JP2000083292A - Condenser microphone with narrow directivity - Google Patents

Abstract

PROBLEM TO BE SOLVED: To adopt a unidirectional condenser microphone whose directivity indicates a cardioid and that is mass-produced for a line type microphone as it is without any acoustic adjustment. SOLUTION: The condenser microphone 10 with narrow directivity includes a unidirectional microphone unit 30 and a sound tube 20 and where the sound tube 20 is partitioned into a front sound volume chamber 21 and a rear sound volume chamber 22 by the microphone unit 30 and the front sound volume chamber 21 and the rear sound volume chamber 22 are connected acoustically with a gap G between an outer circumferential face of the microphone unit 30 and an inner circumferential face of the sound tube 20. In this case, the unidirectional condenser microphone unit whose directivity indicates a cardioid is adopted for the unidirectional microphone unit 30 and an acoustic resistance member 23 to prevent cavity resonance is provided in the rear sound volume chamber 22.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a narrow-directional condenser microphone, and more particularly, to a line-type condenser microphone obtained by combining a unidirectional microphone unit and an acoustic tube having a predetermined length.

[0002]

2. Description of the Related Art A narrow directional microphone is known as a microphone having a small angle at which sound can be collected even in outdoor coverage. These are roughly classified into a line type microphone and a secondary sound pressure gradient type microphone. Here, a line type microphone which is exclusively related to the present invention will be described.

[0003] The line type microphone has, as its basic configuration, a unidirectional (primary sound pressure gradient type) microphone unit having a front acoustic terminal and a rear acoustic terminal.
An acoustic tube (interference pipe) having, for example, a slit-shaped acoustic resistance hole is provided on the peripheral wall of the tube, the acoustic tube is connected to the front acoustic terminal side of the microphone unit, and the rear acoustic terminal faces free space.

In this line type microphone, sound waves from the side or the rear reach the inside of the acoustic tube from the acoustic resistance hole of the acoustic tube and the front opening of the acoustic tube, respectively. The phase difference causes sound wave interference, which weakens sound waves from the side or from the rear. On the other hand, the sound wave from the front opening is
It reaches the diaphragm of the microphone unit without attenuation.

The advantage of the line microphone is that the frequency response characteristic is relatively flat, and the sensitivity and the inherent noise are superior to those of the secondary sound pressure gradient microphone. Line microphones have been widely used in professional video cameras and the like that require high-quality sound pickup because of their evaluation.

[0006] By directing the rear acoustic terminal to the free space, a considerably narrow directivity can be obtained. On the other hand, an external wind noise or the like is picked up. There was a problem that the effect became high. This is due to the fact that the sound wave introduction port of the front acoustic terminal is located at the tip of the acoustic tube, and the distance between the front acoustic terminal and the rear acoustic terminal in a low frequency region is long.

In order to solve this problem, the present applicant has made one proposal in Japanese Patent Application Laid-Open No. 62-11898, which is a conventional example of the present invention, and whose configuration is schematically shown in FIG. A brief description will be given based on this.

That is, in the line type microphone 1, the unidirectional microphone unit 3 is housed inside the rear end side of the acoustic tube 2, and the inside of the acoustic tube 2 is moved by the microphone unit 3 into the front acoustic capacity chamber. 21 and a rear acoustic capacity chamber 22. Note that a slit-shaped acoustic resistance hole 2a is formed on the front side of the acoustic tube 2, and an acoustic resistance material (not shown) is attached to the acoustic resistance hole 2a.

One surface of the microphone unit 3 (FIG. 6)
In FIG. 6, a front acoustic terminal 31 is provided, and on the other surface (right side in FIG. 6), a rear acoustic terminal 32 is provided. A sound wave introduction port 2b for the terminal 32 is provided. Although not shown in detail, this sound introduction port 2
An appropriate acoustic resistance material is also attached to b.

In this case, the outer diameter of the microphone unit 3 is smaller than the inner diameter of the acoustic tube 2. That is,
A predetermined gap G is provided between the outer peripheral surface of the microphone unit 3 and the inner peripheral surface of the acoustic tube 2.
The front acoustic terminal 31 and the rear acoustic terminal 32 of the microphone unit 3 are acoustically connected.

As described above, the rear acoustic terminal 32 of the microphone unit 3 communicates with the free acoustic field through the sound wave introduction port 2b and is acoustically connected to the front acoustic terminal 31 through the gap G. Therefore, the distance between the acoustic terminals in the low frequency range is mainly governed by the distance between the acoustic terminals 31 and 32 of the microphone unit 3, whereby the influence of wind noise and the proximity effect can be reduced. .

[0012]

By the way, the directivity of a line microphone has frequency dependence, and a considerably long acoustic tube is required to obtain good narrow directivity even in a low frequency band.

For this reason, the microphone unit 3 has a single directivity indicating a cardioid in the first place, but when housed in the acoustic tube 2, the microphone unit 3 is designed so as to be almost non-directional. (For example, the acoustic resistance material on the side of the rear acoustic terminal 32) is adjusted.

However, since this kind of acoustic resistance material has a high acoustic resistance value, it is necessary to make quite difficult adjustments to change the cardioid-like directivity to a characteristic close to non-directionality. Tends to occur.

Moreover, this variation is difficult to confirm even by acoustic measurement of the unit alone, and when actually incorporated in an acoustic tube, the variation greatly appears as a variation in directivity especially in a low frequency band.

Therefore, a unidirectional microphone unit for a line microphone can be mass-produced by an automatic assembling machine like a general-purpose unidirectional electret condenser microphone unit whose directivity indicates a cardioid. It is extremely difficult, and in the past, one by one was manufactured by hand, and therefore, there was a problem that cost increase was unavoidable.

The present invention has been made to solve such a problem, and an object of the present invention is to enable a mass-producible unidirectional condenser microphone unit having directivity indicating a cardioid to be used as it is. Another object of the present invention is to provide a narrow directional condenser microphone.

[0018]

SUMMARY OF THE INVENTION To achieve the above object, the present invention provides a unidirectional microphone unit having a front acoustic terminal and a rear acoustic terminal, and an acoustic tube in which the microphone unit is housed. The inside of the acoustic tube is divided into a front acoustic volume room and a rear acoustic volume room by the microphone unit, and a rear acoustic wave inlet for the rear acoustic terminal is provided on the rear acoustic volume room side. In the narrow directional condenser microphone, the front acoustic capacity chamber and the rear acoustic capacity chamber are acoustically connected by a gap between the outer peripheral surface of the microphone unit and the inner peripheral surface of the acoustic tube. As the unidirectional microphone unit, a unidirectional condenser microphone unit having a cardioid directivity is used. Both are characterized by the above-described rear acoustic volume chamber acoustic resistance member for preventing the cavity resonance are provided.

According to this, the front acoustic capacity chamber operates solely to obtain narrow directivity with respect to the microphone unit, whereas the rear acoustic capacity chamber operates as an acoustic load of the microphone unit. Therefore, in a line microphone, a unidirectional microphone unit that can be mass-produced and has directivity indicating a cardioid can be used as it is.

In the present invention, it is preferable that the acoustic resistance material for preventing cavity resonance is made of a cotton-like fiber material filled in the rear acoustic capacity chamber.

In a preferred embodiment, both the front acoustic volume chamber and the rear acoustic volume chamber have substantially the same axial length, and each acoustic tube wall has an axial length in the axial direction. Preferably, a slit-shaped acoustic resistance hole is formed along.

Further, an embodiment is preferably adopted in which a predetermined acoustic resistance is connected in series to an acoustic mass existing in a gap between the outer peripheral surface of the microphone unit and the inner peripheral surface of the acoustic tube. You. This acoustic resistance can be obtained by, for example, a shock mount member (for example, a foamed rubber material) that supports the microphone unit in the acoustic tube.

[0023]

BRIEF DESCRIPTION OF THE DRAWINGS The invention will now be described in more detail with reference to an embodiment shown in the drawings.

FIG. 1 is a side view of a line type microphone 10 according to this embodiment, and FIG. 2 is a cross-sectional view along the axial direction thereof. According to this, the line type microphone 10 includes a cylindrical acoustic tube 20 whose both ends are open. In FIG. 2, the left end of the acoustic tube 20 is the front sound inlet 201, and the right end is the rear sound inlet 202.

In the acoustic tube 20, approximately two acoustic tubes 20 are provided.
The unidirectional microphone unit 30 is housed at a position where the sound volume is divided, so that the front acoustic volume room 21 and the rear acoustic volume room 2 having substantially the same acoustic volume in the same acoustic tube 20.
It is divided into two. The rear acoustic capacity chamber 22 is filled with an acoustic resistance material 23 having an extremely small acoustic resistance value for preventing the cavity resonance. In this embodiment, a cotton-like polyester fiber is used for the acoustic resistance member 23.

Each of the tube walls of the front acoustic volume chamber 21 and the rear acoustic volume chamber 22 has a slit-shaped acoustic resistance hole 21.
Each of the acoustic resistance holes 211 and 221 has an acoustic resistance material 24 made of nylon mesh or the like attached to each of the acoustic resistance holes 211 and 221. The acoustic resistance holes 211 and 221 are provided in the acoustic tube 2.
The slit holes formed partially along the circumferential direction of 0 may be arranged in the axial direction at predetermined intervals.

In the present invention, as the unidirectional microphone unit 30, a condenser microphone unit whose directivity indicates a cardioid is used.
With reference to the cross-sectional view of FIG.

This unidirectional microphone unit 30
Is provided with a cylindrical unit case 31 having a front acoustic terminal hole 311 on the front wall side facing the front sound wave introduction port 201. The front inside the cylindrical unit case 31,
That is, on the side of the front acoustic terminal hole 311, the diaphragm 32 made of, for example, an electret film is housed in a state of being stretched over the holding ring 321.

A fixed pole 33 is arranged behind the diaphragm 32, that is, behind the sound tube 202 on the rear side of the acoustic tube 20 via a spacer ring (not shown) so as to face the diaphragm 32 at a predetermined interval. . The fixed pole 33 is supported by an insulating pedestal 34, and a rear acoustic terminal hole 341 is formed in the insulating pedestal 34. Although only one rear acoustic terminal hole 341 is shown in FIG. 3, a plurality of the rear acoustic terminal holes 341 are provided on the same circumference at predetermined intervals.

Each rear acoustic terminal hole 341 communicates with a back air chamber of the diaphragm 32 (a space between the diaphragm 32 and the fixed pole 33) through a through hole 331 provided in the fixed pole 33. . An acoustic resistance material 35 made of a felt material or the like is provided on the back side of the fixed pole 33 in the communication path, for example, and the directivity is appropriately adjusted at this portion.

A circuit board 36 on which an impedance converter 361 is mounted is disposed on the back side of the insulating pedestal 34, and the fixed pole 33 and the impedance converter 361 are mounted.
Are electrically connected via a relay terminal member 37.

Also in this embodiment, the outer diameter of the microphone unit 30 is smaller than the inner diameter of the acoustic tube 20, and a predetermined gap G is provided between the microphone unit 30 and the acoustic tube 20. Through the gap G, the front acoustic terminal hole 311 and the rear acoustic terminal hole 341 of the microphone unit 30 are acoustically connected.

Thus, the influence of the wind noise and the proximity effect are reduced. In this embodiment, a donut-shaped shock mount 40 made of a suitably compressed sponge material is attached around the microphone unit 30. Through the shock mount 40, the microphone unit 3
0 is stored in the acoustic tube 20. Therefore, the gap G
This is equivalent to connecting the acoustic resistance of the shock mount 40 in series with the acoustic capacity of.

FIG. 4 shows the sound collecting characteristics at 0, 90 and 180 degrees with respect to the sound source of the unidirectional microphone unit 30 used in this embodiment.
Figure 6 shows the polar pattern of Figure 5;

As described above, the unidirectional microphone unit 30 applied to the present invention has a typical cardioid directivity. Like the conventional example described,
It is possible to obtain a narrow directivity similar to that of a line microphone using a microphone unit in which a unidirectional one is specifically adjusted to be non-directional.

FIG. 6 shows the sound pickup characteristics of the line microphone according to this embodiment at 0, 90 and 180 degrees with respect to the sound source, and FIG. 7 shows the polar patterns thereof. Is located at the rear of the microphone unit 30 having a cardioid directivity,
In addition to providing the rear acoustic capacity chamber 22 having substantially the same acoustic capacity as that of the first embodiment, the rear acoustic capacity chamber 22 is filled with, for example, an acoustic resistance material 23 having an extremely small acoustic resistance value for preventing cavity resonance.

The principle of operation of the line microphone 10 will now be described based on the mechanical equivalent circuit shown in FIG.

It should be noted that in FIG.
, The distance from the front sound introduction port 201 of the acoustic tube 20 to the center position of the acoustic resistance hole 221 of the rear acoustic capacity chamber 22 is l2, and the entire length of the acoustic tube 20, that is, the front sound introduction port The distance between 201 and the rear sound wave inlet 202 is 13 and the unit length of the acoustic tube 20 is x.

In this mechanical equivalent circuit, PS is a sound source for the front acoustic terminal of the microphone unit 30 and is represented by the product PS of its sound pressure P and the effective area S of the unit diaphragm 32. PSe- ^jl3cosθ is a rear sound source for the rear acoustic terminal 31 of the microphone unit 30. Further, PSe- ^jxcosθ is a sound source per unit length x of the acoustic tube 20, and PSe- ^j11cosθ is a sound source for the entire front acoustic capacity chamber 21.

In this mechanical equivalent circuit, M0 is the acoustic mass per unit length x of the acoustic tube 20, and when the density of air is ρ and the sectional area of the acoustic tube 20 is S1, it is expressed by the following equation. M0 = ρS (S / S1)

S0 is the stiffness per unit length x of the acoustic tube 20, and is represented by the following equation, where the sound speed is c. S0 = ρc ² S ² / S1

G0 is the leakage conductance per unit length x of the acoustic tube 20, and if the width of the slit-like acoustic resistance holes 211 and 221 is b and the constant of the acoustic resistance is γ, it is expressed by the following equation. Is done. G0 = b / γS ²

Sf is the air stiffness in the front acoustic volume chamber 21, sb is the air stiffness in the rear acoustic volume room 22,
m0 is the mass of the unit diaphragm 32, s0 is the stiffness of the diaphragm 32, r0 is the braking resistance of the diaphragm 32, s1 is the air stiffness of the back air chamber in the unit, and r1 is the unit that gives directivity to the rear acoustic terminal side. , Rb is the acoustic resistance of the acoustic resistance material 24 covered by the acoustic resistance hole 221 at the rear, mb is the mass of the acoustic resistance material 24, and m is the outer peripheral surface of the microphone unit 30. The acoustic mass in the gap G between the acoustic tube 20 and the inner peripheral surface.

In this mechanical equivalent circuit, sf (air stiffness in the front acoustic capacity chamber 21), m (acoustic mass in the gap G), and sb (air stiffness in the rear acoustic capacity chamber 22) generate resonance. Impedance element,
These components constitute a resonance system. If the acoustic resistance of the shock mount 40 is represented by r, in this embodiment,
Since this acoustic resistance r is equivalent to being connected in series to the acoustic mass m of the gap G, the above sf, m and s
b can be effectively damped.

In the present invention, the acoustic resistance R0 of the acoustic resistance material 23 made of, for example, a cotton body of polyester fiber is provided on the rear acoustic capacity chamber 22 side with ^respect to the rear sound source represented by PSe- ^{jl2cos θ.} Is added, and the impedance component per unit length x of the acoustic tube 20 including the acoustic mass M0, the stiffness S0, and the leakage conductance G0 described above is added over the length of the axial length l2.

Thus, even if a condenser microphone unit having a cardioid whose directivity is typical is used as the unidirectional microphone unit 30, a microphone unit in which a unidirectional microphone is adjusted to be nearly omnidirectional is used. The same narrow directivity as that of the conventional line type microphone used can be obtained.

The impedance component added to the rear acoustic capacity chamber 22 side in the present invention operates at a low frequency when viewed from the directional frequency response, and therefore can be treated as a lumped constant instead of a distributed constant in design. .

In the above embodiment, the front acoustic volume chamber 2
1 and the rear acoustic capacity chamber 22 have substantially the same length, but the acoustic resistance of the acoustic resistance hole 221 on the rear acoustic capacity chamber 22 side and / or the acoustic resistance material 2 in the rear acoustic capacity chamber 22 are different.
By adjusting the acoustic resistance of the rear acoustic capacity chamber 2,
2 can be shortened.

[0049]

As described above, according to the present invention,
Since the directivity produced in large quantities by automatic assembly machines can use the cardioid unidirectional condenser microphone unit without any acoustic adjustment, an inexpensive line microphone is provided. You.

Further, the acoustic mass existing in the gap between the outer peripheral surface of the microphone unit that acoustically connects the front acoustic volume chamber and the rear acoustic volume room and the inner peripheral surface of the acoustic tube is reduced. For example, by connecting a predetermined acoustic resistance by a shock mount material in series, it is possible to effectively dampen the resonance caused by the stiffness of the air in both acoustic rooms.

[Brief description of the drawings]

FIG. 1 is a side view showing an embodiment of a narrow directional microphone according to the present invention.

FIG. 2 is a cross-sectional view along the axial direction of an acoustic tube of the narrow directional microphone according to the embodiment.

FIG. 3 is a sectional view of a condenser microphone unit applied to the narrow directional microphone according to the embodiment.

FIG. 4 is a graph showing sound collecting characteristics at 0, 90, and 180 degrees with respect to a sound source of the condenser microphone unit.

FIG. 5 is a graph showing a polar pattern of the condenser microphone unit.

FIG. 6 is a graph showing sound collection characteristics at 0, 90, and 180 degrees with respect to the sound source of the narrow directional microphone according to the embodiment.

FIG. 7 is a graph showing a polar pattern of the narrow directional microphone according to the embodiment.

FIG. 8 is a mechanical equivalent circuit diagram of the narrow directional microphone according to the embodiment.

FIG. 9 is a schematic sectional view showing a line microphone as a conventional example.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 10 Line type microphone (narrow directional microphone) 20 Acoustic tube 21 Front acoustic capacity room 22 Rear acoustic capacity room 211,221 Acoustic resistance hole (slit hole) 23 Acoustic resistance material 30 Microphone unit 31 Unit case 311 Front acoustic terminal hole 32 Diaphragm 33 Fixed pole 34 Insulation pedestal 341 Rear acoustic terminal hole 35 Acoustic resistance material

Claims (4)

[Claims] 1. A microphone comprising a unidirectional microphone unit having a front acoustic terminal and a rear acoustic terminal, and an acoustic tube in which the microphone unit is housed, wherein the inside of the acoustic tube is a front acoustic capacitance by the microphone unit. The rear acoustic capacity room is provided with a rear acoustic wave inlet for the rear acoustic terminal on the side of the rear acoustic volume room, and the front acoustic volume room and the rear acoustic volume room are separated from each other. In a narrow directional condenser microphone acoustically connected by a gap between the outer peripheral surface of the microphone unit and the inner peripheral surface of the acoustic tube, the unidirectional microphone unit has a directivity of cardioid The unidirectional condenser microphone unit of Narrow directional condenser microphone, wherein the acoustic resistance member to stop is provided. 2. The narrow directional condenser microphone according to claim 1, wherein the acoustic resistance material for preventing cavity resonance is made of a cotton-like fiber material filled in the rear acoustic capacity chamber. 3. The front acoustic volume chamber and the rear acoustic volume chamber both have substantially the same axial length, and each of the acoustic pipe walls has a slit-like shape along the axial direction. The narrow directional condenser microphone according to claim 1, wherein an acoustic resistance hole is formed. 4. A predetermined acoustic resistance is connected in series to an acoustic mass existing in a gap between an outer peripheral surface of the microphone unit and an inner peripheral surface of the acoustic tube. Item 5. A narrow directional condenser microphone according to Item 1, 2 or 3.