section being taken along the line 44 of FIGURE 1 when viewed in the ection of the arrows,
FIGURE 5 is a side view of a protective case for the microphone shown in FIGURE 1, the case being partially broken away to show the microphone,
FIGURE 6 is a schematic diagram of the acoustical network of the microphone shown in FIGURES 1 to 5,
FIGURE 7 is a simplified acoustical network of the microphone shown in FIGURES 1 to 5,
FIGURE 8 is a fragmentary front view, partially in section, of a directional electrostatic microphone of the type shown in FIGURES 1 to 5 and having additional features,
FIGURE 9 is a schematic diagram of a circuit which may be used with the microphone shown in FIGURES l to 5,
FIGURE 10 is a curve showing a typical grid current characteristic for an electron tube,
FIGURE 11 is a schematic diagram of a circuit which may be used with the microphone illustrated in FIG- URE 8,
FIGURE 12 is a schematic diagram of a modified circuit which may be used with the microphone illustrated in FIGURE 8,
FIGURE 13 is a schematic diagram of another circuit which may be used with the microphone illustrated in FIGURE 8; and
FIGURE 14 is a schematic diagram of still another circuit which may be used with the microphone illustrated in FIGURE 8.
The structure and construction of a directional electrostatic microphone having the features provided by the present invention are shown in FIGURES 1 to 4. The microphone is enclosed in a case 10 of light-weight metal such as aluminum. The case includes two separable cylindrical parts It and I3 which are joined to each other, as at a joint 14. The bottom one 13 of the case parts is closed at its lower end. This closed end is provided with a centrally located hole. A cylindrical, internally threaded boss 12 projects from the bottom of the case. Electrical connections to the elements of the microphone may be brought through the boss 12 and the hole in the bottom of the case. The major portion of the elements of the electrostatic transducer are located in the top part ll of the case It). The bottom of the case may containcomponents of an electrical circuit for operating the transducer. In the drawing, a small electron tube 24 is shown as being located within the case.
The electrostatic transducer includes a vibratile diaphragm l6. This diaphragm may be a disc of flexible plastic material. A polyester plastic material, such as Mylar (a trademark of the Dupont Company), may be used. The diaphragm 16 shown in FIG. 1 of the drawings is a disc of Mylar material which is coated with a conductive material on at least one surface thereof. In one embodiment, the surface which is coated faces outward of the case so that it may cooperate with the other elements of the electrostatic transducer. A conductive material, such as gold, may be coated onto the surface of the Mylar sheet by any suitable coating technique, such as sputtering. The diaphragm is secured to the top of the casing 16: by means of a clamping ring 18. This clamping ring may also be made of some conductive material such as aluminum. A plurality of screws 20, which are spaced at equal distances from each other around the ring 13 are used to fasten the ring and diaphragm 16 to the top of the case 10.
An electrode 22 cooperates with the diaphragm 16. This electrode is illustrated as a disc-shaped plate of conductive material, such as brass. Another term descriptive of the electrode 22 is back-plate. The top surface of the back-plate electrode 22 is spaced closely adjacent to the rear surface of the coated diaphragm 16. A plurality of holes extends through the electrode 22. A largest hole is located at the center of the electrode 22 and is a blind hole which extends from the back surface of the disc. The holes are arranged along circular paths at successively greater radial distances from the center of the disc. The ones of the holes which are along the path at next to the largest radial distance from the center of the disc are blind holes 26 and extend only a predetermined distance into the disc from its front surface. The arrangement of holes, and particularly the blind holes 26, provides desirable acoustical characteristics for the microphone. The edge of the electrode 22 at the front surface thereof is slightly beveled. This slight bevel also improves the acoustical characteristics of the microphone.
The electrode 22 is supported by a bushing 28 of insulating material, such as nylon. This bushing 28 also supports most of the other acoustical elements of the microphone. An acoustical resistance element provided by a disc 30 of porous material is disposed directly against the rear surface of the electrode 22. This acoustical impedance element is selected to provide an impedance characteristic which is substantially resistive. An element of controlled porosity has been found suitable. For example, the material of the disc 30 may be filter paper of a selected grade. Ordinary chemical laboratory quality filter paper may be found suitable.
It will be noted that the bushing 28 is provided with a plurality of apertures 32 in the walls thereof. The axis of each of these apertures is in the same plane. This plane is located at a predetermined distance behind the diaphragm 16, the electrode 22, and the acoustical resistance element provided by the filter paper disc 30. The apertures 32 in the bushing 28 are exactly aligned with apertures 34 in the walls of the casing Ill. Sound waves may be conducted to the rear surface of the diaphragm through the apertures 32 and 34, the resistance element 30, and the back-plate electrode 22. Sound waves may be conducted through the apertures 32 and 34 into the cavity defined at the rear of the case 10 by way of other acoustical elements 36, 38 and 40 and the element 50. These elements permit sound pressure to be developed at the rear of the diaphragm which is related to the sound pressure developed at the front surface of the diaphragm in a predetermined manner. The relationship between the sound pressures is such that the desirable directional characteristics for the electrostatic microphone are obtained. The apertures 32 and 34 together with the filter paper disc 30, the apertured electrode 22 and the acoustical elements 36, 33, 40 and 543 which communicate the apertures 32 and 34 with the cavity at the rear of the case 10 comprise means for shifting the phase of the sound pressure developed at the rear surface of the diaphragm 16 with respect to the sound pressure at the front surface of the diaphragm. As will be more fully set forth hereinafter, certain of these elements present an acoustical network having a substantially resistive impedance characteristic.
The other acoustical resistance elements, which may be provided by discs 36, 38 and 40 of filter paper, similar to the disc 3d of filter paper used for the element at the rear of the electrode 22, are disposed behind the apertures 32 and 34 to provide an acoustical network which is substantially resistive. The acoustical resistance elements provided by discs 30 and '36 are spaced from each other by means of other discs 42 and 44 of foam rubber. The acoustical resistance element discs 36 and 38 are spaced from each other by further discs 46 and 48 of foam rubber material. The foam rubber discs are substantially equal in diameter to the inside diameter of the bushing 28. A circular plate 56 of insulating material, such as nylon, is disposed between discs 38 and 40 of a filter paper. This circular plate 59 is ported with a plurality of apertures. A centrally located hole extends through all of the discs of filter paper 30, 36, 38 and 4! the discs of foam rubber 42, 4-4, 46 and 43 and the apertured plate 50. A bolt 52 of conductive material extends through the plate and the disc element and this screws into the centrally disposed blind hole of the electrode 22. There is thereby provided a unitary structure of relatively inexpensive, easily manufactured and assembled elements which provide the desired acoustical elements of the microphone. The entire structure may be readily inserted into the top portion of the case 10 and fastened into place by means of the set screws 54.
The foam rubber discs, 42, '44, 46 and 48 provide the requisite, predetermined spacing between the acoustic held resistance elements provided by the filter paper discs 30, 36, 3S and 40. Any material, such as some plastics, having equivalent acoustical and mechanical properties, may be used in place of foam rubber in constructing the discs 42, 44, 46 and 48. The discs 42, 44, 46 and 48 have an acoustical impedance substantially less than the acoustical impedance provided by the filter paper discs. Therefore, the discs 42, 44, 46 and 48 have a negligible effect upon the acoustical characteristics of the microphone. However, the discs insure proper assembly of the other elements of the microphone in a convenient, inexpensive and eflicient manner.
The electron tube 24 is mounted on the closed end of a hollow cylindrical member 56 which is closed at only one end. This member 56 is made of insulating material, such as nylon, since it functions as a socket for the tube 24. The cylindrical member is disposed in the bottom part 13 of the casing 10 and rests upon the bottom casing. A post 58 of insulating material such as nylon is inserted into the end of the cylindrical member 56. A lead 60 is held by the post. This lead 60 may be the grid lead for the tube 24. Thus, the lead 60 is held securely and vibration of the sensitive elements of the tube is prevented which might otherwise cause microphonics. The other leads 62 are inserted through socket members 64 in the bottom of the cylindrical member 56. Conductors 66 in a cable 68' are connected to the socket members 64. The cable 68 is mounted in a plug 70 which is screwed into the boss 12 on the bottom of the case 10. The conductors 66 in the cable 68 are connected to other components of the circuit for the microphone. These elements will be discussed later.
Referring now to FIG. 5, the case 10 of the microphone is shown suspended within a protective outer casing 72. This casing is constructed of thin sheet metal and is perforated. The inside of the casing is covered with a porous cloth screen 74. By reason of the perforated casing 72 and the cloth screen 74, the microphone is protected against wind noise of the type encountered in the motion of the microphone through the air on a boom. The boom support '76 is connected to the casing 72.
The microphone is supported Within the perforated casing '74 by means of a yoke structure '78. This yoke structure includes a circular strap 86 having bifurcated elements 32 extending from opposite edges thereof. The bifurcated elements 82 are connected to the inside of the casing by means of yieldable supports, such as rubber bands 84. The microphone is therefore protected against external vibration. The cable 68 connects to the microphone element.
In operation, the microphone will be positioned with its cylindrical axis in the direction of the sources of sound which are to be picked up. For example, the microphone may be turned so that its cylindrical axis points toward the performing actor on the sound stage with the end of the microphone having the diaphragm facing the actor. Unwanted noise is likely to be produced in a random manner due to production equipment around the sound stage. This noise will arise from sources all around the microphone and particularly at the back thereof. The desired sound will first strike the front surface of the diaphragm 16. After a phase and time delay due to the displacement of the apertures 34 and 32 behind the diaphragm 16 and the acoustical network in the microphone, the sound waves will reach the rear surface of the diaphragm 16. For sounds along the cylindrical axis of the microphone and emanating from the front of the diaphragm lid, sound pressures will be produced at the front and rear surfaces of the diaphragm 16 having a maximum phase displacement with respect to each other. Thus, significant forces will be applied to the diaphragm 16 and the diaphragm will vibrate at relatively large amplitudes for sounds coming from the front of the microphone and along its cylindrical axis. Unwanted sounds and noise coming from the rear of the microphone will be subjected to a phase and time delay in passing through the apertures and acoustical network at the rear of the diaphragm. A similar phase and time delay will occur before such unwanted sounds reach the front of the diaphragm. Therefore, the sound pressures established at the front and rear surfaces of the diaphragm for unwanted sounds will be substantially in phase. Consequently, the diaphragm will not vibrate due to such unwanted sounds or will vibrate at a considerably smaller amplitude than for sounds which are to be picked up. The maximum sensitivity of the microphone will therefore be for sounds from the front of the microphone which are directed along the cylindrical axis thereof. Because of the symmetrical disposition of the apertures, the microphone will be uniformly unresponsive to sounds from all sides which are incident upon the diaphragm due to sources at the rear of the microphone. The microphone will be uniformly sensitive to sound from sources at the front thereof. The microphone may therefore be'considered to have a uniangular response characteristic. Because the axis of maximum sensitivity of the microphone corresponds to the axis of the microphone assembly, the directional, electrostatic microphone provided by the present invention may be termed a uniaXial microphone. Uniaxial microphones of the electromagnetic type, which may have ribbon vibratile elements, are available. Such microphones are illustrated in Olson et al., Patent No. 2,680,787, issued June 8, 1954. The uniaxial electrostatic microphone provided by the present invention is therefore directly compatible with sound systems which may use uniaxial electromagnetic microphones, and will be especially useful where the features of small-size, light weight and directivity of the uniaxial electrostatic microphones are desired.
It follows from the above discussion that the microphone provided in accordance with the present invention is responsive to the difference in sound pressure on the opposite surfaces of the diaphragm 16. In other words, the microphone is a sound pressure gradient responsive microphone. The pressure gradient is established by the phase shift introduced in the apertures 32, 34 and phase shift means, including the acoustical elements behind the diaphragm. Since phase shift imposed on acoustical waves by such phase shift means varies in accordance with the frequency of such waves, it would seem that the pressure gradient, which is the actuating sound pressure for the microphone, would be related to the frequency. It is desirable that the response of the microphone be independent of the frequency of the actuating sound waves. Expressed in other Words, it is desirable that there be a constant relationship between the actuating sound pressure and the open circuit voltage out ut of th microphone.
In previous directional electrostatic microphones which are dependent upon pressure gradient of the actuating sound, complex acoustical networks were combined with the vibrating elements of the microphone to provide the requisite uniform response with respect to the frequency of the actuating sound waves. Another alternative was additional circuitry. The former increases the cost of the microphone and makes repair difiicult. The latter requires careful and special design of the amplifiers associated with the microphone. Thus, prior directional electrostatic microphones may not be compatible with the available amplifiers in the sound studio. In accordance with the present invention, the requisite constant relationship between the voltage output of the microphone and the actuating sound pressure and a uniform response with respect to frequency is obtained with the acoustical network provided by simple acoustical elements, such as the filter paper disks 30, 36, 3-3 and 40, which are inexpensive to build and easy to maintain. These acoustical elements provide an acoustical network which is resistance controlled, in an acoustic sense. The combination of an electrostatic transducer and phase shift means including the resistance controlled acoustical network provides a feature of the present invention.
In the electrostatic transducer of the illustrated micro phone, the diaphragm 16, which is coated with a conductive material, and the back-plate electrode 22 provide two plates of a capacitor. A potential gradient will be established between these elements 16 and 22 by means of circuitry to be described hereinafter. The open circuit voltage developed by the electrostatic transducer is given y where e=open circuit output voltage of the transducer, in volts;
e =polarizing voltage or potential gradient between the elements, in volts;
a=spacing between the surfaces of the elements, in centimeters; and
x =arnplitude of vibration of the movable element,
which is the diaphragm 16, in centimeters.
It appears from Equation 1 that there is a constant relationship between the amplitude of vibration of the diaphragm l6 and the open circuit output voltage of the transducer. In accordance with the present invention, the actuating sound pressures in the microphone are conrolled so that forces for actuating the diaphragm are related to the sound pressure in a manner to establish a constant relationship between the actuating sound pressures and the open circuit output voltage of the microphone, notwithstanding that the microphone is a gradient microphone and the intensity of the actuating sound pressures is proportional to frequency. Thus, the microphone will have a uniform response with respect to the frequency of the actuating sound waves.
The actuating force on the diaphragm may be expressed as follows:
fMD=i 1P where,
f =actuating force, in dynes; K =constant of the vibrating system;
2" p:sound pressure, in dynes per square centimeter; w=21rf; f=frequency of the actuating sound waves, in cycles per second; and A=area of the diaphragm, in square centimeters.
The amplitude of vibration of the diaphragm 15 is given by:
f MD r (3) where, x =amplitude of vibration of the diaphragm 16, in centimeters, as was the case in Equation 1; and z =mechanical impedance characteristic of the vibrating system of the transducer, in mechanical ohms.
Combining Equations 2 and 3, the amplitude of vibration of the diaphragm 16 will be:
Equation 4 shows that the amplitude of vibration of the diaphragm will be independent of the frequency of the actuating sound waves, if the mechanical impedance characteristic z is not dependent on frequency and therefore is a mechanical resistance. Thus, a resistance controlled vibrating system, as is provided by the acoustical elements disposed behind the diaphragm 16 in the microphone illustrated in connection with FIGS. 1 to 4, will have an output voltage which has a constant relationship with the actuating sound pressure. Consequently, the response of the microphone will be uniform with respect to frequency.
The resistance controlled nature of the electrostatic uniangular microphone shown in FIGS. 1 to 4 may be illustrated in connection with the schematic diagrams of the acoustical network thereof which are shown in FIG. 6.
In that network:
p is the sound pressure at the diaphragm 16;
p is the sound pressure at the apertures 34, 32;
M is the mass of the diaphragm 16, and the air load on the front of the diaphragm;
C is the acoustical capacitance of the diaphragm;
r is the acoustical resistance behind the diaphragm;
M is the inertance of the apertures 32, 34;
r is the acoustical resistance of the apertures;
r is the acoustical resistance of the termination, which includes the elements behind the aperture; and
C is the acoustical capacitance of the volume of air in the case it In order to provide the uniform response and directivity of the microphone with respect to frequency, the acoustical resistance r of the first branch -86 is the controlling acoustical impedance in that branch86. For the same reason, the acoustical resistance r in the parallel branch 88 is the controlling acoustical resistance in that branch. Under these conditions, the acoustical network can be simplified by eliminating the acoustical elements which have a negligible eifect on the system. The simplified acoustical network is shown in FIG. 7. This acoustical network may be referred to in analyzing the operation of the vibrating system of the electrostatic uniangular microphone provided by the present invention. Thus, the volume current in the branch 86 due to the pressure p is given in equation:
h m Aa 'l'j z m +1 2 213 (5) where,
X =volume current, in cubic centimeters per second;
p =actuating pressure at the diaphragm of the microphone, in dynes per square centimeter;
r =acoustical resistance behind the diaphragm, in
r =acoustical resistance of the termination, in acoustical ohms; and M =the inertance of the apertures, in grams per (centimeter) The volume current in the branch =86 due to the pressure 11 is given by: t
X =the volume current, in cubic centimeters per sec 0nd; and
p -the actuating pressure at the apertures of the microphone, in dynes per square centimeter.
In order to determine the characteristics of the actuating sound pressures on the microphone, it will be assurned that the reference point of zero phase is at the front surface of the diaphragm 16 of the microphone. Then, the expression for p may be written:
p the amplitude of p in dynes per square centimeter, and t=time, in seconds.
7 The expression for p is written:
p =amplitude of p in dynes per square centimeter;
d=elfective acoustic path from the diaphragm to the aperture, in centimeters;
A=wavelength of the actuating sound wave, in centimeters; and
6=angle between the axis of the microphone and the direction of the incident sound wave.
The resultant volume current in branch 86, which is the volume current of the diaphragm 16, is given by:
If (a) the pressures, p and p are equal, (b) the distance d is small compared to the wave length of actuating sound, and (c) the values of r r and M are selected to provide a cardioid pattern, in the well known manner as, for example, set forth in the text, Elements of Acoustical Engineering, by H. F. Olson, 2nd ed., pp. 26465, the volume displacement of the diaphragm 16 can be expressed as X =K p (.5+.5 cos 0) (10) where,
X =volume displacement of the diaphragm in cubic centimeters; and
K =constant of the acoustical and electrical systems.
The amplitude of the diaphragm is given by:
Combining Equations 10 and 11, the open circuit output voltage developed by the microphone is given by:
9 effects are illustrated on page 20 of the aforementioned text. The response of the microphone as a function of the azimuth of the incident sound with the directivity maximized by the employment of phase diffraction eflects is given by:
e=K e p (/l+.6 cos 8 cos (13) where, K =the sensitivity constant of the acoustical and electrical systems.
An examination of Equation 13 shows that the direc tivity of the microphone is greater than that of a simple cardioid microphone. For example, at ninety degrees with respect to the axis of the microphone, the response is down eight decibels from that at zero degrees or on the axis of the microphone as compared to six decibels for the cardioid pattern.
The frequency response of the microphone may be improved in the high frequency region by means of the blind holes 16 which are arrayed in the back-plate electrode 22. The presence of the blind'holes in the closed area at the back surface of the diaphragm i6 establishes an auxiliary acoustical network which has a negligible effect upon the operation of the electrostatic transducer and its vibrating system except in the high frequency region of the response characteristic of the microphone. In that region, the auxiliary network produces a resonance effect which improves the sensitivity of the microphone. Thus, the disclosed improved uniangular directional microphone has a uniform response with frequency for all audio frequencies and is compatible with high fidelity sound and recording systems.
The microphone illustrated in connection with FIGS. 1 to 4 may be used with conventional circuity to derive the output voltage therefrom. One such circuitry is shown in FIG. 9 of the drawings. In F210. 9, the microphone is schematically illustrated as comprising the casing 1d, the diaphragm 16 and the electrode 22. The electrode 22 is positioned in back of the diaphragm 16 and may be referred to as the back-plate electrode of the electrostatic transducer. The casing. 10 of the microphone is connected to a point of reference potential, such as ground. The diaphragm 16 which is coated with a conductive material is also connected to ground since it is in contact with the casing 10. The physical connection of the diaphragm to the casing It} is shown in greater detail in FIG. l. The circuit shown in FIG. 9 includes the vacuum tube 24, having a plate 9% grid 92, a cathode 94 and a heater or filament 96. The filament may be connected to a source of heating voltage, which is desirably a direct current source. The grid 92 is connected to the back plate electrode 22. The plate 90 is connected to a source of operating voltage shown on the drawing at 3+. The cathode 94 is connected to ground through a cathode resistor 93 and to the primary winding of an output transformer 16% through a coupling capacitor 192.. It will be observed that the grid connection to the backplate 22 may be by way of the lead 69 which is shown in FlGURE 1.
As the diaphragm 16 vibrates, the distance between the adjacent surfaces of the diaphragm 16 and the back plate electrode 22 varies in accordance with the actuating sound pressure. This variation causes the voltage between the plates of the capacitor constituted by the diaphragm 16 and to correspondingly vary as the capacity changes. This varying voltage appears at the grid 92. The tube circuit functions as a cathode follower to provide an output voltage across the cathode resistor 98. The direct current voltage across the cathode resistor 93 is blocked by the coupling capacitor 103 so that only alternating current passes through the primary winding of the transformer 1%. The useful alternating current output signal of the microphone may be derived across the secondary winding of the transformer Till).
The potential gradient across the electrostatic transducer from the back plate electrode 22 to the diaphragm 35.6 is established when the operating potential is applied to the tube 24. The electrostatic transducer functions as a capacitive element in the circuit, and charges to establish the polarizing voltage thereacross. However, the time required for the capacitor to charge to its operating voltage may be appreciable. As may be observed from Equation 1, the output voltage of the electrostatic trans ducer is proportional to the potential to which the electrodes thereof are charged. Until this potential is sufliciently great, the voltage output of the transducer and the signal output of the circuit is not of sufficient magnitude to properly operate amplifiers which are associated therewith.
With the circuit of FIGURE 9, the amount of time required to charge the electrostatic transducer may be appreciable. An analysis of the grid current-grid voltage characteristics of the vacuum tube 24-, such as shown in FIGURE 16, will determine the reason for the charging time. A free grid," i.e., a grid with no external connection, must assume the condition of zero grid current. Such a point is indicated as, A, in FIGURE 10. The free grid voltage for the majority of electron tubes is negative with respect to the cathode and is small and only a few volts in magnitude. The grid 92, in FIGURE 9, does not have a conductive return path connected thereto. Therefore, it is a free grid in the direct current sense and must ultimately assume the Zero grid current condition, A in FTGURE 10. With a small negative grid voltage, appreciable current fiow takes place through the tube 24 from plate to cathode, A large voltage drop occurs across the cathode resistor 98. This voltage drop raises the cathode substantially positive in voltage with respect to ground. Since the grid 92. and the transducer element 22 will have assumed a small negative voltage with respect to the cathode 9d, the transducer element 22 will be charged to approximately the same voltage as the cathode 94-.
In the interval preceding activation of the circuit of FIGURE 9 by applying 3+ to the plate 9%, all elements of the circuit will be at zero or ground potential. At the instant of applying 2-]- to the plate the transducer element 22 and the grid 92 must, by the nature of the transducer capacitance, remain at substantially ground potential. The plate current of the vacuum tube 24 will therefore, in that instant, increase to a value which will raise the cathode 9d to a voltage approximating the grid cut off potential of the vacuum tubef In the first instant of activation of the circuit of FIGURE 9, the grid 94 will, therefore, have a voltage several volts negative with respect to cathode 94. This negative grid voltage will be at a point such as B in the grid current curve depicted in FIGURE 10. The resulting grid current is quite small under these conditions. in the interval following the instant of activation of the circuit of FIGURE 9, the electrostatic transducer capacitance is charged by the flow of grid current from grid 94 to the back plate electrode 22. Since the grid current, as described above, is quite small the charging time of the transducer capacitance must be appreciable.
improved circuits for decreasing the charging time may be provided. Illustrative embodiments of such circuits are shown in FIGURES 11, 12, and 13 of the drawings. The circuits of FIGURE-5 12 and 13 have the additional feature of causing the voltage across the elements 16 and 22 of the electrostatic transducer to increase to an extent that they are larger than the polarizing voltages available with the circuit shown in FIGURE 9. It will be noticed, however, that with the circuits shown in FIGURES 11, 12, and 13, the diaphragm 16 of the microphone is not connected to ground. Instead, it is connected to the source of operating potential.
In the embodiment of the microphone shown in FIG- URE 8, means are provided for insulating the diaphragm 1 1 16 so that it may be connected to a source of high voltage and at the same time eliminate any hazard of electrical shock to the user of the microphone. The parts of the embodiment of the microphone shown in FIGURE 8 which are similar to the corresponding parts of the microphone shown in FIGURES 1 to 4 are designated with like reference numerals. It will be appreciated, however, that the improved circuits shown in FIGURES 11 to 13 of the drawings are useful with any electrostatic transducers.
Referring to FIGURE 8, there is shown an electrostatic uniangular microphone similar to the microphone shown and described in connection with FIGURES 1 to 4. This microphone includes a case having an electrostatic transducer in the upper part thereof and electrical components, such as an electron tube M2 in the lower part thereof. The electrostatic transducer includes a diaphragm 16 coated on at least one surface with a conductive material. A back plate electrode 22 is spaced adjacent the uncoated surface of the diaphragm 16. Apertures 32, 34 are provided in the case. The interior of the case in back of the diaphragm contains similar acoustical elements to those shown in FIGURE 1 for introducing sound to the back surface of the diaphragm and establishing the directional response characteristic of the microphone. The acoustical operation of this microphone was discussed above.
The diaphragm 16 is supported on top of the case 10 in a manner to be insulated from the case. A ring 104 of insulating material which may be an insulating plastic, such as Bakelite, is disposed at the top of the case. The diaphragm rests on this ring. Another ring 106 of conductive material such as copper is placed on top of the diaphragm and overlies the plastic ring 1414. Another ring of insulating material, such as a plastic material, is placed on top of the conductive ring. Thus, the edge of the diaphragm 16 and the contact ring 166 are sandwiched between the plastic rings and are insulated by means of these rings from the case. A plurality of screws 1% extend through holes in the rings 1%, 106, and 108, and in the diaphragm. The diameter of the holes is greater than the diameter of the screws so that the screws are not in contact with the conductive ring 106. These screws are attached to the material at the top of the case 10. The screws 116 may be equally spaced from each other around the rings as was the case with the screws 20 shown in FIGURES 1 and 2.
A tab 112 of conductive material is attached to the conductive ring 1%. A lead 114 is connected to this tab by means of soldering. A terminal cover 116 of insulating material, which may be cemented to the plastic rings 1% and 1118 encloses the soldered junction. A screen 118 of stiffened cloth may be used to cover the entire top portion of the microphone. This screen 118 may be secured to the top of the microphone by means of the screws 11d. Thus, the entire unit is electrically insulated and isolated so as to eliminate any shock hazard.
The lead 114'. may be connected to the cable at the bottom of the microphone case. A plug connection similar to that shown in FIGURE 1 may be used. This lead 114, as will be observed from the circuit diagrams of FTGURES 11 to 13, is connected to the source of operating potential for the microphone.
In FIGURE 11, a microphone of the type shown in FIGURE 8 is schematically illustrated as including the diaphragm 16, the back plate electrode 22 and the case 10. The lead 114 is connected to the diaphragm and is insulated from the case as in the diaphragm. The circuit of FIGURE 11 includes a power supply 12% for providing low voltage direct current for operating the filament of the electron tube 1412. The power supply 12) also provides higher voltage direct current for operating the circuit and the microphone. The power supply transformer 12% has a primary winding which may be connected to the power lines and low voltage and high voltage secondary windings and 132, respectively; The low voltage alternating current across the low voltage winding 13% is rectified by means of a pair of diodes 134. The rectified alternating current from the diodes is filtered in a resistance-capacitance filter circuit 136. The filament 138 of the tube 162 is connected across the output of the filter 136. Thus, the tube 162 is heated by direct current so as to prevent the introduction of hum into the circuit.
High voltage direct current is provided by a half wave rccifier power supply including a diode 140 which operates into a filter network 154 The filter network 159 is a multi-stage filter of the resistance capacitance type so that a substantially pure direct current voltage output is obained. The high voltage terminal of the high voltage supply is labeled +8 and the low voltage terminal of the power supply is labeled B. The high voltage terminal of the power supply is connected to the plate 152 or" the tube 1%2 and simultaneously to the diaphragm 16 of the electrostatic transducer in the microphone. The low voltage terminal, indicated at B is connected to the cathode 154 of the tube 102 through a cathode resistor 156. The signal voltage output of the circuit is obtained across the cathode resistor through a blocking capacitor 158 and an output transformer 160.
In operation, the electrostatic transducer is immediately charged upon energization of the circuit, as by connecting the transformer 123' primary winding to the power line. A high polarizing voltage is established between the diaphragm 16 and the back plate electrode 22. When the high voltage from the power supply, indicated at +13 is applied to the diaphragm 16, the capacitive electrostatic transducer initially acts as a short circuit and applies a positive voltage to the grid 162 of the tube 162. As may be seen in FlGURE 10, a positive grid causes a large amount of grid current to flow and the transducer element immediately charges to a sufficiently high polarizing voltage to permit proper operation of the electrostatic microphone. The plate current, through the tube, is initially higher than normal, since the grid 162 is positive. However, the bias voltage developed across the cathode resistor 156 is not sufficient to cut oft conduction through the tube before the capacitive transducer element is charged to requisite polarizing voltage by grid current conduction.
When the circuit reaches its quiescent operating condition, the grid voltage will return to a magnitude such that grid current conduction is zero. This operation takes place because of the characteristics of the grid 162 as a free grid in the circuit as explained above.
After initial energization of the circuit of FIGURE 11, the polarizing voltage across the transducer elements 16 and 22 will be approximately equal to the voltage drop across the plate to cathode of the electron tube, 102. The free grid potential is small compared to the potential of the cathode 154. More accurately, the polarizing voltage will be equal to the supply voltage at +13, added to th evoltage from the grid 162 to the cathode 154, less the voltage drop across the cathode resistor. The last voltage will be substantial in magnitude. Consequently the polarizing voltage will be correspondingly less than the supply voltage.
In the circuit of FIGURE 9, the electrostatic transducer eventually becomes charged to a polarizing voltage equal to the voltage drop across the cathode resistor 98, less the bias voltage from the grid to the cathode. In other words, the polarizing voltage in the circuit of FIGURE 9, will be approximately equal to the voltage drop across cathode resistor 98. This is because the grid to cathode voltage of the vacuum tube 24 will be small as explained above.
The circuit of FIGURE 11 is an improvement over the circuit of FIGURE 9 inasmuch as the transducer charg ing time is considerably reduced in the circuit of FIG- URE 11 as compared to the charging time of the circuit of FIGURE 9. The polarizing potential developed in 13 either circuit will be substantially less in magnitude than the potential of +8.
A consideration of Equation 1 will Show that it is desirable to have a polarizing potential as large as pos sible. A circuit which will increase the polarizing voltage to approximately 13+ potential is shown in FIGURE 12 which schematically illustrates the microphone shown in FIGURE 8 having the diaphragm 16 and the back plate electrode 2' The back plate electrode is connected to the grid 164 of a vacuum tube 16d. The plate 163 of the tube in; is connected to the source of operating voltage indicated at B-lthrough a plate current limiting resistor 170. The primary winding 172 of an output transformer 17 i is connected between the 8- terminal of the source of operating voltage and the cathode 176 of the tube res. The 3- terminal may be grounded, if desired. The filament 178 of the tube 166 may be heated by direct current voltages in the manner shown in FIG- URE 11. A capacitor 189 is connected between the B- terminal and the plate so as to shunt any transients, occuring in the power supply around the tube and winding 172 of the output transformer. In other words, the resistor 17% and capacitor 18% function in the same man her as the last section of the filter circuit 1.5% shown in FEGURE 10.
A high polarizing voltage is immediately established between the diaphragm is and the back plate electrode 22 when the operating voltages are applied to the circuit. The operation of the circuit in quickly charging the electrostatic transducer to operating voltage is similar to that described in FIGURE ll. The polarizing voltage developed across the transducer elements of this circuit is greater than was the case with the circuit shown in FIG- URE 11. When the circuit reaches its quiescent operating condition, the grid current conduction ceases and a bias voltage develops between the grid 16% and the oathode 176 which is sufficient to cause grid current cut off. The polarizing voltage across the transducer elements is equal to the voltage of the source of operating voltage at 13+, plus the bias voltage which is developed between the grid 164 and the cathode 176, since there is'substantially no direct-current voltage drop across the primary winding 172 of the output transformer 174. The bias voltage between the grid 164 and the cathode 176 is only a few volts. Therefore, the polarizing voltage developed across the transducer will be approximately equal to the voltage of the source of operating voltage at B+. This higher polarizing voltage results in a higher output voltage from the microphone.
FIGURE 13 shows another embodiment of the improved circuit for operating an electrostatic microphone. This circuit is particularly advantageous when the source of operating voltage for the microphone is a battery or batteries. The microphone elements are shown schematically as including the diaphram 16, back plate electrode 22 and case Ill. The diaphragm 16 is connected over lead 114 to the positive terminal of a battery 182 which provides the source of high operating voltage for the circuit and microphone. The manner of connection of the microphone elements and the disposition of the lead 114 is illustrated in FIGURE 8 of the drawings. The back plate electrode 22 is connected over the lead of to the grid 1&3. of a tube 185. The plate 13$ of the tube 186 is connected to the battery 182 by way of a filter circuit including the current limiting resistor 19% and the shunt capacitor 192. This resistance-capacitance circuit operates in the same manner as the circuit of resistor 170 and capacitor 186 shown in FIGURE 12. The filament 1% functions as the cathode of the tube. It is a feature of this circuit that the filament is connected to the battery 1% which supplies the filament heating power through the primary windings 198 and 20b of an output transformer. 292. The signals from the circuit are obtained across the secondary winding 2594 of the transformer 202. The primary windings 198 and 2&0 are connected, in bucking relationsmp, as indicated by the dots at the ends thereof which are shown on the drawing, in series with the battery 1% and the filament 94. Thus, the filament current does not produce flux in the core of the transformer. The windings 1% and 2% are in parallel with the plate circuit of the tube. Thus, signals passing through the tube and the windings 198 and 200 in its cathode circuit will be reflected in the output voltage across the secondary winding 2%. if desired, the windings 1558 and 2% may be wound in a 'bifilar manner. The polarizing voltage is established in the electrostatic transducer in a rapid manner as was the case for the circuit illustrated in FIGURE 12. Similarly, the voltage be tween the diaphragm l6 and the back plate electrode 22 will be a higher polarizing voltage than possible with prior circuits as was explained in connection with FIG- URE 12.
Another circuit incorporating improvements in addition to the improvements provided by the circuits shown in FIGURES 11 to 13 is illustrated in FIGURE 14. This circuit is similar in most respects to the circuit shown in FIGURE 12 except that (1) connections are made to ground instead of to B and (2) a resistor 205 is connected between the grid 164 and ground.
In certain applications of an electrostatic microphone, for example in motion picture studio work, the micro phone may be exposed to explosive sound, such as gunshot sounds. After exposure to severe gunshot sounds the microphone may suffer a momentary decrease of sensitivity. Such decrease may possibly be attributable to the effect of the gunshot sound on the capacitive transducer element of the microphone which causes the grid 164 of the tube 156 to become positive. Upon becoming positive, the grid draws current. The capacitive transducer element charges so that the back plate 22 becomes sufficiently negative to reduce signal transmission through the tube 166 for a short interval of time.
The provision of the grid resistor 2%" permits the transducer element to discharge and return to its normal charge immediately after the gunshot sound terminates. Thus, the interval of decreased microphone sensitivity is alleviated. in all other respects the circuit of FlIGURE 14 operates similarly with the circuit of FIGURE 12.
What is claimed is:
1. An electrostatic microphone comprising a diaphragm having one surface thereof exposed to the ambient, an electrode cooperative with said diaphragm having a surface thereof opposed to the other surface of said diaphragm and spaced therefrom to define a gap therebetween, a structure for supporting said diaphragm and said electrode, one end of said structure being disposed adjacent the surface of said electrode opposite from said gap defining surface thereof, said structure having an opening therein in said end thereof, said surface of said electrode opposite said gap defining surface thereof being adjacent said opening to define a cavity at said end of said structure, a plurality of sheets of acoustical resistance material disposed in said cavity and spaced from each other and said surface of said element opposite to said gap defining surface thereof, and said structure having an aperture therein communicating with said cavity for introducing sound waves into said gap through said cavity and said acoustical resistance elements.
2. In an electrostatic microphone including a vibratile diaphragm a plate having surfaces spaced from each other to define a therebetween, a structure for providing directional characteristics for said microphone, said structure having an opening at the end thereof, said diaphragm and said plate being disposed adjacent said end of said structure to define a cavity at said open end of said structure, said cavity communicating with said gap, said structure having a plurality of openings therein spaced from each other around the sides thereof and communicating with said cavity, a plurality of thin members of porous material providing acoustical resistance being disposed in said cavity, said members being spaced from each other and said plate, at least one of said members being 15 disposed across said cavity between said openings and said plate, and at least one of said members being disposed between said openings and the end of said cavity opposite from said plate.
3. The invention as set forth in claim 2 wherein spacing members are disposed between adjacent ones of said acoustical resistance members, said spacing members being constituted of material having greater porosity than said thin members.
4. An electrostatic microphone comprising a diaphragm,
a plate electrode having a surface opposed to a surface of said diaphragm and spaced therefrom to define a gap therebetween, a housing for said diaphragm and said plate electrode, said housing having an opening at one end thereof extending longitudinally of said housing, said dia' phragm and said plate electrode being disposed across said one end of said housing to define a cavity in said housing including said opening, a sheet of material having a pre determined porosity disposed across said cavity adjacent the side of said plate opposite from said gap defining side thereof, a plurality of sheet members of material having a predetermined porosity disposed across said cavity spaced from said first-named sheet member and spaced from each other, spacer members of material having a porosity much greater than the porosity of said sheet members disposed between adjacent ones thereof, said structure having a plurality of openings therein disposed below said diaphragm and communicating with said cavity, and said openings also being disposed between a pair of adjacent ones of said sheet members. 7 5. An electrostatic microphone comprising a diaphragm, a back plate electrode spaced from one surface of said diaphragm to define a gap therebetween, a housing, said diaphragm being secured at. one end of said housing, said housing being hollow at at least one end thereof, a' sleeve disposed within said housing, a plurality of sheets of material having a predetermined porosity for presenting to sound waves an acoustical impedance which is substantially resistive, a perforated terminating plate, said plate electrode, said sheets and said terminating plate being carried by said sleeve in spaced relation to each other with one of said sheets being disposed adjacent said plate electrode, two of said sheets disposed on opposite sides of said performed terminating plate, and the other of said sheets being disposed between said plates, spacer members of material having foam porosity being disposed between said last mentioned sheet and the ones of said sheets adjacent thereto, a plurality of apertures in said housing and disposed around said housing at a predetermined distance be low said diaphragm, said sleeve having a plurality of apertures therein, said plate electrode having a plurality of holes therein for communicating said cavity with said gap, and said aperture in said sleeve, said aperture in said housing and said apertures in said plate electrode communicating sound waves into said gap through said spacer members and said sheets.
6. The invention as set forth in claim 5 wherein said terminating plate, said sheets, said spacer members and said plate electrode have aligned holes therethrough, and a bolt extending through said holes for assembling said plates, sheets and spacers with said sleeve as a unitary structure. 7
7. An electrostatic microphone comprising a diaphragm of flexible plastic material coated on at least one surface thereof with a conductive material, a hollow cylindrical housing, a back plate electrode disposed in said housing adjacent to said uncoated surface of said diaphragm to define a gap therebetween, said back plate electrode having a plurality of perforations therethrough, a sleeve of insulating material disposed in said housing for supporting said back plate electrode adjacent to said diaphragm, a perforated terminating plate disposed in said housing below said sleeve and defining a cylindrical cavity with the wall of said sleeve and the surface of said back plate electrode opposi e to Said gap defining surface, said housing having a plurality ofapertures spaced from each other circumferentially around the wall thereof, said sleeve having a plurality of apertures spaced from each other circumferentially around the Wall thereof, said apertures in said housing and in said sleeve communicating with each other and said cavity, said cavity communicating with said gap through said perforations in said back plate elecrode, a plurality of discs of filter paper, one of said discs being disposed in juxtaposition with said opposite surface of said back plate electrode, the other of said discs being spaced from each other and parallel to the one of said discs in juxtaposition with said back plate electrode, a plurality of discs of foam rubber material disposed be tween adjacent ones of said filter paper discs and filling said cavity, and said cavity communicating with the interior of said hollow housing through said perforations in said terminating plate.
8. The invention as set forth in claim 7 including a bolt of conductive material extending through said terminating plate and said discs into said back plate electrode for fastening said terminating plate and said back plate to said sleeve and for assembling said discs therebetween, an electrical circuit element disposed in said housing below said back plate, and a lead connecting said element to said bolt.
9. The invention as set forth in claim 7 wherein said back plate electrode has a plurality of blind holes therein extending from said gap defining surface thereof.
10. An electrostatic microphone comprising 21 diaphragm, a housing of conductive material, said diaphragm being coated on one side thereof with conductive material, a strip of material disposed at the edge of said diaphragm in contact with the conductive coating thereon, a pair of rings of insulating material, said diaphragm and said conductive strip being sandwiched between said rings to provide an insulated assembly, means for securing said assembly to the end of said housing, said housing being hollow at at least said end thereof, a back plate electrode being supported in said housing with a surface thereof opposed to said diaphragm and spaced therefrom to define a gap therebetween, a plurality of apertures in said housing near said end thereof for communicating sound waves through said housing into said gap, and means providing an acoustical network having an impedance characteristic which is substantially resistive disposed in said housing behind said back plate electrode in the path of said sound waves into said gap.
11. The invention as set forth in claim 10 including a circuit element disposed in said housing, and electrical connections to saidgelement, said electrical connections being provided by a lead extending from said contact strip into said housing.
12. A directional electrostatic microphone comprising a diaphragm, an electrode spaced from said diaphragm, opposed surfaces of said electrode and said diaphragm delining a gap therebetween, a hollow structure open at one end thereof, said diaphragm closing said structure to define a cavity therein, the surface of said diaphragm opposite to said gap defining surface being disposed facing outwardly of said structure, said electrode being disposed in said structure with said gap communicating with said cavity, said structure having an aperture in the side thereof for communicating sound waves into said cavity, an acoustical resistance element'disposed in said cavity between said aperture and said gap, and another acoustical resistance element being disposed across said cavity between said aperture and the end thereof opposite to said open end to confine a predetermined air volume within said cavity in said structure.
13. A directional electrostatic microphone comprising a diaphragm, an electrode spaced from said diaphragm to define a gap therebetween, a hollow cylindrical housing open at one end thereof, said diaphragm being disposed across said one end of said housing with said electrode in said housing for defining an enclosed volume in said housing having said gap in communication therewith, said housing having a plurality of apertures spaced from each other around the wall thereof and disposed between the ends of said housing, a disc of acoustical resistance material disposed in said enclosed volume across said housing between said apertures and said electrode, and another disc of acoustical resistance material being disposed across said housing in said enclosed volume between said apertures and the end of said housing opposite said one end thereof.
14. In an electrostatic microphone including an electrode and a diaphragm having a front surface facing the atmosphere and a rear surface facing said electrode, which diaphragm is movable with respect to said electrode under the pressure of incident sound waves on said front surface, means for establishing sound pressure on said rear surface having a predetermined relationship to the sound pressure of said incident sound waves on said front surface, said sound pressure establishing means comprising means for defining a chamber behind said diaphragm, and at least one acoustical element in said chamber separating said enclosed volume into two separate portions a first of which is between said diaphragm and said element and a second of which is separated from said first portion by said element, said chamber having an opening to the atmosphere from said first portion for communicatin-g sound waves from the atmosphere directly into said first portion and through said element into said second portion, said element presenting an acoustical impedance to said sound waves passing into said second portion which is substantially resistive and greater in magnitude than any other acoustical impedance in said chamber defining means.
15. A directional electrostatic microphone comprising a diaphragm element and electrode element having surfaces opposed to and spaced from each other to define a gap therebetween, said diaphragm being a vibratile element, a structure for supporting said diaphragm and said electrode elements so that said diaphragm element vibrates under sound pressure due to incident sound waves, said structure having means cooperative with said elements for defining a space containing a volume of air which space is closed except for an opening to the atmosphere, said electrode being supported in said structure with said gap communicating with said space, and at least one member disposed in said space in the path of sound waves communicated through said opening into said space, said member presenting an acoustical impedance which is substantially resistive and greater in magnitude than the acoustical impedance presented by those portions of said structure disposed in said path, said opening being disposed between said member and said electrode element.
.16. In an electrostatic microphone including a diaphragm element and electrode element having surfaces spaced from each other to define a gap therebetween,
said diaphragm element being movable with respect to said electrode element to vary the size of said gap in response to sound waves incident on the surface of said diaphragm element opposite to said gap defining surface thereof, a structure associated with said elements for providing directional response in said microphone, said structure having a cavity therein, said cavity being disposed behind said electrode element, said electrode being disposed in said structure with said gap communicating with said cavity, a member providing an acoustical resistance disposed across said cavity in spaced relation to said electrode element and defining a space containing a volume of air in said cavity, said stnlcture having an opening entering said cavity between said acoustical resistance member and said other element for communica-ting sound waves into said cavity whereby to establish a path for sound waves into said space and into said gap, and said acoustical resistance member cooperating with said cavity to provide an acoustical network in said cavity having an impedance which is substantially resistive.
17. A directional electrostatic microphone comprising a diaphragm element and an electrode element having opposed surfaces spaced from each other to define a gap therebetween, a structure for supporting said elements so that said diaphragm element vibrates with respect said electrode element under sound pressure due to sound waves, said structure having a cavity therein containing said electrode element and closed by said diaphragm element, said electrode element being perforated whereby said gap is in sound communicating relationship with said cavity through said electrode, at least one member of acoustical resistance material disposed in said cavity and defining space containing a volume of air therein between said acoustical resistance member and an end of said cavity opposite to said diaphragm element, said structure having an aperture therein for introducing sound waves into said cavity from the atmosphere, said aperture entering said cavity between said electrode element and said space, said acoustical resistance member being disposed between said aperture and said opposite end of said cavity whereby said sound waves pass through said acoustical resistance member into said space.