JP2022120305A - Capacitor microphone - Google Patents

Abstract

To provide a back electret-type capacitor microphone that can still suppress static pressure characteristics from deteriorating even when reducing stiffness of a vibration film.SOLUTION: A capacitor microphone 10 comprises: a cylindrical enclosure 11; a vibration film 12 that blocks an opening part 15 at a cylindrical end of the enclosure 11; a back electrode 13 which has an electrode main body 21 having an electret film 24 arranged to oppose to the vibration film 12 in the enclosure 11 and a rod part 22 extending from the electrode main body 21 toward an opposite side of the vibration film 12; and a first insulation member 14, fixed to the enclosure 11, which lies between an inner peripheral surface of the enclosure 11 and an outer peripheral surface of the rod part 22 and supports the back electrode 13. When a space between the vibration film 12 and the first insulation member 14 is set as a back air chamber 35 and a frequency range of sound waves that can be measured is set as a frequency through design, a length H of the back air chamber which is an interval between the vibration film 12 and the first insulation member 14 is larger than a half-wave length of a sound wave corresponding to an upper-limit frequency that is an upper limit of the frequency through design.SELECTED DRAWING: Figure 1

Description

The present invention relates to a back electret condenser microphone.

2. Description of the Related Art Measurement microphones are known as detectors that are used to accurately measure the volume of sound (sound pressure) in soundproofing measures, noise measurement, and the like. As such a measurement microphone, for example, there is a back electret condenser microphone as disclosed in Patent Document 1.

A back electret condenser microphone has a tubular housing, a vibrating membrane that closes a tubular end opening of the housing, and a back electrode arranged in the housing. The back electrode has an electrode body having an electret film arranged to face the vibrating membrane, and a rod portion extending from the central portion of the electrode body toward the opposite side of the vibrating membrane. The back electrode is fixed to the housing on the side opposite to the vibrating membrane with respect to the electrode main body, and is supported by an insulating member interposed between the inner peripheral surface of the housing and the outer peripheral surface of the rod portion. In a condenser microphone, when the vibrating membrane vibrates due to external sound pressure, the distance between the vibrating membrane and the electret membrane changes, and the capacitance changes accordingly. Then, the sound pressure is measured by detecting the change in the capacitance.

JP 2015-192167 A

The sensitivity of the back electret condenser microphone described above can be increased by increasing the electret potential and reducing the tension of the diaphragm, thereby reducing the stiffness of the diaphragm (= tension of diaphragm/diameter of diaphragm). is. However, if the stiffness of the vibrating membrane is reduced, the effect of the stiffness of the space (back air chamber) between the vibrating membrane and the insulating member (= volume of the back air chamber/bulk modulus of air in the back air chamber) cannot be ignored. Gone. Specifically, when the atmospheric pressure fluctuates, the stiffness of the back air chamber changes, and accordingly the sensitivity of the condenser microphone changes. That is, the static pressure characteristics of the condenser microphone deteriorate. Therefore, in the back electret condenser microphone, a technique is desired that can suppress the deterioration of the static pressure characteristics even if the stiffness of the vibrating membrane is reduced.

A back electret condenser microphone that solves the above problems has a cylindrical housing, a vibrating membrane that closes a cylindrical end opening of the housing, and an electret membrane that is arranged inside the housing so as to face the vibrating membrane. a back electrode having an electrode main body and a rod portion extending from the electrode main body toward the opposite side of the vibrating membrane; fixed to the housing and between an inner peripheral surface of the housing and an outer peripheral surface of the rod portion; and an insulating member that is interposed in and supports the back electrode. When the space between the vibrating membrane and the insulating member is the back air chamber, and the frequency range of the measurable sound wave is the design frequency, the length of the back air chamber, which is the distance between the vibrating membrane and the insulating member, is the design frequency. It is larger than the half wavelength of the sound wave corresponding to the upper frequency limit, which is the upper frequency limit.

According to the above configuration, since the length of the back air chamber is longer than half the wavelength of the sound wave corresponding to the upper limit frequency, the volume of the back air chamber is larger than that in the case where the length of the back air chamber is less than or equal to half the wavelength of the sound wave corresponding to the upper limit frequency. can be increased. As a result, since the stiffness of the back air chamber is reduced, deterioration of static pressure characteristics can be suppressed even if the stiffness of the vibrating membrane is reduced.

In the condenser microphone configured as described above, it is preferable that the insulating member is a first insulating member, and that a second insulating member having air permeability is provided between the electrode main body and the first insulating member.

When sound pressure acts on the vibrating membrane from the outside, sound waves caused by the sound pressure propagate through the back air chamber. Further, when the length of the back air chamber is made larger than half the wavelength of the sound wave corresponding to the upper limit frequency, the vibrating membrane resonates under the influence of the reflected wave of the sound wave propagating in the back air chamber and reflected by the first insulating member. (resonance phenomenon). In this respect, according to the above configuration, the second insulating member is arranged between the electrode main body and the first insulating member. This makes it difficult for the sound wave propagating in the back air chamber to reach the first insulating member and makes it difficult for the reflected wave reflected by the first insulating member to reach the vibrating membrane. You can reduce the impact. As a result, deterioration of measurement accuracy due to reflected waves can be suppressed while suppressing deterioration of static pressure characteristics.

In the condenser microphone configured as described above, it is preferable that the second insulating member has a sound absorbing property.
Since the second insulating member has a sound absorbing property as in the above configuration, not only sound waves reaching the first insulating member but also reflected waves reflected by the first insulating member can be attenuated. As a result, it is possible to reduce the influence of the reflected wave on the vibration of the vibrating membrane, so that there is an effect of suppressing the resonance phenomenon of the vibrating membrane.

In the condenser microphone configured as described above, it is preferable that the second insulating member includes a reflector that partitions the back air chamber into two or more spaces communicating with each other.
According to the above configuration, part of the sound wave propagating in the back air chamber can be reflected by the reflecting plate before reaching the first insulating member. As a result, the proportion of sound waves reflected by the first insulating member in the sound waves propagating in the back air chamber can be reduced. As a result, the influence of the reflected wave on the vibration of the vibrating membrane can be reduced. Furthermore, since the second insulating member includes the sound absorbing material and the reflecting plate, it is possible to more effectively suppress the influence of the reflected waves on the vibration of the vibrating membrane.

In the condenser microphone configured as described above, the reflector is provided so as to be supported by the rod portion and in contact with the inner peripheral surface of the housing, and has a communication hole penetrating through the reflector. Preferably, the diameter of the holes is less than 1/10 of the wavelength corresponding to the upper limit frequency.

According to the above configuration, the propagation path of sound waves propagating toward the first insulating member side of the reflector can be limited to the communication hole. Further, since the diameter of the communication hole is less than 1/10 of the wavelength corresponding to the upper limit frequency, it is possible to further effectively suppress the influence of the reflected wave on the vibration of the vibrating membrane.

In the condenser microphone configured as described above, the rod portion is composed of a plurality of partial rods that are linearly connected, and the reflecting plate has a through hole into which the rod portion is inserted, and the reflecting plate is composed of two portions that are connected to each other. It is preferable that it is supported by the rod portion by being clamped by the rod.

According to the above configuration, the reflecting plate can be assembled to the rod portion of the back pole by connecting the two partial rods to be connected and the two partial rods while aligning the positions of the reflecting plates. . Therefore, by supporting the back electrode on the housing through the first insulating member, the reflector can be easily arranged at a predetermined position in the back air chamber.

1 is a cross-sectional view showing a schematic configuration of a first embodiment of a condenser microphone; FIG. FIG. 5 is a cross-sectional view showing a schematic configuration of a second embodiment of a condenser microphone; FIG. 5 is a cross-sectional view showing a schematic configuration of a condenser microphone according to a third embodiment; FIG. 11 is an exploded perspective view for explaining a method of assembling a back electrode having a reflector in the third embodiment; The perspective view which shows an example of a reflector in a modification.

(First embodiment)
A first embodiment of a condenser microphone will be described with reference to FIG.
As shown in FIG. 1, a condenser microphone (hereinafter simply referred to as a microphone) 10 includes a housing 11, a vibrating membrane 12, a back electrode 13, and a first insulating member .

The housing 11 has a substantially cylindrical shape. The housing 11 is made of metal, for example. The housing 11 is provided with mounting portions for mounting various members.
The vibration film 12 closes the cylinder end opening 15 at the first end of the housing 11 . The vibrating membrane 12 is, for example, a thin metal membrane. The vibrating membrane 12 is protected by a grid 16 covering the first end of the housing 11 so as to prevent external contact. The grid 16 is provided with a plurality of grid through-holes 17 so that sound pressure from the outside acts correctly on the vibrating membrane 12 .

The back electrode 13 is arranged inside the housing 11 . The back electrode 13 has conductivity. The back electrode 13 has an electrode main body 21 and a rod portion 22 .
The electrode body 21 has a substantially disk shape. The electrode body 21 is arranged such that its central axis coincides with the central axis of the housing 11 and its outer peripheral surface is separated from the inner peripheral surface of the housing 11 .

The electrode body 21 has a plurality of body through holes 23 . The plurality of main body through-holes 23 reduce the air resistance between the electrode main body 21 and the vibrating membrane 12 so that the vibrating membrane 12 can accurately vibrate against sound pressure from the outside.

The electrode body 21 has an electret film 24 arranged to face the vibrating film 12 . In the electret film 24 , when the vibrating film 12 vibrates due to external sound pressure, the capacitance changes according to the distance from the vibrating film 12 .

The rod portion 22 extends from the central portion of the electrode body 21 toward the opposite side of the diaphragm 12 . The rod portion 22 is arranged such that its central axis coincides with the central axis of the housing 11 . The rod portion 22 has a terminal attachment portion 25 and an enlarged diameter portion 26 . The terminal mounting portion 25 is provided at the end opposite to the electrode main body 21 . An output terminal 27 that outputs the capacitance of the electret film 24 to a signal line (not shown) is attached to the terminal attachment portion 25 . The output terminal 27 is attached to the terminal attachment portion 25 by screwing, for example. The enlarged diameter portion 26 is provided at a position closer to the terminal attachment portion 25 . The rod portion 22 is supported by the housing 11 via the first insulating member 14 sandwiched between the output terminal 27 and the enlarged diameter portion 26 and a lock nut 32 to be described later.

The first insulating member 14 is arranged inside the housing 11 . The first insulating member 14 has an annular shape. The first insulating member 14 has an outer diameter larger than that of the electrode body 21 . The first insulating member 14 has a rod through-hole 28 through which the terminal attachment portion 25 penetrates in the central portion thereof. The first insulating member 14 is attached to the back electrode 13 by attaching the output terminal 27 to the terminal attachment portion 25 with the terminal attachment portion 25 penetrating the rod through hole 28 .

The first insulating member 14 is attached to the housing 11 while being attached to the back electrode 13 . In other words, the back electrode 13 is attached to the housing 11 in an assembled state in which the first insulating member 14 and the output terminal 27 are assembled to the rod portion 22 .

A fitting portion 31 to which the first insulating member 14 is fitted is formed in the housing 11 . The first insulating member 14 is fitted into the fitting portion 31 by inserting the back electrode 13 from the electrode main body 21 side into the housing 11 from the opposite side of the vibrating membrane 12 . The first insulating member 14 is assembled to the housing 11 with a lock nut 32 arranged inside the housing 11 . Thereby, the back electrode 13 is supported by the housing 11 with the first insulating member 14 interposed between the outer peripheral surface of the rod portion 22 and the inner peripheral surface of the housing 11 .

In the microphone 10 having such a configuration, the vibrating membrane 12 vibrates according to the sound pressure acting from the outside. When the vibrating membrane 12 vibrates, the capacitance between the vibrating membrane 12 and the electret membrane 24 changes according to the distance between the vibrating membrane 12 and the electret membrane 24 . Then, the sound pressure is measured by detecting the change in the capacitance. Here, the frequency range of sound waves measurable by the microphone 10 is called the design frequency. Of the design frequencies, the lower limit of the measurable frequency is called the lower limit frequency f1, and the upper limit of the measurable frequency is called the upper limit frequency f2.

In the state where the back electrode 13 is assembled to the housing 11, the space between the vibrating membrane 12 and the first insulating member 14 is called a back air chamber 35, and the gap between the vibrating membrane 12 and the first insulating member 14 is called a back air chamber. It is called air chamber length H. The back air chamber length H is set to a length longer than the half wavelength λ2 of the sound wave corresponding to the upper limit frequency f2. The half-wavelength λ2 is a half-wavelength value obtained by dividing the speed of sound c in the standard atmosphere by the upper limit frequency f2.

(Action)
According to the microphone 10 described above, since the back air chamber length H is longer than the half wavelength λ2 of the sound wave corresponding to the upper limit frequency f2, the back air chamber 35 is larger than the case where the back air chamber length H is equal to or less than the half wavelength λ2. Volume can be increased. As a result, the stiffness of the back air chamber (=the volume of the back air chamber/the bulk modulus of air in the back air chamber) is reduced. As a result, even if the stiffness of the vibrating membrane 12 (=tension of the vibrating membrane/diameter of the vibrating membrane) is reduced, the degree of influence of static pressure can be reduced, and deterioration of static pressure characteristics can be suppressed.

The inventors conducted an experiment on sensitivity using a microphone whose back air chamber length H is equal to or less than half wavelength λ2 as a comparison object. According to the experimental results, the back air chamber length H is about three times the length of the back air chamber of the comparison target microphone. I was able to

Effects of the first embodiment will be described.
(1-1) By making the length H of the back air chamber larger than the half-wavelength λ2 of the sound wave corresponding to the upper limit frequency f2, it is possible to improve the sensitivity of the microphone 10 and suppress deterioration of static pressure characteristics.

(1-2) By setting the back air chamber length H to about three times the length of the back air chamber of the microphone to be compared, the deterioration of the static pressure characteristics can be suppressed while obtaining the sensitivity equal to or higher than that of the comparison target. can be done.

(Second embodiment)
A second embodiment of the microphone will be described with reference to FIG. The microphone of the second embodiment has the same main configuration as the microphone of the first embodiment. Therefore, in the second embodiment, portions different from the first embodiment will be described in detail, and portions similar to those in the first embodiment will be denoted by the same reference numerals, and detailed description thereof will be omitted.

In the microphone 10 of the first embodiment, deterioration of the static pressure characteristics is suppressed by making the back air chamber length H larger than the half wavelength λ2 of the sound wave corresponding to the upper limit frequency f2. However, if the back air chamber length H is greater than the half wavelength λ2, there is concern that the reflected wave reflected by the first insulating member 14 will affect the vibration of the vibrating membrane 12, causing a resonance phenomenon within the design frequency. . The microphone 10 of the second embodiment suppresses deterioration of the static pressure characteristics and suppresses the influence of the reflected wave reflected by the first insulating member 14 on the vibration of the vibrating membrane 12 .

As shown in FIG. 2, microphone 10 has a second insulating member in back air chamber 35 . The second insulating member is a sound absorbing material 41 that has air permeability in addition to sound absorbing properties. The sound absorbing material 41 is arranged around the rod portion 22 between the electrode main body 21 and the first insulating member 14 . The sound absorbing material 41 is preferably positioned closer to the first insulating member 14 . The sound absorbing material 41 can be fixed to the rod portion 22 and the first insulating member 14 by, for example, adhesion. An example of such a sound absorbing material 41 is glass wool. Other examples of the sound absorbing material 41 may be any material that has insulation and air permeability, such as polyurethane sound absorbing material and polyester sound absorbing material.

(Action)
The sound absorbing material 41 absorbs sound waves propagating in the back air chamber 35 due to vibration of the vibrating membrane 12, specifically, sound waves propagating from the vibrating membrane 12 toward the first insulating member 14, as well as the first insulating member. The reflected wave reflected at 14 can be attenuated.

According to the microphone 10 of the second embodiment, the following effects can be obtained in addition to the effects described in (1-1) above.
(2-1) By disposing the sound absorbing material 41 in the back air chamber 35, the amplitude of the reflected wave reaching the vibrating membrane 12 is reduced, so that the influence of the reflected wave on the vibration of the vibrating membrane 12 is suppressed. be able to. As a result, it is possible to suppress deterioration in measurement accuracy due to reflected waves.

(2-2) Since the sound absorbing material 41 disposed in the back air chamber 35 has insulating properties, even if the sound absorbing material 41 is in contact with the rod portion 22 and the housing 11, the vibration film 12 and the electret film 24 can be accurately detected.

(2-3) Since the second insulating member is the sound absorbing material 41 such as glass wool, the influence of the reflected waves on the vibration of the vibrating membrane 12 can be suppressed with a simple configuration. Further, the sound absorbing material 41 may be fixed to the rod portion 22 and the first insulating member 14 by adhesion or the like, or wound around the rod portion 22 when the back electrode 13 is assembled to the housing 11 . Therefore, the second insulating member can be easily arranged.

(Third Embodiment)
A third embodiment of the microphone will be described with reference to FIGS. 3 and 4. FIG. The microphone of the third embodiment has the same main configuration as the microphone of the first embodiment. Therefore, in the third embodiment, portions different from the first embodiment will be described in detail, and portions similar to those in the first embodiment will be denoted by the same reference numerals, and detailed description thereof will be omitted.

In the microphone 10 of the third embodiment, as in the microphone 10 of the second embodiment, in addition to suppressing the deterioration of the static pressure characteristics, the reflected wave reflected by the first insulating member 14 is caused by the vibration of the vibrating membrane 12. reduce its impact.

As shown in FIG. 3, the microphone 10 has a reflector 50 as a second insulating member. The reflector 50 divides the back air chamber 35 into two spaces, a first back air chamber 51 and a second back air chamber 52 , which are arranged in the extending direction of the rod portion 22 . The reflector 50 has an annular shape. The reflecting plate 50 is supported by the rod portion 22 at its central portion, and arranged so that its outer peripheral edge portion is in close contact with the inner peripheral surface of the housing 11 . An example of the reflector 50 is made of sapphire, for example. The reflecting plate 50 may be made of any insulating material, and other examples thereof include ceramics, polytetrafluoroethylene, and glass. In addition, the reflector 50 preferably has a sound absorbing property in order to suppress generation and amplitude of reflected waves on the reflector 50 . This can be realized by adhering a sound absorbing material such as glass wool to the surface of the reflector 50 .

The reflecting plate 50 has a communication hole 53 penetrating through the reflecting plate 50 as a communication passage for communicating the first back air chamber 51 and the second back air chamber 52 . The hole diameter of the communication hole 53 is preferably less than 1/10 of the wavelength corresponding to the upper limit frequency f2.

The rod portion 22 is composed of a first partial rod 56 and a second partial rod 57 that are partial rods that are connected to each other. The first partial rod 56 and the second partial rod 57 function as the rod portion 22 by being linearly connected. The reflecting plate 50 is supported by the rod portion 22 by being sandwiched between the first partial rod 56 and the second partial rod 57 .

The first partial rod 56 extends from the central portion of the electrode body 21 toward the opposite side of the diaphragm 12 . The first partial rod 56 has a first connecting portion 58 to which the second partial rod 57 is connected at the end opposite to the electrode main body 21 . The first connecting portion 58 is a male thread in this embodiment. The first partial rod 56 has an enlarged diameter portion 59 between the electrode body 21 and the first connecting portion 58 that engages the top surface 50 a of the reflector 50 . A rod insertion hole 60, which is a through hole into which the first connecting portion 58 is inserted, is formed in the central portion of the reflecting plate 50. As shown in FIG. The first partial rod 56 is configured such that the first connecting portion 58 protrudes from the bottom surface 50b of the reflector 50 in a state where the enlarged diameter portion 59 is engaged with the top surface 50a of the reflector 50 . The first back air chamber length H1, which is the distance between the vibrating membrane 12 and the reflector 50, is preferably less than or equal to the half wavelength λ2 of the sound wave corresponding to the upper limit frequency f2.

The second partial rod 57 has the above-described terminal mounting portion 25 and enlarged diameter portion 26 on the side opposite to the first partial rod 56 . The second partial rod 57 has a second connecting portion 62 that is connected to the first connecting portion 58 of the first partial rod 56 at the end on the first partial rod 56 side. The second connecting portion 62 is a female thread in this embodiment. The second partial rod 57 has an outer diameter larger than that of the rod insertion hole 60 of the reflector 50 at the second connecting portion 62 . That is, when the second connecting portion 62 is connected to the first connecting portion 58 , the end surface of the second connecting portion 62 contacts the bottom surface 50 b of the reflector 50 .

A method of assembling the back electrode 13 having the reflector 50 will be described with reference to FIG.
As shown in FIG. 4, in the back pole 13, the first connecting portion 58 of the first partial rod 56 is inserted into the rod insertion hole 60 of the reflecting plate 50, and the second connecting portion 62 is attached to the first connecting portion 58. concatenated. Thereby, the back plate 13 is assembled with the reflecting plate 50 attached to the rod portion 22 . Then, the first insulating member 14 and the output terminal 27 are attached to the housing 11 in an assembled state in which the rod portion 22 is assembled.

(Action)
According to the microphone 10 described above, the back air chamber 35 is divided by the reflector 50 into the first back air chamber 51 and the second back air chamber 52 . As a result, the sound wave propagating through the back air chamber 35 can be reflected by the reflecting plate 50 before reaching the first insulating member 14 . As a result, the proportion of sound waves reflected by the first insulating member 14 in the sound waves propagating through the back air chamber 35 can be reduced. In addition, the natural vibration frequency of the air column in the back air chamber 35 can be increased to the upper limit frequency f2 or higher of the design frequency.

According to the microphone 10 of the third embodiment, the following effects can be obtained in addition to the effects described in (1-1) above.
(3-1) The back air chamber 35 is divided by the reflector 50 into a first back air chamber 51 and a second back air chamber 52 that communicate with each other. According to such a configuration, it is possible to increase the natural vibration frequency of the air column in the back air chamber 35 to the upper limit frequency f2 or higher of the design frequency while reducing the proportion of sound waves reflected by the first insulating member 14 . As a result, the influence of the reflected wave on the vibration of the vibrating film 12 can be suppressed, and as a result, deterioration of the measurement accuracy caused by the reflected wave can be suppressed.

(3-2) Since the length H1 of the first back air chamber is equal to or less than the half wavelength λ2 of the sound wave corresponding to the upper limit frequency f2, it is possible to suppress the influence of the wave reflected by the reflecting plate 50 on the vibration of the vibrating membrane 12. .

(3-3) In general, in a circuit that generates a wave such as a sound wave, in order for the circuit to be approximated as a lumped constant circuit instead of a distributed constant circuit, the circuit dimensions of the circuit must be equal to the wavelength of the wave. It is required to be several percent to several tens of percent.

In the above configuration, the hole diameter of the communication hole 53 that communicates the first back air chamber 51 and the second back air chamber 52 is the wavelength corresponding to the upper limit frequency f2, that is, the smallest wavelength of the measurable sound wave wavelengths. It is set to less than 1/10.

With such a configuration, the communication hole 53 portion can be approximated as a lumped constant circuit. As a result, when designing the microphone 10, the communicating hole 53 can be treated as a lumped constant circuit, so that various dimensional values that suppress the occurrence of the resonance phenomenon can be easily derived. As a result, the influence of the reflected wave on the vibration of the vibrating membrane 12 can be suppressed more effectively.

(3-4) The reflecting plate 50 is supported by the rod portion 22 by being sandwiched between the first partial rod 56 and the second partial rod 57 . According to such a configuration, by attaching the back electrode 13 to the housing 11 , the reflector 50 can be positioned with respect to the vibrating membrane 12 . As a result, the reflector 50 can be easily arranged at a predetermined position in the back air chamber 35 .

The embodiment described above can be implemented with the following modifications. The embodiments described above and the following modified examples can be combined with each other within a technically consistent range.
- In the microphone 10 of the third embodiment, a plurality of reflectors 50 may be supported by the rod portion 22 . In this case, if the reflecting plate 50 is the n-th reflecting plate (n is an integer equal to or greater than 1) from the electrode main body 21 side, the rod portion 22 is composed of n+1 partial rods. Further, if the n+1 partial rods are the k-th rod portion (k is an integer equal to or greater than 1) from the electrode main body 21 side, the k-th rod portion and the k+1-th rod portion are connected, and the k-th rod portion and the k+1-th rod portion are connected. The n-th reflector (at this time, n is the same as k) is sandwiched by the rod portion.

- In the microphone 10 of the third embodiment, the number of the communication holes 53 formed in the reflector 50 may be two or more.
- In the microphone 10 of 3rd Embodiment, the reflector 50 should just have a communication path which connects the two spaces which this reflector 50 divides. Therefore, the communication path is not limited to the communication hole 53 passing through the reflector 50, and may be, for example, a communication recess 65 extending in the thickness direction of the outer peripheral edge of the reflector 50 as shown in FIG. In this case, the maximum length of the communicating recess 65 in the cross-sectional direction is preferably less than 1/10 of the wavelength corresponding to the upper limit frequency f2. The maximum length is, for example, the diameter when the communicating recess 65 has a semicircular cross section.

- In the microphone 10 of the third embodiment, the connection method of the partial rods is not limited to screw joint. For example, other mechanical joining methods such as press fitting or riveting may be used, or metallurgical joining methods such as brazing may be used.

- In the microphone 10 of the third embodiment, the reflecting plate 50 is snap-engaged with an engagement recess formed in the rod portion 22 by inserting the rod portion 22 into the rod insertion hole 60, for example. It may be supported by the portion 22 .

- In the microphone 10 of the third embodiment, the sound absorbing material 41 may be arranged in at least one of the first back air chamber 51 and the second back air chamber 52 . With such a configuration, it is possible to further effectively suppress the influence of the reflected wave on the vibration of the vibrating membrane 12 .

- In the microphone 10 of the second embodiment, the sound absorbing material 41 may be arranged in the space between the electrode main body 21 and the first insulating member 14 . Therefore, for example, the sound absorbing material 41 may be arranged so as to fill the entire space, or the sound absorbing material 41 may be arranged in places in the space.

DESCRIPTION OF SYMBOLS 10... Capacitor microphone 11... Housing 12... Vibration membrane 13... Back electrode 14... First insulating member 15... Tube end opening 16... Grid 17... Grid through hole 21... Electrode body 22 Rod portion 23 Body through hole 24 Electret film 25 Terminal attachment portion 26 Expanded diameter portion 27 Output terminal 28 Rod through hole 31 Fitting portion 32 Lock nut 35 Back air chamber 41 Sound absorbing material 50 Reflector 50a Top surface 50b Bottom surface 51 First back air chamber 52 Second back air chamber 53 Communication hole 56 First portion Rod 57...Second partial rod 58...First connecting portion 59...Expanded diameter portion 60...Rod insertion hole 62...Second connecting portion.

Claims (6)

A back electret condenser microphone,
a tubular casing;
a vibrating film that closes a cylinder end opening of the housing;
a back electrode having an electrode body having an electret film arranged opposite to the vibration film in the housing and a rod portion extending from the electrode body toward the opposite side of the vibration film;
an insulating member fixed to the housing and interposed between the inner peripheral surface of the housing and the outer peripheral surface of the rod portion to support the back electrode;
When the space between the vibrating membrane and the insulating member is a back air chamber, and the frequency range of measurable sound waves is the design frequency,
A capacitor microphone in which a back air chamber length, which is a distance between the vibrating membrane and the insulating member, is longer than a half wavelength of a sound wave corresponding to an upper limit frequency of the design frequency. The insulating member is a first insulating member,
2. The condenser microphone according to claim 1, further comprising a second insulating member between said electrode body and said first insulating member. The condenser microphone according to claim 2, wherein the second insulating member has sound absorbing properties. 4. The condenser microphone according to claim 2, wherein the second insulating member includes a reflector that partitions the back air chamber into two or more spaces communicating with each other. The reflector is
It is supported by the rod portion and is provided so as to contact the inner peripheral surface of the housing, and has a communication hole that penetrates the reflector,
5. The condenser microphone according to claim 4, wherein the diameter of said communication hole is less than 1/10 of the wavelength corresponding to said upper limit frequency. The rod part
Composed of a plurality of partial rods connected in a straight line,
The reflector is
6. The condenser microphone according to claim 4, wherein the rod part has a through-hole inserted therein and is supported by the rod part by being sandwiched by two partial rods connected to each other.

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