US3082298A

US3082298A - Frequency independent directional condenser microphone

Info

Publication number: US3082298A
Application number: US10718A
Authority: US
Inventors: Gorike Rudolf
Original assignee: AKG Akustische und Kino Geraete GmbH
Current assignee: AKG Acoustics GmbH
Priority date: 1959-03-04
Filing date: 1960-02-24
Publication date: 1963-03-19
Anticipated expiration: 1980-03-19
Also published as: DE1121116B; GB911866A; NL249090A; AT209396B

Description

March 19, 1963 R. GORIKE 3,082,298

FREQUENCY INDEPENDENT DIRECTIONAL CONDENSER MICROPHONE Filed Feb. 24, 1960 3 Sheets-Sheet 1 H6. 1 DIRECTIONAL PA T7RN F01? PRIOR ARI cwvM/vsm MICROPHONE FREQUENCY RESPONSE (URI/E6 F01? PRIOR RT H6 2 comm/W51? MICROPHDNE A7 VAR/0V5 ANGLES 6F sou/v0 INCIDENCE 0 45" 90 150" IN VENTOR ATTORNEY R. GORIKE March 19, 1963 FREQUENCY INDEPENDENT DIRECTIONAL CONDENSER MICROPHONE Filed Feb. 24, 1960 3 Sheets-Sheet 2 INVENTOR Papa/.1" I M ATTORNEY March 19, 1963 R. GORIKE 3,082,298

FREQUENCY INDEPENDENT DIRECTIONAL CONDENSER MICROPHONE Filed Feb. 24. 1960 3 Sheets-Sheet 3 00 I INVENTOR @ZOl-F 66!!!! BY QM. f -ma;

ATTORNEY atent Patented Mar. 19, 1963 time The known directional condenser microphones have only a small self-capacitance (about 30400 picofarads). To obtain an adequate output voltage it is necessary, therefore, to combine such microphones in a unit with the first amplifier stage or to connect them only by very short leads with the associated amplifier. This results in one case in a relatively large microphone casing and in the other case in a reduction of the output voltage if the cable capacitance approaches the self-capacitance of the microphone. An increase of the dimensions of such a microphone button for a directional condenser microphone with the aim of increasing the self-capacitance has been opposed by the fact that the diameter of the button must be approximately of the order of magnitude of the Wavelength of the highest frequency to be transmitted in order to obtain a directional pattern which is largely independent of frequency because only in this case is the pressure gradient at the diaphragm linear up to the highest frequency to be transmitted. In accordance therewith the usual directional condenser micro phones have a button about 1.5-3 centimeters in diameter and consequently only a low microphone capacitance.

On the other hand, condenser microphones have been disclosed which have a diaphragm area of about 40 square centimeters and a capacity of up to about 1000 picofarads.

Iowever, these microphones designed as pressure transmitters do not have a frequency-independent directional pattern. Owing to their high self-capacitance they may be arranged at a distance from the amplifier and may be connected to the latter by a long cable (up to meters and more).

As contrasted therewith the invention relates to a directional condenser microphone which has a frequencyindependent directional pattern as well as a high selfcapacitance and which for this reason is particularly suitable for use with portable tape recorders because, as in the case of a dynamic microphone, it may be connected to the recording apparatus by a long cable whereas, compared with the dynamic microphone, it has the advantage that sound phenomena taking place at high speed (transients) are transduced more accurately. The directional condenser microphone according to the invention belongs to the type in which the diaphragm is exposed to the sound field on both sides and at least one phase shifting member is provided to provide for directional effect. The microphone according to the invention is characterized in that the diaphragm has an area of at least 10 square centimeters and that the maximum dimension of the diaphragm, that is, the largest side of the diaphragm, in the case of a rectangular diaphragm, or the diameter of the diaphragm, in the case of a circular diaphragm, is much larger than the wavelength of the highest frequency of the transmitted range and that the upper limiting frequency of the phaseshifting element approximates the frequency at which the pressure gradient reaches its highest value.

If a bidirectional eifect is desired, frictional acoustic resistance is provided between the diaphragm and electrode either alone or in combination with frictional acoustic resistance disposed directly on the rear of the cathode.

In the drawings which partly illustrate the state of the art and partly illustrate embodiments of the invention,

FIG. 1 shows the directional pattern of a condenser microphone which is designed as a pressure transmitter and has a large diaphragm in accordance with the prior art,

FIG. 2 shows frequency response curves of the conventional condenser microphone for various angles of sound incidence,

FIG. 3 is an exploded view showing an illustrative embodiment of the invention having a unidirectional pattern,

FIG. 4 shows the microphone formed from the parts of FIG. 3 in assembled condition,

FIG. 5 shows an illustrative embodiment of the present invention having two diaphragms,

FIG. 6 shows a microphone according to the invention having a bidirectional pattern,

FIG. 7 shows a microphone according to the invention having a prearranged diaphragm for protection against moisture,

FIG. 8 shows a microphone embodying the inventionwhich can be changed over from a unidirectional pattern to an omnidirectional pattern,

FIG. 9' shows a microphone according to the invention for stereophonic recording,

FIG. 10 shows the directional pattern of the microphone illustrated by FIG. 9,

FIG. 11 shows a modification of the microphone shown in FIG. 9,

FIG. 12 shows the directional pattern of the micro phone of FIG. 11,

FIGS. 13 and 14 show frequency response curves in rectangular coordinates for microphones in accordance with this invention,

FIG. 15 is a showing of the frequency response curves in polar coordinates,

FIG. 16 shows a microphone in accordance with this invention having an elongated, narrow diaphragm.

FIG. 17 shows the associated directional pattern for higher frequencies, and

FIG. 18 shows a microphone embodying the invention in the form of a hexagonal pencil.

The subject matter of the invention will be explained more fully with reference to the above-mentioned figures.

As has already been stated initially hereinbefore, condenser microphones constituting pressure transmitters having a large diaphragm are already known. FIG. 1 shows in polar coordinates, for different frequencies, the directional pattern of a microphone having a diaphragm size of 4.5 X 9 centimeters. It is apparent that the directional effect increases with an increase in frequency. FIG. 2 shows the frequency curves of the same microphone for various angles of sound incidence in polar coordinates.

. With such a transmitter the sound character of the sound recording will depend on the distance of the sound source from the microphone because the ratio of direct to indirect sound becomes dependent on distance. Besides, the sound recordings made with such microphone are unnatural owing to an exaggerated reverberation. For this reason it has become generally accepted that the transmitter must be small relative to the wavelength of the highest frequency to be transmitted in order to obtain a frequency-independent directional pattern.

The invention discards this opinion because the measures according to the invention, which will be described hereinafter, enable even a condenser microphone having a relatively very large diaphragm to be designed for a frequency-independent directional pattern. The invention relates to a-microphone the diaphragm of which is exposed to the sound field on both sides and which has a sufliciently frequency-independent directional pattern in spite of the large area of the diaphragm. For this reason a perfect sound recording can be made even in rooms having poor acoustic characteristics. It has also been found that cables having a length of up to 20 meters resulted in a loss of only about 4 decibels. This does not involve a modification of the frequency response, as is known, because only a voltage division occurs between the microphone capacitance and the cable capacitance.

The invention provides further the use of a second diaphragm, which may be electrically effective or ineffective. In the former case different directional patterns can be adjusted in a manner known per se by a change of the polarizing voltage.

By the provision of two diaphragms the microphone may be converted to a stereophonic microphone operating on two separate channels.

Besides, the microphone may also be used as a speaker, e.g., for dictating apparatus. Owing to the high capacitance of the microphone a transformer may be connected thereto or a transistor amplifier may be used which is assembled with the microphone to form a unit and which may also generate the polarizing voltage and which generates such a high audiofrequency output voltage that the alternating voltage can be transmitted on unshielded lines.

It is known that the directional pattern of a pressure transmitter is spherical as long as the wavelength of the sound is small relative to the dimensions of the transmitter. Beyond this range the pressure builds up to provide a directional effect which increases with the frequency. It is also known that the pressure gradient as a driving force in a diaphragm exposed to the sound field on both sides rises linearly until half the wavelength of the sound approximately equals the sound detour around the microphone, i.e. the sound path from the center of the front side of the diaphragm to the center of the rear side of the diaphragm. The two effects described are obtained almost at the same frequency so that the microphone begins to show a directional effect due to a build-up of pressure where the pressure gradient reaches its highest value. As the limits are not definite, the transition is not strictly defined so that it appears to be more suitable to consider the phenomena which occur considerably above and below the critical frequency and then to draw conclusions regarding the behavior at the limiting frequency.

At a frequency at which the sound detour corresponds approximately to the wavelength the driving force effective at the diaphragm due to the pressure gradient is theoretically zero. At the same frequency, however, the build-up of pressure on the front side of the diaphragm is already effective so that the resultant driving force at the diaphragm is larger than zero. Practical experiments have shown that disturbing effects are virtually absent. With a diaphragm having an area of about 40 square centimeters the critical frequency is about 2000 cycles per second.

As is apparent from FIGS. 1 and 2 a unidirectional effect is already obtained above 2000 cycles per second. For this reason measures must be taken to provide for a unidirectional effect below 2000 cycles per second. If the diaphragm has a bidirectional pattern the directional effects are obtained above approximately 2000 cycles per second on both sides of the diaphragm.

In connection with small microphones, it is known to provide an unidirectional pattern by an RC member and to provide a bidirectional effect by an R member disposed in back of the diaphragm. This is also utilized according to the invention but these measures are effective according to the invention only in the frequency range below approximately 2000 cycles per second.

In FIG. 3 a first illustrative embodiment of the invention is shown. The figure shows the components of a condenser microphone having a unidirectional pattern. One or more layers of a sheetlike acoustic frictional resistance 2, e.g., filter paper, are inserted in a sheet metal casing 1 having a perforated bottom. A trough 3 consisting, e.g., of plastic and having also a perforated bottom is placed on this resistance. An elastic sealing frame 4 of plastic is inserted in the trough. The perforated electrode 5 with parallel spacing threads 6 consisting, e.g., of beads having a thickness of about 20 microns, rests on tional pattern for 0, 90 and 180.

the sealing frame 4. The diaphragm 7 is placed on the electrode 5 and stressed by being adhesively connected at its rim. On the side facing away from the electrode the diaphragm is vapor-coated with a metal. The front cover 8 of the housing with the protective grid or with openings forms the closure. The cable 9, which may be plastic insulated and have a shielded core and a length of about 1.5 meter or more leads to the amplifier.

In FIG. 4 the microphone of FIG. 3 is diagrammatically shown in assembled condition. The diaphragm 7 is supported by the threads 6 and by the rim of the electrode 5. The cavity 13 and the frictional acoustic resistance 2 form the phase-shifting member, which according to the invention is effective at frequencies below 2000 cycles per second and provides for a unidirectional pattern in this range.

FIG. 5 shows a microphone having two

diaphragms

7a and 7b. One or both of said diaphragms may be electrically effective. It is known that different directional patterns can be set by electrical remote control where two electrically conducting diaphragms are provided. A frictional resistance 1:) may be arranged between the electrodes 5a and 51').

A microphone having a bidirectional pattern is shown in FIG. 6. The diaphragm 17 of this microphone contains only a perforated electrode 18 as an acoustically effective element. The frictional resistance is formed between the diaphragm and the perforated electrode. An additional frictional resistance 19 may be provided. For protection against moisture, a microphone as shown in FIG. 4 may be provided with a slack, thin resilient diaphragm 20 (FIG. 7). To vary the directional pattern it is possible in accordance with the invention to provide a mechanically actuable slide 21 (FIG. 8). When the openings of the housing 22 are closed the undirectional pattern is changed to an omnidirectional pattern. The latter pattern, however, has the above-described disadvantages of being dependent on frequency, although this may he desirable for achieving certain acoustic effects. The mechanism may also serve to obtain the highest fidelity.

The microphone according to FIG. 5 may also be used for stereophonic recording if the two diaphragms are connected to separate transmission channels I and II, as is shown in FIG. 9. En this case the microphone is oriented so that the diaphragm surfaces face the sound source. The directional pattern is shown in FIG. 10. For stereophonic recording two individual microphones according to the invention may also be used which are disposed close to each other for intensity recording and for AB stercophonic recording are spaced from each other and arranged at an angle of relative to each other.

The diaphragms may be inclined relative to each other. This is an arrangement which has already been provided in small microphones (FIG. 11). The patterns will obviously also be inclined, as is shown in FIG. 12. In FIG. 13 the frequency response curves are shown which are obtained with the microphones according to the invention for a sound incidence at 0, 45, 90, and whereas FIG. 14 shows the frequency response curves for a microphone according to the invention having a bidirec- FIG. 15 shows in polar coordinates the characteristics of a unidirectional microphone according to the invention. Without departing from the invention the diaphragm may have any desired proportions. For instance, a circular or oval form may be chosen provided that in accordance with the invention the area of the diaphragm is sufficient to ensure a capacitance of approximately 1500 picofarads or more. If the shape according to FIG. 16 is chosen a directional pattern as shown in FIG. 17 is obtained for the higher sound frequencies. This may be desirable for certain applications. FIG. 18 is an enlarged view showing a condenser microphone in the form of, a hexagonal pencil. A hexagonal sleeve 21 is perforated on all side faces and serves as an electrode. It is wound with spacing threads 22 and the diaphragm 23- is stretched on the outside over these threads on three adjacent side surfaces. The three side faces opposite to the diaphragm 23 are lined inside the sleeve with filter paper 24, a textile insert or the like forming an acoustic resistance. The ends of the sleeve are closed by plugs, not shown.

I claim:

1. A directional condenser microphone for transmitting a predetermined frequency range; comprising at least one diaphragm having front and rear faces with a minimum area of square centimeters and a maximum dimension substantially larger than the wavelength of the highest frequency of said range, both of said facesbeing exposed to the action of the sound field; support means carrying said diaphragm; means constituting a frictional acoustic resistance; said support means, said rear face of the diaphragm and said means constituting a frictional acoustic resistance defining a cavity; and said cavity and said frictional acoustic resistance being operative as a phase-shifting element only up to a predetermined frequency in said range at which one-half of the wavelength of sound is less than the effective length of the sound path from the center of said front face of the diaphragm around the microphone to the center of said rear face of the diaphragm.

2. A directional condenser microphone as in claim 1; wherein said support means includes a perforated electrode disposed at the rear face of the diaphragm and having spacing threads on the surface of the electrode facing toward said diaphragm, said diaphragm being secured, at the periphery thereof, to said electrode.

3. A directional condenser microphone as in claim 2; wherein said means constituting a frictional acoustic resistance includes porous sheet material disposed at the side of said electrode facing away from said diaphragm.

4. A directional condenser microphone as in claim 3; wherein said support means further includes means spacing said sheet material from said electrode.

5. A directional condenser microphone as in claim 3; wherein said sheet material is against said side of the electrode facing away from the diaphragm.

6. A directional condenser microphone as in claim 3; wherein said sheet material is filter paper.

7. A directional condenser microphone as in claim 3; wherein said sheet material is textile fabric.

8. A directional condenser microphone as in claim 3; wherein said perforated electrode is in the form of a hollow sleeve having said spacing threads wound thereon, said diaphragm covering a portion of the outer surface of said perforated sleeve, and said sheet material covering another portion of the surface of said sleeve.

9. A directional condenser microphone as in claim 8; wherein said sleeve is of hexagonal cross-section and has six flat outer surface areas, said diaphragm covering three of said surface areas and said sheet material covering the remaining three of said surface areas.

10. A directional condenser microphone as in claim 2; wherein said spacing threads are of silk.

11. A directional condenser microphone as in claim 2; wherein said spacing threads are of plastic.

12. A directional condenser microphone as in claim 1; further comprising an additional diaphragm disposed at the side of said phase-shifting element remote from the first mentioned diaphragm.

13. A directional condenser microphone as in claim 12; wherein said additional diaphragm is electrically ineffective.

References Cited in the file of this patent UNITED STATES PATENTS 1,753,137 Seibt Apr. 1, 1930 1,776,112 Edelman Sept. 16, 1930 2,387,845 Harry Oct. 30, 1945 2,686,847 Aamodt Aug. 17, 1954 2,787,671 Grosskopf et al Apr. 2, 1957 2,852,620 Schoeps et al. Sept. 16, 1958 2,920,140 Morgan Jan. 5, 1960 FOREIGN PATENTS 571,870 Great Britain Sept. 12, 1945 884,516 Germany July 27, 1953 920,311 Germany Nov. 18, 1954 OTHER REFERENCES Acoustics, Beranek, 1954, pp. 149- 150.

Claims

1. A DIRECTIONAL CONDENSER MICROPHONE FOR TRANSMITTING A PREDETERMINED FREQUENCY RANGE; COMPRISING AT LEAST ONE DIAPHRAGM HAVING FRONT AND REAR FACES WITH A MINIMUM AREA OF 10 SQUARE CENTIMETERS AND A MAXIMUM DIMENSION SUBSTANTIALLY LARGER THAN THE WAVELENGTH OF THE HIGHEST FREQUENCY OF SAID RANGE, BOTH OF SAID FACES BEING EXPOSED TO THE ACTION OF THE SOUND FIELD; SUPPORT MEANS CARRYING SAID DIAPHRAGM; MEANS CONSTITUTING A FRICTIONAL ACOUSTIC RESISTANCE; SAID SUPPORT MEANS, SAID REAR FACE OF THE DIAPHRAGM AND SAID MEANS CONSTITUTING A FRICTIONAL ACOUSTIC RESISTANCE DEFINING A CAVITY; AND SAID CAVITY AND SAID FRICTIONAL ACOUSTIC RESISTANCE BEING OPERATIVE AS A PHASE-SHIFTING ELEMENT ONLY UP TO A PREDETERMINED FREQUENCY IN SAID RANGE AT WHICH ONE-HALF OF THE WAVELENGTH OF SOUND IS LESS THAN THE EFFECTIVE LENGTH OF THE SOUND PATH FROM THE CENTER OF SAID FRONT FACE OF THE DIAPHRAGM AROUND THE MICROPHONE TO THE CENTER OF SAID REAR FACE OF THE DIAPHRAGM.