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JPH11331978

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This translation is machine-generated. It cannot be guaranteed that it is intelligible, accurate,
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DESCRIPTION JPH11331978
[0001]
BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a
narrow directional microphone, and more particularly to the technology of a narrow directional
microphone which can shorten an acoustic pipe (interference pipe) without enhancing a
proximity effect. It is.
[0002]
2. Description of the Related Art A narrow directional microphone is known as a microphone
with a small angle that can be picked up even in outdoor news gathering. There are line type
microphones and secondary sound pressure gradient type microphones roughly, but here, line
type microphones 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, and, for example, a slit in which an acoustic resistance material is attached to a pipe
peripheral wall. The acoustic tube is connected to the front acoustic terminal side of the
microphone unit, and the rear acoustic terminal is exposed to free space.
[0004]
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In this line type microphone, the sound waves from the side or the back reach the acoustic tube
from the acoustic resistance hole of the acoustic tube and the front opening of the acoustic tube
respectively, but from the phase difference due to the difference in their arrival paths Sound
wave interference occurs, which attenuates the sound waves from the side or the back.
On the other hand, sound waves from the front opening reach the diaphragm of the microphone
unit without attenuation.
[0005]
The advantage of this line type microphone is that the frequency response characteristic is
relatively flat, and the sensitivity and intrinsic noise are superior to those of the second sound
pressure gradient type microphone. Line type microphones are often used in professional video
cameras and the like where high quality sound collection is required.
[0006]
By the way, by making the rear acoustic terminal face free space, a fairly narrow directivity can
be obtained, but on the other hand, it picks up external wind noise etc. Also, the proximity effect
that distorts the low range when the sound source is close is high. There was a problem of
becoming This is because the sound wave introduction port of the front acoustic terminal is the
tip of the acoustic tube, and the distance between the front acoustic terminal and the rear
acoustic terminal in the low band becomes long.
[0007]
In order to solve this point, the present applicant has proposed one proposal in Japanese Patent
Application Laid-Open No. 62-118698. This is taken as a conventional example of the present
invention, and the configuration thereof is outlined based on the schematic cross sectional view
of FIG. Explain it.
[0008]
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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 divided by the same
microphone unit 3 into the front acoustic capacity chamber 21 and the rear. It is divided into an
acoustic capacity room 22.
A slit-like 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.
[0009]
The front acoustic terminal 31 is provided on one side (left side in FIG. 4) of the microphone unit
3, and the rear acoustic terminal 32 is provided on the other side (right side in FIG. 4). A sound
wave inlet 2 b for the rear acoustic terminal 32 is bored on the side of the rear acoustic capacity
chamber 22. Although not shown in detail, an appropriate acoustic resistance material is
attached to the sound wave inlet 2b.
[0010]
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, and the front
acoustic terminal 31 and the rear acoustic terminal 32 of the microphone unit 3 are acoustically
Connected.
[0011]
Thus, the rear acoustic terminal 32 of the microphone unit 3 communicates with the free sound
field via the acoustic wave inlet 2 b and is also acoustically connected to the front acoustic
terminal 31 through the gap G. The distance between the acoustic terminals in the low band 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]
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The above-described prior art technique is effective when the acoustic tube 2 is relatively long.
However, particularly in the case of being mounted on a video camera, the following problems
occur when the acoustic tube 2 is shortened in recent years as the video camera is made smaller
and lighter.
[0013]
The cause will be described based on the machine equivalent circuit of FIG. First, PS is a sound
source for the front acoustic terminal 31 and is represented by the product of the sound pressure
P and the effective area S of the unit diaphragm. Further, PSe-jkl cos θ is a sound source for the
rear acoustic terminal 31, and in this case, l is the distance between the front opening of the
acoustic tube 2 and the acoustic inlet 2b at the rear.
[0014]
Z is the total impedance of the acoustic tube 2, sf is the air stiffness in the front acoustic capacity
chamber 21, sb is the air stiffness in the rear acoustic capacity chamber 22, m0 is the mass of
the unit diaphragm, s0 is the stiffness of the unit diaphragm, r0 is the braking resistance of the
unit diaphragm, s1 is the air stiffness of the back air chamber of the unit, r1 is the acoustic
resistance giving directivity to the rear acoustic terminal 31, and rb is the acoustic resistance
material placed on the acoustic wave inlet 2b at the rear. The acoustic resistance, mb is the mass
of the acoustic resistance material, and m is the acoustic mass in the gap G between the outer
peripheral surface of the microphone unit 3 and the inner peripheral surface of the acoustic tube.
[0015]
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) are
impedance elements that generate resonance. These constitute a resonant system.
[0016]
The total impedance Z of the acoustic tube 2 increases in proportion to the axial length if the
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inner diameter is the same.
Therefore, when the acoustic tube 2 is long, the impedance of the resonance system due to the
above sf, m and sb can be ignored, and the influence of the resonance on the frequency response
and directivity is extremely small.
[0017]
On the other hand, when the acoustic tube 2 is shortened, the total impedance Z approaches the
impedance of the resonance system due to the above sf, m and sb, so the influence of the
resonance becomes remarkable, and the frequency response and directivity are improved.
Irregularities are likely to occur.
[0018]
Therefore, conventionally, an acoustic resistance material having a relatively low acoustic
resistance per unit area is covered on the acoustic wave inlet 2 b at the rear portion to damp the
above-mentioned resonance.
However, if the acoustic resistance is too large, it becomes equivalent to opening the portions rb
and mb in the mechanical equivalent circuit of FIG. 5, and it becomes difficult for the sound wave
to enter from the rear sound inlet 2b. Also, the resonance due to sf, m and sb can not be
suppressed.
[0019]
An example of measurement of frequency response and directivity when the acoustic resistance
is too large is shown in FIG.
According to this, it is understood that the directivity in the middle to low frequency range of
200 to 500 Hz is not good. In addition, in the high frequency band of 2 to 6 kHz, the rise of the
characteristic is observed, and it is shown that the directivity is poor also in this band.
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[0020]
On the contrary, if the acoustic resistance of the acoustic resistance material applied to the rear
sound wave inlet 2b is too small, it is equivalent to the rb and mb portions being shorted in the
mechanical equivalent circuit of FIG. As a result, although sb is removed from the abovedescribed resonant system, resonance is still generated by the sf and m impedance elements.
[0021]
An example of measurement of frequency response and directivity when the acoustic resistance
is too small is shown in FIG.
In this case, although the directivity in the middle and low band is improved compared to the
measurement example of FIG. 6A, the rise of the characteristic is seen in the high band of 2 to 10
kHZ and the directivity is bad I understand.
[0022]
Because of this, conventionally, when shortening the acoustic tube 2, the acoustic resistance
material to be applied to the sound wave inlet 2b in the rear part is selected by trial and error,
which requires considerable time and labor. It would be desirable to improve on this point.
[0023]
The present invention has been made to meet such a need, and its object is to provide an acoustic
tube which can reliably suppress resonance by a simple means without losing resistance to wind
noise and the like. To provide a short narrow directional microphone.
[0024]
In order 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. And the acoustic tube is divided into the front acoustic capacity
chamber and the rear acoustic capacity chamber by the microphone unit, and the front acoustic
terminal of the microphone unit and the rear acoustic terminal are acoustically connected. A
predetermined gap is provided between the outer peripheral surface of the microphone unit and
the inner peripheral surface of the acoustic tube, and a sound wave inlet for the rear acoustic
terminal is provided on the rear acoustic capacity chamber side of the acoustic tube. In a narrow
directional microphone provided with a microphone, the acoustic mass existing in the gap
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between the outer peripheral surface of the microphone unit and the inner peripheral surface of
the acoustic tube is used. And it is characterized in that the predetermined acoustic resistor is
connected in series.
[0025]
According to this configuration, the acoustic resistance r is connected in series in the resonance
system of the air stiffness sf in the front acoustic capacity chamber, the acoustic mass m in the
gap, and the air stiffness sb in the rear acoustic capacity chamber in the above mechanical
equivalent circuit. The resonance of the same resonance system is effectively damped.
[0026]
In the present invention, the acoustic resistance is provided by an acoustic resistance material of
a predetermined material, but the acoustic resistance material is between the gap and the rear
acoustic capacity chamber, in the gap, or the gap and the front acoustic capacity chamber It may
be placed anywhere between
[0027]
Further, it is preferable that the above-mentioned acoustic resistance material is made of an
elastic body of an open cell compressed in a predetermined manner, and according to this, it is
possible to expect an anti-vibration effect on the microphone unit.
As such an elastic body, a sponge rubber which is easy to obtain, good in processability
(moldability) and inexpensive is preferable.
[0028]
BEST MODE FOR CARRYING OUT THE INVENTION Next, in order to better understand the
technical concept of the present invention, a preferred embodiment thereof will be described
with reference to the schematic sectional view of FIG. 1 and the mechanical equivalent circuit of
FIG. Do.
[0029]
Also in the line microphone 1A according to this embodiment, the unidirectional microphone unit
3 is housed inside the rear end side of the acoustic tube 2, and the microphone unit 3 makes the
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inside of the acoustic tube 2 a front acoustic capacity chamber 21 and the like. It is divided into a
rear acoustic capacity chamber 22.
[0030]
The general dimensions of the acoustic tube used in the line microphone are, for example, an
inner diameter of 18 mm and an axial length of about 390 mm, while the dimensions of the
acoustic tube 2 in this embodiment are an inner diameter of 18 mm and an axial length of 162
mm. Its length is less than half that of ordinary ones.
[0031]
A slit-like acoustic resistance hole 2a is formed on the front side of the acoustic tube 2, and an
acoustic resistance material (for example, nylon mesh # 508 manufactured by NBC) (not shown)
is attached to the acoustic resistance hole 2a. There is.
The acoustic resistance holes 2a may be formed by axially arranging slit holes formed partially
along the circumferential direction of the acoustic tube 2 at predetermined intervals.
[0032]
Further, as in the prior art, the front acoustic terminal 31 is provided on one side (left side in FIG.
1) of the microphone unit 3, and the rear acoustic terminal 32 is provided on the other side
(right side in FIG. 1). A sound wave inlet 2 b for the rear acoustic terminal 32 is bored on the side
of the rear acoustic capacity chamber 22 of the acoustic tube 2.
For example, nylon mesh # 200 manufactured by NBC is attached as an acoustic resistance
material to the sound wave introduction port 2b.
[0033]
Also in this embodiment, the outer diameter of the microphone unit 3 is smaller than the inner
diameter of the acoustic tube 2.
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That is, a predetermined gap G is provided between the microphone unit 3 and the acoustic tube
2, and the front acoustic terminal 31 and the rear acoustic terminal 32 of the microphone unit 3
are acoustically connected by the gap G.
This reduces the influence of wind noise and the proximity effect.
[0034]
The acoustic resistance material 4 is disposed in the rear acoustic capacity chamber 22 so as to
cover the gap G.
In this embodiment, the acoustic resistance material 4 is formed of a suitably compressed sponge
material (for example, HR50 manufactured by Bridgestone, compression ratio 1/5) and formed
into a donut shape having an outer diameter of 18 mm, an inner diameter of 10 mm and a
thickness of 3 mm. It is done.
[0035]
FIG. 2 shows a mechanical equivalent circuit of the line microphone 1A.
Here, assuming that the acoustic resistance of the acoustic resistance material 4 is r, this acoustic
resistance r is equivalent to being connected in series to the acoustic mass m of the gap G, and
according to the above sf, m and sb The resonance can be effectively damped.
[0036]
FIG. 3 shows an example of measurement of frequency response and directivity of this
embodiment.
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According to this, it can be seen that the frequency response and directivity are both significantly
improved and good over the entire range from the low band to the high band as compared with
the conventional example of FIGS. 6 (a) and 6 (b).
[0037]
Although the acoustic resistance material 4 is disposed in the rear acoustic capacity chamber 22
in the above embodiment, it may be disposed in the gap G or on the front acoustic capacity
chamber 21 side. Further, this acoustic resistance material 4 can also have a role as a shock
mount of the microphone unit 3. The material of the acoustic resistance material 4 may be other
than a sponge, and for example, a non-woven fabric, the nylon mesh illustrated above, etc. are
applicable.
[0038]
As described above, according to the present invention, in order to reduce the influence of wind
noise and proximity effect, a predetermined gap is provided between the outer peripheral surface
of the microphone unit and the inner peripheral surface of the acoustic pipe. In the narrow
directional microphone in which the front acoustic terminal and the rear acoustic terminal of the
microphone unit are acoustically connected, the acoustic mass m present in the gap between the
outer peripheral surface of the microphone unit and the inner peripheral surface of the acoustic
tube The resonance system of the air stiffness sf in the front acoustic capacity chamber, the
acoustic mass m in the gap, and the air stiffness sb in the rear acoustic capacity chamber in the
mechanical equivalent circuit by a simple configuration in which predetermined acoustic This is
equivalent to connecting the acoustic resistance r in series, and the resonance of the same
resonance system can be effectively damped.
[0039]
Therefore, it is possible to inexpensively provide a high-performance narrow directivity line
microphone that is easy to assemble and has good frequency response and directivity even if the
acoustic tube is short.
[0040]
In addition, by using an elastic body (preferably, a sponge that is appropriately compressed) of a
predetermined number of compressed cells as the acoustic resistance material of the acoustic
resistance r, vibration isolation of the microphone unit can also be achieved.
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