GB2218303A

GB2218303A - Directional microphone

Info

Publication number: GB2218303A
Application number: GB8905924A
Authority: GB
Inventors: Shigeo Mori
Original assignee: PRIMO CO Ltd
Current assignee: PRIMO CO Ltd
Priority date: 1988-04-20
Filing date: 1989-03-15
Publication date: 1989-11-08
Also published as: JPH01268398A; DE3907895A1; GB8905924D0; FR2630610A1; JP2541621B2

Abstract

The microphone comprises a casing 1 having a plurality of acoustic openings 2 and a linear pressure-gradient microphone device 3 enclosed within the casing. The acoustic openings are provided in the front surface of the casing and a main acoustic axis a which is perpendicular to a diaphragm 4 of the microphone device is arranged to be substantially parallel with the front surface of the casing. An acoustic shielding member 10 is provided between the outer surface of the microphone device and the inner surface of the casing so as to divide the casing into first and second spaces 1A, 1B. Since the pressure gradient across the diaphragm varies in dependence on Cos theta where theta is the angle of incidence of a sound wave on the microphone, the microphone has directional characteristics. …<IMAGE>…

Description

c i 2r ,- 18303 DIRECTIONAL MICROPHONE The present invention relates to a

directional microphone which can detect an object sound without being influenced by ambient noises and which enables a sound to be clearly transmitted, the microphone being provided in, e.g., a speaker/transmitter of a telephone handset.

In association with the recent electronization of telephones, new types of microphone such as the dynamic type, the electrostatic type and so on have been used in place of the conventional carbon type microphone which is provided in the speaker/transmitter of a handset.

However, almost of these new types of microphones are nondirectional, so that unrelated ambient sounds are mixed with voice to be transmitted upon transmission thereof and the voice therefore often becomes unclear.

Under these circumstances, there has been a demand for the provision of a close-taking type microphone having a performance or characteristics equal to or superior to those of the carbon type microphone which has nonlinear characteristics, and which can minimize a detected level of any ambient noises.

However, if a pressure gradient is given to the conventional close-talking type microphone unit using only one microphone, proper openings allowing the sound waves to pass through must be formed in the side or rear surface of the casing of the speaker/transmitter. Unless acoustic openings are formed, the required directional characteristics cannot be properly obtained.

This point will now be described hereinbelow with reference to Figs. 1 to 3.

Fig. 1 is a vertical sectional view of a conventional microphone. Figs. 2(a) to 2(c) are diagrams showing pressure gradients of the microphone. Fig. 3 is a graph showing frequency characteristics of the microphone.

In Fig. 1, reference numeral 1 denotes a microphone casing. Acoustic openings 2 are formed in the front surface (the surface on the right side in the drawing) of the microphone casing 1. A linear pressure gradient microphone device (hereinafter, simply referred to as a device)'3 is enclosed in the casing 1 in such a manner that an acoustic main axis a (a central axis which is vertical to a diaphragm 4) is vertical to the front surface of the casing which has the acoustic openings 2.

It is now assumed that sound pressure which acts on the front surface of the diaphragm 4 of the device 3 is defined by P, when a sound wave arrives in parallel with the main axis a of the device 3 (the incident angle 6 of the sound wave arriving in this direction is set at 0). When the incident sound wave is a plane wave, sound pressure P r at an acoustic terminal 5 provided on the rear surface of the device 3 has the same amplitude as that of the sound pressure P, but has a phase which is delayed by kd e radian (k = 2n/X X is the wavelength of a sound) relative to that of the sound pressure P, due to an external mean path difference d e. Further, the sound pressure P r changes to 11 sound pressure P2 whose phase is delayed by kd i radian because of the path difference in the device 3 (only the internal path difference is shown in Fig. 1) and an acoustic impedance for phase shift provided in the device 3. The sound pressure P2 acts on the rear surface of the diaphragm 4. Fig. 2(a) shows a pressure gradient given by the relationship between these sound pressures. That is, there is a phase difference of the kd e radian between the sound pressure P, which acts on the surface of the diaphragm 4 and the sound pressure P r at the acoustic terminal 5 of the device 3. There is a phase difference of kd 1 radian between the sound pressure P2 which acts on the rear surface of the diaphragm and the sound pressure P r Therefore, a pressure gradient, P = P1 - P2, which acts to drive the diaphragm, is expressed by the length of line 7.

When the incident angle e of the sound wave is set at 90, the pressure gradient P is expressed by a line 8 in Fig. 2(b). When e = iso., the pressure gradient P is expressed by a line 9 in Fig. 2(c). There are not such large differences between the lengths of the lines 7, 8 and 9.

The frequency characteristic curves of this microphone are shown in Fig. 3. Output levels produced by the different incident angles change due to a diffraction phenomenon of the sound wave in a high-frequency range.

However, the output levels hardly change at all in a lowfrequency range.

In a conventional microphone which is constructed and behaves in the manner mentioned above, even if the incident angle e of the sound wave changes, the pressure gradient 1 $1 - 4 hardly changes at all because the differences between the lengths of the lines 7 to 9 are very small. Therefore, even when it is only the acoustic sound near the region of the incident angle of, e.g., 9 - 0 0 that is to be detected, ambient noises are also detected as they are. In other words, this microphone displays hardly any directional property at all.

In order to provide the microphone 1 having such a structure with a good directional property, proper openings must be formed in the side surface, rear surface and the like of not only the casing 1 but also the speaker/transmitter of, for example, a telephone handset. As a result, it is apparent that the outside appearance of the casing will be spoiled from the aesthetic viewpoint.

The present invention has been achieved in order to alleviate the foregoing problems.

According to the present invention, there is provided a directional microphone comprising:

a hollow casing having a plurality of acoustic openings, said openings generally facing a particular direction, a linear pressure-gradient microphone device having a diaphragm with front and rear surfaces, said microphone device being placed within said casing, acoustic shielding means extending between the outer surface of said microphone device and the inner surface of said casing so as to divide the interior of said casing into two spaces; i said microphone device and said acoustic shielding being arranged so that the front surface of said diaphragm is open to one of said spaces and the rear surface of said diaphragm is open to the other of said spaces, each space having at least one acoustic opening opening into it; said microphone device being orientated such that a main acoustic axis, perpendicular to said diaphragm is inclined to said particular direction.

The microphone device may be arranged so that said main acoustic axis is perpendicular to said predetermined duration or at an angle of substantially 600 to said predetermined direction.

In a microphone of the present invention, the main acoustic axis of the diaphragm of the microphone device is arranged to be parallel with or inclined away from one surface, for example, a front surface of the casing, and the acoustic shielding member is provided to shut out the sound pressure passing through the space between the inner surface of the casing and the outer surface of the microphone device, so that a change in pressure gradient which acts on the diaphragm increases in response to any change in incident angle of the sound wave and thus the microphone can be provided with clear directional characteristics.

According to the microphone of the present invention, it is not necessary to form any acoustic opening in the side or rear surface of the casing enclosing the microphone device and hence not necessary to form any openings in the side or rear surfaces of a handset or the like of a telephone system, which handset or the like 1 encloses the casing. It is possible, therefore, to provide a close- talking type speaker/transmitter such as a telephone handset without spoiling the aesthetic appearance thereof.

In a microphone of the present invention, the acoustic openings are limited in position to one surface, for example, the front surface of the casing. Thus, the microphone device can be enclosed in the speaker/transmitter without regard to the positions of the acoustic openings, in contrast with the conventional directional microphone, and thus can be used in the form of a set-in mode, such as being set into the upper surface of a desk so that a user can talk without using either hand.

The invention will be further described by way of non-limitative example, with reference to the accompanying drawings in which; Fig. 1 is a vertical sectional view of a conventional microphone; Figs. 2(a) to 2(c) are diagrams showing pressure gradients of the conventional microphone.

Fig. 3 is a diagram showing frequency characteristics of the conventional microphone; Fig. 4 is a vertical sectional view of a microphone in accordance with a first embodiment of the present invention; Figs. 5(a) to 5(c) are diagrams showing pressure gradients of the first embodiment shown in Fig. 4; c j Fig. 6 shows output-frequency characteristic curves of the first embodiment, which are given at different incident angles; Fig. 7 shows output-frequency characteristic curves of the first embodiment, which are given at different incident angles and at different distances from a sound source; Fig. 8 is a diagram showing directional characteristics of the first embodiment; Fig. 9 is a schematic diagram showing a state in which the microphone of the invention is provided in a telephone handset and is used therefore; Fig. 10 is a vertical sectional view of a microphone in accordance with a second embodiment of the present invention; Fig. 11 shows output-frequency characteristic curves of the second embodiment, which are given at different incident angles; and Fig. 12 is a diagram showing directional characteris- tics of the second embodiment.

Figs. 4 to 9 schematically show the first embodiment of the present invention.

In Figs. 4 to 9, parts and components which are the same as or similar to those shown in Figs. 1 and 2 are designated by the same reference numerals. The first embodiment will now be described in detail with reference to the drawings.

T 1 The linear pressure gradient microphone device 3 is enclosed in the casing 1 with the main acoustic axis a arranged to be parallel with the front surface of the casing 1. The acoustic openings 2 are solely formed in the front surface of the casing 1. An acoustic shielding member 10 is provided between the outer surface of the device 3 and the inner surface of the casing 1 so as to divide the inside of the casing 1 into a space 1A surrounding the front portion of the device 3 and a space 1B surrounding the rear portion of the device 3, so that sound pressure passing through the space between the inner surface of the casing 1 and the outer or side surface of the device 3 can be shut out.

It is assumed that the incident angle e is 0 when a sound wave arrives from a position in front of the device along the lines parallel with the main acoustic axis a of the device 3, e is 90 when a sound wave arrives in the direction perpendicular to the front surface of the casing having the acoustic holes 2, and e is 180 when a sound wave arrives from the rear of the device along the lines parallel with the axis a.

When e = C, the sound pressure P2 which acts on the rear surface of the diaphragm has the same amplitude as that of the sound pressure P, which acts on the front surface of the diaphragm 4 of the device and has a phase which is delayed by (kd i + kd e) radian relative to that of the sound pressure P,. The pressure gradient P which acts to vibrate the diaphragm 4 can be obtained by P = P1 - P2 = 2P, sin k(d i +d e) 2 17 1 J 1 1 -g- (k = 2jr/,X: A is a wavelength of sound) Fig. 5(a) shows this state and the pressure gradient P indicated by a line V.

is When e = 9C, a sound wave arrives simultaneously at the front surface of the diaphragm and at the acoustic terminal 5 of the device 3 and the sound pressure P, at this time is equal to P r The phase of the sound pressure P2 is delayed by kd i radian relative to that of the sound pressure P, due to the internal path difference of the device 3 and an acoustic impedance for phase shift. Therefore, the pressure gradient P (= P, - P2) becomes as shown by a line 8' in Fig. 5(b) and is smaller than the pressure gradient P in Fig. 5(a).

When e = 1800, a sound wave first reaches the acoustic terminal 5 at the rear position of the device 3. The phase of the sound pressure P, is delayed by only the kd e r The phase of the sound pressure P2 is delayed by kd i radian relative to that of the sound pressure P r Therefore, the sound pressure P decreases as shown by a line 9' in Fig. 5(c).

Thus, for an incident sound wave arriving from the direction of e, the mean path difference d e is only reduced radian relative to that of the sound pressure P by a distance which is cose times as long as that in the case where e = 0. Therefore, for the general incident angle e, the pressure gradient P which acts on the diaphragm 4 is obtained by P = 2 P, sin k(d i +d e cosE)) 2 When (d +d cose) is sufficiently smaller than the wave i e length A of a sound wave, it is regarded as follows.

sin k(d +d cose) i e k 2 Therefore, 2 (d i +d e cose) P = 2P, x 1 (d +d cose) 2 i e = P1 k (d. i +d e cose) d.

= P1 M ( 1 + cose) e d e + cose) where, u = P, kd, B = d' e d e Since x and B can be regarded as constants, bY setting proper values for a and B, for instance, B = 1, i.e., d e d i in the above equations, P- =a(l+cose) is obtained. Since the output voltage of the microphone is proportional to the pressure gradient which acts on the diaphragm, a microphone in which the magnitude of pressure gradient varies depending on the incident angle e based on the term (1+cos8) can indicate unidirectional characteristics.

Fig. 6 shows output-frequency characteristic curves for the incident direction of the sound wave when the delays d e and d i of the path are set to have proper values. Fig. 7 shows output-frequency characteristic curves when a sound source is set at a close position and at a remote position relative to the device. Fig. 8 is a diagram showing directional characteristics. In Fig. 8, a solid line 11 indicates the case of a frequency of 1 kHz and a broken line 12 represents the case of a frequency of 500 Hz- 11 I When the device 3 is assembled in a handset of a telephone and used as shown in Fig. 9, since the mouth of a user which acts as the sound source is remarkably close to the microphone, the sound wave which enters the device 3 considered to be spherical wave. A proximity effect thus occurs due to this spherical wave. The microphone is more sensitive to a sound wave of long wavelength, i.e., to a low-frequency range, and thus its output level can rise.

The dotted/dashed line 13 in Fig. 7 shows an example of a proximity characteristic of a microphone (the incident angle 6 a of the sound wave is set at about 450) in a case where the microphone is provided in a telephone handset and used as illustrated in Fig. 9.

The solid line 14 in Fig. 7 represents an output- frequency characteristic curve when e = 0 and the microphone is 50 cm away from the sound source. The broken line 15 indicates an output- frequency characteristic curve when e = 90 and the microphone is 50 cm away from the sound source. The solid line 16 shows an output-frequency charac- teristic curve when E) is 180 and the microphone is 50 cm away from the sound source. When the user speaks into the speaker/transmitter using the microphone according to the invention, even if an undesirable sound wave enters from a remote position at the same level as the voice level, the output level of the voice to be transmitted is higher than that of such undesirable sound wave. Consequently, the voice can be clearly transmitted.

Figs. 10 to 12 show a second embodiment of the invention, in which the main acoustic axis a of the device 3 is 4 -12inclined by an angle of 30 away from the front surface of the casing 1. Fig. 10 shows a vertical sectional view of a microphone in accordance with the second embodiment. Fig. 11 shows output-frequency characteristic curves of the second embodiment. Fig. 12 is a diagram showing directional characteristics of the second embodiment.

According to the second embodiment, the device 3 shown in the first embodiment is enclosed in the casing and is inclined away from the main acoustic axis a by 30. The output-frequency characteristic curves in this case are shown in Fig. 11 and the directional characteristics are shown in Fig. 12. In Fig. 12, the solid line 17 indicates the case of a frequency of 1 kHz. The broken line 18 represents the case of 500 Hz. Although the required directional property can be obtained, their characteristics at any of these frequencies are asymmetrical with respect to E) = 0.

1 - 1 1

Claims

- 13 CLAIMS

1. A directional microphone comprising:

a hollow casing having a plurality of acoustic openings, said openings generally facing a particular direction, a linear pressure-gradient microphone device having a diaphragm with front and rear surfaces, said microphone device being placed within said casing, acoustic shielding means extending between the outer surface of said microphone device and the inner surface of said casing so as to divide the interior of said casing into two spaces; said microphone device and said acoustic shielding being arranged so that the front surface of said diaphragm is open to one of said spaces and the rear surface of said diaphragm is open to the other of said spaces, each space having at least one acoustic opening opening into it; said microphone device being orientated such that a main acoustic axis, perpendicular to said diaphragm is inclined to said particular direction.

2. A microphone according to claim 1 wherein said main acoustic axis is at a right angle to said particular direction.

3. A microphone according to claim 1 wherein said main acoustic axis is at substantially 600 to the said particular direction.

4. A microphone device according to any preceding claim wherein said casing has a substantially - 14 planar front face and said acoustic openings are provided in said front face.

5. A microphone constructed and arranged to operate substantially as hereinbefore described with reference to, and as illustrated in, figures 4 to 12 of the accompanying drawings.

6. A telephone including a microphone according to any one of the preceding claims.

Published 1989 atThe Patent Office, State House, 66 71 High Holt)r33, LondorxWClR 4TP. Further copies maybe obtainedfromThe Patent OfficeSales Branch. St Mary Cray. Orpington. Kent BR5 3RD. Printed by Multiplex techniques ltd. St Mary Cray. Kent, Con. 1/87