JP2541621B2 - Directional microphone - Google Patents

Description

TECHNICAL FIELD The present invention relates to a directional microphone capable of capturing a target sound without being bothered by ambient noise,
For example, it can be used to obtain a directional microphone that can be incorporated into a telephone handset to enable clear speech.

(Prior Art) With the digitization of telephones in recent years, new types of microphones such as dynamic type and electret type have been used instead of the conventional carbon type microphones incorporated in the handsets. It is coming.

However, since these new types of microphones are almost omnidirectional, undesired surrounding sounds are often mixed in during transmission to obscure the transmission.

Therefore, there is an increasing need for a close-talking microphone that has a performance equal to or better than that of a carbon microphone that has a non-linear characteristic and hardly picks up ambient noise.

However, when trying to give a sound pressure gradient to a close-talking type microphone unit using only one conventional microphone,
A suitable opening for transmitting sound waves must be provided on the side or back of the case of the transmitter, and almost no directivity can be obtained without this acoustic opening.

 This point will be described below with reference to FIGS.

FIG. 10 is a vertical sectional view of the microphone, FIG. 11 is a diagram showing sound pressure gradient, and FIG. 12 is a frequency characteristic diagram.

In FIG. 10, 1 is a sound hole 2 on the front surface (right side of the drawing)
In the microphone case, the primary sound pressure gradient microphone element 3 is housed. The microphone element (hereinafter simply referred to as element) 3 is installed in the case 1 with its acoustic main axis a (central axis perpendicular to the diaphragm 4) being perpendicular to the front surface of the case having the sound hole 2.

Now, parallel to the main axis a of the element 3 (incident angle θ in this direction is 0
Suppose that the sound pressure acting on the front surface of the diaphragm 4 of the element 3 is P ₁ when the sound wave arrives at (°), the sound pressure P _{r at} the acoustic terminal 5 on the back surface of the element 3 is If
Due to the external mean path difference d _e , the amplitude is equal to the sound pressure P ₁ and only the phase is kd _e radian. It will be delayed. Vibration further a sound pressure P ₂ in which the phase is delayed kd _i radian for the phase transition for the acoustic impedance provided on the path difference and the element in the element (in FIG. 10 shows only the internal path difference) It acts on the back of the plate 4. The relationship between each of these sound pressures is illustrated by a diagram showing the sound pressure gradient.
11 It looks like Figure a. That is, between the sound pressure P ₁ acting on the surface of the diaphragm 4 and the sound pressure P _{r at} the acoustic terminal 5 of the element, kd
_Since there is a phase difference of _e radians and there is a phase difference of kd _i radians between the sound pressure P ₂ acting on the back surface of the frequency and the sound pressure P _r , the sound pressure gradient P = P that drives the diaphragm. _1- P ₂ is represented by the length of line 7.

When the incident angle θ of the sound wave is 90 °, the sound pressure gradient P is represented by the line 8 in FIG. 11b, and when θ = 180 °, it is represented by the line 9 in FIG. 11c. There is little change in the length of lines 7, 8 and 9.

Also, the frequency characteristic of this microphone is as shown in FIG. 12, and the magnitude of the output due to the difference in the incident angle changes in the high frequency range due to the diffraction phenomenon of the sound wave.
Almost no in the low frequency range.

(Problems to be Solved by the Invention) The conventional microphone configured and operating as described above produces a sound so that the difference in length between the lines 7, 8 and 9 is small even if the incident angle θ of the sound wave changes. Since the change in the pressure gradient is small, for example, even when it is desired to capture only the sound near the θ = 0 ° direction, ambient noise is also captured with the same strength. That is, there is almost no directivity.

In order to provide the microphone 1 with a clear directivity in such a configuration, in the case of the case 1 and the telephone set, it is necessary to appropriately perforate the side face, the back face, etc. of the handset, which impairs the appearance of the case. I will be sleeping.

(Means for Solving the Problem) According to the present invention, a single element is housed in a case having a sound hole in the front surface thereof with the acoustic main axis a parallel or inclined to the front surface of the case, and between the outer surface of the element and the inner surface of the case. A microphone having directivity is configured by acoustically dividing the inside of the case into two with an acoustic shielding material interposed therebetween.

(Function) The acoustic principal axis a of the element is parallel to or inclined to the front surface of the case, and the inner surface of the case and the outer surface of the element are cut off to separate sound waves acting on the front and back of the diaphragm of the element. The change in the sound pressure gradient acting on the diaphragm increases as the angle of incidence changes, and clear directivity can be given to the microphone.

(Embodiment) FIGS. 1 to 6 schematically show a first embodiment of the present invention, FIG. 1 is a longitudinal sectional view of a microphone, and FIGS. 2a, 2b and 2c are diagrams showing sound pressure gradient, and FIG. The figure shows the frequency characteristic diagram when the incident angle changes, Fig. 4 shows the frequency characteristic diagram when the incident angle θ and the distance to the sound source change, Fig. 5 shows the directional characteristic diagram, and Fig. 6 shows It is a schematic diagram showing a state where it is built in and used in a handset of a telephone.

The same parts as those in FIGS. 10 and 11 are designated by the same reference numerals and will be described below.

The primary sound pressure gradient microphone element 3 is installed in the case 1 with the acoustic principal axis a parallel to the front surface of the case 1 in which the sound hole 2 is formed. In addition, an acoustic shield 10 is interposed between the outer surface of the element 3 and the inner surface of the case 1 to divide the case 1 into a space surrounding the front part of the element 3 and a space surrounding the rear part thereof.

The incident angle θ = 0 ° when the sound wave arrives from the front of the element parallel to the acoustic main axis a of the element 3, and θ = 90 ° when the sound wave arrives at right angles to the front surface of the case where the sound hole 2 is provided. When sound waves arrive from the rear, θ = 180 °.

When θ = 0 °, the sound pressure P ₂ acting on the back surface of the diaphragm is
Only the phase is delayed by (kd _i + kd _e ) radians with the same amplitude as the sound pressure P ₁ acting on the front surface of the diaphragm 4 of the element, and the sound pressure gradient P for vibrating the diaphragm 4 is Becomes FIG. 2a shows this state, and the sound pressure gradient P is shown by the line 7 '.

In the case of θ = 90 °, the acoustic element 5 of the diaphragm and the element 3
A sound wave arrives at the same time, and the sound pressure at that time is P ₁ = P _r , and the sound pressure P ₂ is delayed by Kd _i radians due to the internal path difference of the element 3 and the acoustic impedance for phase transition. The pressure gradient P = P ₁ -P ₂ is as shown by the line 8 ′ in FIG. 2B, which is smaller than the sound pressure gradient P in FIG. 2A.

When θ = 180 °, the sound wave first reaches the acoustic element 5 behind the element 3, and the sound pressure P ₁ is delayed in phase by Kd _i radians with respect to the sound pressure P _r , and the sound pressure P _{2 is} generated. Since Kd _i radians lag behind the pressure P _{r, the} sound pressure P becomes smaller as indicated by line 9'in FIG. 2c.

In this case, the average path difference d _e for the incident sound wave from the θ direction becomes cos θ times shorter than in the case where θ = 0 °. Therefore, for a general incident angle θ, the sound pressure gradient P acting on the diaphragm 4 is And when d _i + d _e cos θ is sufficiently smaller than the wavelength λ of the sound wave, Because I guess Becomes Where α = P ₁ kd _e , Is.

Since α and β can be regarded as constants, α,
If β is an appropriate value, for example β = 1, that is, d _e = d _i , then P = α
(1 + cos θ). The output voltage of the microphone is proportional to the sound pressure gradient acting on the diaphragm.
A microphone in which the magnitude of the sound pressure gradient changes depending on the incident angle θ with a term of + cos θ becomes unidirectional.

FIGS. 3 and 4 show the frequency characteristic diagram (FIG. 3) with respect to the incident direction of the sound wave when the delays d _e and d _i of the strokes are set to appropriate values, and when the sound source is brought close to each other. And a frequency characteristic diagram (Fig. 4) when separated from each other, and Fig. 5 shows a directional characteristic diagram. In FIG. 5, the solid line 11 shows the case where the frequency is 1 KHz, and the broken line 12 shows the case where the frequency is 500 Hz.

When the element 3 is incorporated in a telephone handset and used as shown in FIG. 6, the sound source incident on the element 3 is considered to be a spherical wave because the mouth of the sound source comes very close to the microphone. Therefore, the proximity effect due to the spherical wave is generated, the sensitivity to the sound wave having a long wavelength, that is, the low frequency is improved, and the output is increased.

The chain line 13 in FIG. 4 is used when the handset of the telephone illustrated in FIG. 6 is incorporated (the incident angle θa of the sound wave is about 45 °).
2 illustrates an example of the proximity characteristic of the microphone.

The solid line 14 in FIG. 4 shows the frequency characteristic when θ = 0 ° and 50 cm away from the sound source, the broken line 15 shows the frequency characteristic when 50 cm away from the sound source and θ = 90 °, and the solid line 16 shows 50 cm away from the sound source θ = 180
When the transmitter that uses the microphone of the present invention is brought close to the mouth to transmit, the output of the transmitted voice is higher than that even if there is a sound wave incident at the same level from a distance. It can be seen that the transmission of voice can be performed with high clarity because it becomes high.

7 to 9 show a second embodiment of the present invention, in which the acoustic principal axis a of the element 3 is inclined by 30 ° with respect to the front surface of the case 1. FIG. 7 is a longitudinal sectional view of the microphone, FIG. 8 is a frequency characteristic diagram, and FIG. 9 is a directional characteristic diagram.

In this embodiment, the element 3 of the first embodiment is installed in the case with an inclination of 30 °, and the frequency characteristic at that time is as shown in FIG. 8 and the directional characteristic is as shown in FIG. . In FIG. 9, the solid line 17 indicates the case of 1 KHz, and the broken line 18 indicates the case of 500 Hz. There is directivity at any frequency, but it is left-right asymmetric.

(Effects of the Invention) (1) Since it is not necessary to provide an acoustic opening on the side surface or the back surface of the case for accommodating the element, it is necessary to provide an opening also on the side surface or the back surface of the handset or the like of a telephone which further incorporates this case. In addition, the appearance of a close-talking type transmitter such as a handset is not deteriorated.

(2) Since the acoustic opening of the transmitter is not provided, the mounting position of the built-in microphone is unlikely to be restricted in relation to the position of the opening unlike the conventional directional microphone.

(3) Therefore, the microphone can be embedded in the transmitter to be attached, or the microphone can be embedded on the upper surface of the desk and used for speaking without sitting while sitting in front of the desk.

[Brief description of drawings]

1 to 6 show a first embodiment of the present invention, FIG. 1 is a longitudinal sectional view of a microphone, FIGS. 2a, 2b and 2c are diagrams showing sound pressure gradients, and FIG. Frequency characteristic diagram when it changes, Fig. 4 shows frequency characteristic diagram when the incident angle and the distance to the sound source change, Fig. 5 shows directional characteristic diagram, and Fig. 6 is built in the handset of the telephone. FIG. 7 to FIG. 9 show a second embodiment, FIG. 7 is a longitudinal sectional view of the microphone, FIG. 8 is a frequency characteristic diagram, FIG. 9 is a directional characteristic diagram,
10 to 12 show a conventional example, FIG. 10 is a longitudinal sectional view of a microphone, and FIGS. 11 a, b, and c are diagrams showing sound pressure gradient, FIG.
The figure is a frequency characteristic diagram. 1: case, 2: sound hole, 3: microphone element, 4: diaphragm, 5:
Acoustic terminals, 7, 7 ', 8, 8', 9, 9 ': wire, 10: acoustic shielding material, 11: solid line, 12: broken line, 13: chain line, 14: solid line, 15:
Dashed line, 16, 17: solid line, 18: broken line.

Claims (1)

(57) [Claims] 1. A primary sound pressure gradient microphone element (3) is housed in a microphone case (1) which is provided with a sound hole (2) on the front surface and hermetically seals the others, and a diaphragm of the microphone element (3) ( 4) The acoustic main axis (a) perpendicular to 4) is tilted in parallel with the front surface of the microphone case having a sound hole, or is tilted to the extent that directivity can be obtained. Acoustic shield (1
A directional microphone that is closed by 0) and divides the inside of the microphone case (1) into a space surrounding the front part and a space surrounding the rear part of the microphone element (3).