US 3031538 A
Description (OCR text may contain errors)
R. GCBRIKE 2 Sheets-Sheet 1 INVENTOR. RUDOLF GORIKE ATTORNEY.
April 24, 1962 SMALL-SIZE CONDENSER MICROPHONE Filed April 2, 1958 4 6 8 I25 Hz.cps
United States Patent Ofi 3,031,538 Patented Apr..24, 1962 ice 3,031,538 SMALL-SIZE CONDENSER MICROPHONE Rudolf Gtirike, 15 Gregor Mendelstrasse, Vienna, Austria The functioning of unidirectional condenser microphones may be explained in two ways, either with the aid of a phase-shifting member on the rear side of the diaphragmthis explanation is more in agreement with the physical processes-or with reference to the superposition of the pressure component and pressure gradient component acting as driving forces on the diaphragm. Whereas the latter explanation is not entirely in accordance 'with the actual physical facts, it is convenient because it is easily understood, .and hence will be adopted hereinafter.
In order to obtain in a unidirectional condenser microphone a suflicient insensitivity to a sound incidence at 180, the pressure and pressure gradient components must be equal in amount and opposed in phase.
Microphone constructions have been disclosed which have given good results with a diaphragm about 3 cm. in diameter. The endeavor to reduce the diaphragm in diameter in order to provide small microphones, however, involves difiiculties which are due to the following phenomena:
In the condenser-type pressure transmitter the sensitivity of the microphone for a constant thickness of the diaphragm is independent of the diameter of the diaphragm as long as the detrimental capacitance is negligible and the working resistance is sufficiently high. For instance, if the diameter of the diaphragm is reduced to one-half, the mass of the diaphragm Will be one-fourth and, for the same resonant frequency, the restoring force will also be one-fourth. Owing to the reduction of the diaphragm area to one-fourth the driving force will also be one-fourth so that the elongation of the diaphragm will be equal and with it the ratio between the change in capacitance and the capacity at rest.
On the other hand, in the condenser-type pressure gradient transmitter the sensitivity varies in direct proportion with the diameter because the driving force depends on the length of the sound path from the front face to the rear face of the diaphragm.
The attempt to reduce the diaphragm diameter for the purpose of providing small microphones, as are particularly desired for television, has involved difiiculties in the fulfilment of the physical requirements for achieving a unidirectional effect. It is known that the vibrating system of an elongation transmitter must be mass-controlled for the sound pressure component and resistancecontrolled for the pressure gradient component. If the diaphragm is reduced in size, the frictional damping must be reduced until the two components become equal in magnitude. This results in anexcessively steep flank of the resonance curve of the frictionally damped system, the resonant frequency of which must lie in the middle of the transmitted frequency range.
The German patent specification No. 924,325 describes a microphone having such action and comprising a diaphragm carrier and a counterelectrode which is separated by an ,air, gap from the diaphragm carrier and behind which an air chamber is preferably provided, the air gap communicating with the outside air through one or more acoustically ineffective openings. The length of the sound path from the front face to the rear face of the diaphragm, which determines the pressure gradient, corresponds only to the diameter of the diaphragm. In such a condenser microphone, a reduction of the diaphragm diameter, for example, to 1.8 cm., results in a decrease in sensitivity for sound incidence at 0 and 180 that is suificient only in the medium frequency range.
Constructions are already known which attempt to overcome this disadvantage by providing around the diaphragm an annular detour disc having a diameter of about 3.2 cm. in order to increase the pressure gradient. However, this results in an increase in the size of the microphone, which is undesirable. For this reason it has been attempted to make the detour disc of a transparent material such as plastics.
The present invention adopts another measure for in: creasing the pressure gradient in condenser microphones without an appreciable increase in the diameter of the microphone. Tests have shown that, the pressure gradient will be increased if the surface of the detour disc is folded rearwardly about the line Where the disc is fixed and a narrow annular air gap is defined between the hollow cylinder formed by the disc and the peripheral surface of the microphone. The width of this air gap must be such that the friction is adequate to prevent the formation of standing waves, on the one hand, and that it provides optimum conditions for the passage of sound, on the other hand.
Thus, in accordance with an aspect of the invention, a condenser microphone having a preferential response in one direction comprises a diaphragm having front and rear faces, which are both exposed to the sound field of the microphone, amicrophone body assembled with and extending rearwardly of said diaphragm, and a hollow structure surrounding said microphone body and defining with the periphery of the latter a narrow annular passage having a length which is approximately equal to the diam eter of said diaphragm and which exposes the rear face of said'diaphragm to said sound field.
The above, and other objects, features and advantages of the invention, will be apparent in the following detailed description of illustrative embodiments thereof which is to be read in connection with the accompanying drawings, wherein:
FIGS. 1 and 2 are frequency response curves for known condenser microphones having diaphragms of different diameters;
FIG. 3 is a-vertical, longitudinal sectional view of a condenser microphone embodying the present invention;
FIG. 4 is a transverse sectional view taken along the line IV-IV of FIG. 3; I
FIGS. 5 and 6 are views similar to that of FIG. 3, but showing two additional embodiments of the invention;
FIGS. 7 and 8 are frequency response curves for microphones of the kind shown in FIG. 6; and
FIG. 9 is a view similar to that of FIG. 6, but showing a detail modification of that embodiment of the invention.
Referring to the drawings in detail, and initially to FIG. 1, it will be seen that the'frequency response curves for a known condenser microphone having a diaphragm diameter of 3.2 cm. show a decrease in sensitivity of at least 12 decibels between a sound incidence of 0 and a microphone While permitting reduction of the dimension of the latter, the embodiment of the invention illustrated in FIGS. 3 and 4 has the rim of the diaphragm 1 affixed, e.g., with adhesive, to the end face of a metal sleeve 2.
' The electrode 3 is closely spaced from the diaphragm 1 and may consist, e.g., of a plate formed with bores.
electrode 3 is held in a sleeve 4 of insulating material inserted in the metal sleeve 2, with the sleeves 2 and 4 forming a hollow structure. A microphone body formed by a cylinder 5 of insulating material is supported in the sleeve 4 by means of spacing arms or radial webs 6 (FIG. 4) to define a narrow annular gap 7, which is interrupted only by the Webs 6 extending parallel to the axis of the cylinder. Thus, the sound waves which enter from the rear into the gap 7 can freely reach the air chamber 9 defined between electrode 3 and microphone body 5 and can pass from chamber 9 through the bores in the electrode 3 to the rear face of the diaphragm.
It is obvious that the annular gap 7, rather than being defined between microphone body 5 and sleeve 4, may alternatively be defined between the outside cylindrical surface of the sleeve 4 and the inside cylindrical surface of the metal sleeve 2. The annular gap 7, rather than being of uniform width, as shown, could be conically enlarged from its end disposed near the diaphragm toward the rear.
In the condenser microphone illustrated in FIG. 5, the diaphragm 10 is aflixed to the front end edge of the metal sleeve 11. The electrode 12 defining an internal cavity 13 is held by a bolt 14 and a nut 15 to the insulating plate 16, which may consist, e.g., of quartz. A cylindrical annular clearance 23 forming a radial inner portion of an annular passage is defined between the outside peripheral surface of the electrode 12 and the inside peripheral surface of the sleeve 11. The quartz plate 16 is secured in the sleeve 11 by means of an annular nut 17 engaging internal threads formed in that sleeve. A hollow structure in the form of a sleeve 18 has an internal annular bead 19, which fits over the sleeve 11 so that an annular gap 20 forming the radially outer portion of an annular passage is formed between the outside surface of the sleeve 11 and the inside surface of the sleeve 18.
The sleeve 11 is provided behind the annular head 19 of sleeve 18 with regularly peripherally spaced radial openings 21 which establish communication between the clearance 23 and gap 20 forming the radially inner and outer portions of the annular passage. Sound entering the back end 22 of the narrow annular gap 20 passes along the latter and through the radial openings 21 to the annular clearance 23, and then along the latter to reach the rear face of the diaphragm at 24. The narrow air gap between the diaphragm 10 and the electrode 12 serves as an acoustic frictional resistance. In the microphone of FIG. 5, the electrode 12, plate 16 and that portion of sleeve 11 which extends behind electrode 12 form a microphone body, while that portion of sleeve 11 which surrounds the electrode 12 forms a partition between gap 20 and clearance 23.
The embodiment of FIG. 6 comprises the same elements as the microphone of FIG. 5 and additional means for connecting the cavity 25 behind the electrode through an acoustic frictional resistance to the outside air at the rear of the microphone. To this end, a disc 26 having holes 27 is formed with a forward annular extension 28 screwed into the electrode 30. A disc 31 is disposed in back of disc 26 and has a rearwardly directed screw-threaded sleeve 32 in threaded engagement with the holder 33 for the electrode 30 and an air gap 34 acting as an acoustic frictional resistance is defined between the disc 26 and the disc 31. Sound waves entering from the rear through the opening 35 of sleeve 32. can pass through the acoustic frictional resistance defined by gap 34 into the cavity 25 and from the latter through the bores of electrode 30 to the rear face of the diaphragm. This additional driving force acting on the rear face of the diaphragm effects a greater decrease in sensitivity for higher frequencies incident at 180.
According to the theory of the effect of the viscosity of the air in narrow tubes, in which the wall exercises a great resistance on the air in contact therewith, the damping effected by the viscosity influences the velocity of the sound wave.
The dimensions of the annular passage for the sound may be selected to provide such an effect that, in the case of sound coming from the rear, the wave which travels around the diaphragm and reaches the front face of the microphone is approximately equal in amplitude and opposed in phase to the wave which passes through the annular passage to the rear side of the diaphragm. The narrow passage for the sound has the additional advantage that standing waves (pipe resonances) are avoided by an adequate friction on the Walls of the sound passage. 7
The length of the sound passage is preferably selected to correspond approximately to half the wavelength of the lowest frequency at which the diaphragm has a directional effect. This corresponds to a length which is approximately equal to the diameter of the diaphragm. The internal width of the annular sound passage of a practical embodiment was 0.3 mm. and good results were obtained when the width was varied between 0.5 mm. and 0.1 mm.
A microphone constructed according to FIG. 6 has the frequency response curves shown in FIG. 7 for a sound incidence at 0 and respectively.
Slight irregularities may occur in the frequency response curve for sound incidence at 0 due to interference effects in the case of short wavelengths. FIG. 8 shows such frequency response curve for a small microphone according to FIG. 6, with existing irregularities. An increase of about 2 decibels is observed at a frequency in the range between 4000 cycles and 10,000 cycles, and a drop of about 3 decibels occurs at a frequency adjacent that of increase. 1 I 5 In order to eliminate this disadvantage the sleeve which defines the annular passage is formed according to the invention with one or more small radial openings, so that the sound waves may enter the annular passage at the back end and also through these radial openings and the length of the sound path between the front and rear faces of the diaphragm is varied so that the pressure gradient as a driving force is equalized.
Thus, as shown in FIG. 9, in a microphone of the type described with reference to FIG. 6, the sleeve 18 which defines the outer wall surface of the annular gap 20 opening at the back end, as at 22, is formed with radial openings 36. The equalizing effect of these openings is influenced by their number, form, size and arrangement, and the optimum effect can be empirically determined.
1. A condenser microphone having a preferential response in one direction and comprising a diaphragm having a front face directly exposed to the sound field and a rear face, a microphone body extending rearwardly of said diaphragm, a hollow structure surrounding said microphone body and having a front face secured to the periphery of said diaphragm, a perforated electrode spaced behind said diaphragm and rigidly connected to said hollow structure, and means radially spacing said hollow structure from said microphone body to define an annular passage therebetween, said rear face of the diaphragm being exposed to the sound field by way of said annular passage, said annular passage having a length which is approximately equal to the diameter of said diaphragm, and the radial thickness of said annular passage being between 0.1 mm. and 0.5 mm.
2. A condenser microphone as in claim 1, further comprising means cooperating with the rear side of said electrode to define an air chamber therebetween, said annular passage communicating with said air chamber.
3. A condenser microphone having a preferential response in one direction and comprising a diaphragm having a front face directly exposed to the sound field and a rear face, a microphone body extending rearwardly of said diaphragm, a hollow structure surrounding said microphone body and having a front end face secured to the periphery of said diaphragm, a perforated electrode spaced behind the diaphragm and rigidly connected to said hollow structure, and spacing means radially spacing said hollow structure from said microphone body to define an annular passage therebetween, said annular passage having a length which is approximately equal to the diameter of said diaphragm, said rear face of the diaphragm being exposed to the sound field by way of said annular passage, said spacing means including a tubular partition extending between said microphone body and hollow structure and dividing said annular passage into radially inner and outer portions, said tubular partition having radial openings establishing communication between said radially inner and outer portions of the annular passage,
said radially inner portion being open at its front end to said rear face of the diaphragm and closed at its rear end, said radially outer portion being longer than said radially inner portion and being closed at its front end and open at its rear end to the sound field.
4. A condenser microphone as in claim 1; wherein said annular passage has a radial thickness of approximately 0.3 mm.
5. A condenser microphone as in claim 1; further comprising means cooperating with said rear face of the diaphragm to define a low-volume air chamber therebetween, and acoustic frictional resistance means bypassing said annular passage and exposing said low-volume air chamber to the sound field at the back of the microphone.
References Cited in the file of this patent UNITED STATES PATENTS 2,297,211 Gerlach Sept. 29, 1942 2,445,821 Brewer July 27, 1948 2,787,671 Grosskopf et a1. Apr. 2, 1957 2,852,620 Schoeps et a1. Sept. 16, 1958 FOREIGN PATENTS 420,390 Great Britain Nov. 30, 1934 1,084,139 France Ian. 17, 1955 1,088,157 France Mar. 3, 1955