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JPH09102995

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DESCRIPTION JPH09102995
[0001]
BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a
superdirectional microphone suitable as a sound collection microphone mounted on a video
camera or the like.
[0002]
2. Description of the Related Art In recent years, with the spread of video cameras, the demand
for superdirective microphones for picking up distant sounds S / N well is increasing.
[0003]
An example of the above-described conventional superdirective microphone will be described
below with reference to the drawings.
[0004]
FIG. 4 is a block diagram showing a conventional superdirective microphone.
In FIG. 4, 11 is an acoustic tube, 12a to 12d are slit-like openings provided in the acoustic tube
11, 13 is an acoustic resistance provided to cover the openings 12a to 12d, and 14 is a
nondirectional microphone unit , 10 is a line microphone composed of an acoustic tube 11,
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apertures 12a to 12d, an acoustic resistor 13 and a nondirectional microphone unit 14, 18 is an
opening, 41 is a line microphone 10 on the back of the line microphone 10 on substantially the
same central axis A unidirectional microphone unit arranged to have a central axis at 42, 42 is a
low pass filter, 43 is a high pass filter, and 44 is an adder.
[0005]
The operation of the conventional superdirective microphone configured as described above will
be described below.
[0006]
The sound wave that has reached the acoustic tube 11 enters from the aperture 18 at the tip and
simultaneously enters the interior of the acoustic tube 11 from the slit-like apertures 12 a to 12
d through the acoustic resistor 13.
The sound wave incident on the inside of the acoustic tube propagates inside the acoustic tube
and is collected by the nondirectional microphone unit 14.
[0007]
In the line microphone 10, as the incident direction of the sound waves deviates from the 0 °
direction shown in FIG. 4, mutual cancellation of the sound waves occurs inside the acoustic tube
11, and the sensitivity drops sharply.
FIG. 5 shows the characteristics of the line microphone 10 when an acoustic tube of 10 cm in
length is used.
[0008]
Also from FIG. 5, this line microphone 10 exhibits non-directivity in the low region where the
wavelength is long and the mutual cancellation of the sound waves does not occur inside the
acoustic tube 11, but the directivity is extremely sharp in the high region where the mutual
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cancellation of the sound waves occurs. I know that I have. As described above, the line
microphone 10 is excellent in the directivity in the high region but is insufficient in the
directivity in the low region. Therefore, in the conventional example of FIG. I improve the
directional characteristics of the area.
[0009]
The output of the line microphone 10, that is, the output of the nondirectional microphone unit
14 is input to the high pass filter 43, and only the high-pass component with excellent directivity
is extracted. On the other hand, the output of the unidirectional microphone unit 41 is input to
the low pass filter 42, and the low frequency component is extracted. In the adder 44, the high
frequency component and the low frequency component are mixed and output to the outside as a
microphone output.
[0010]
The output of the adder 44, that is, the characteristics of the microphone of the conventional
example shown in FIG. 4 is shown in FIG. It can be seen from FIG. 6 that a superdirective
microphone can be realized which is unidirectional in the low band and sharper in the high band.
[0011]
However, in the conventional configuration as described above, since an element component
having a very large variation in characteristics, that is, a unidirectional microphone unit, is used,
it is excellent as a superdirective microphone. There is a problem that it is extremely difficult to
stably realize the range characteristics. Since superdirective microphones are directivity-oriented
microphones, the deterioration of directivity becomes a very serious problem.
[0012]
Even if the unidirectional microphone unit 41 is selected, the cost of the selection is high, and as
is apparent from FIG. 4, since it requires a combining circuit consisting of two filters and one
adder, it is rather expensive. Had the problem of becoming a microphone.
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[0013]
In view of the above problems, the present invention replaces the single microphone unit with a
nondirectional microphone unit and processes the output of the nondirectional microphone unit
with a low pass filter having a first-order cutoff characteristic from the line microphone output.
An object of the present invention is to provide a superdirective microphone which is extremely
excellent in directivity characteristics and is inexpensive by adopting a new structure of
subtraction.
[0014]
In order to achieve the above object, the superdirective microphone of the present invention
comprises an acoustic tube having a leak, and the inside of the acoustic tube provided at one end
of the acoustic tube. A line microphone composed of a first nondirectional microphone unit for
collecting propagating sound waves; and a second nondirectional microphone unit provided
behind the first nondirectional microphone unit A low pass filter having a first-order cutoff
characteristic for filtering the output of the second nondirectional microphone unit, and the low
pass filter from the output of the first nondirectional microphone unit incorporated in the line
microphone And a subtractor for outputting the subtraction result to the outside.
[0015]
Furthermore, in addition to the above configuration, the superdirective microphone of the
present invention has a structure in which a low-pass filter having first-order cutoff
characteristics is provided with a function of adjusting the cutoff frequency.
[0016]
Furthermore, in addition to the above configuration, in the superdirective microphone of the
present invention, the second nondirectional microphone unit is configured to be movable along
the axial direction of the acoustic tube.
[0017]
BEST MODE FOR CARRYING OUT THE INVENTION The present invention is configured by using
the line microphone, the second nondirectional microphone unit, the low pass filter having the
first-order cutoff characteristics, and the subtractor described above. An ultradirective
microphone having excellent directivity over a wide frequency band can be supplied
inexpensively and with less variation in characteristics.
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[0018]
Furthermore, by providing the low-pass filter with the function of adjusting the cutoff frequency,
it is possible to compensate for variations in the characteristics of the line microphone, and a
superdirectional microphone with less variation in characteristics is realized.
[0019]
Furthermore, by providing the position adjustment function in the second nondirectional
microphone unit, it is possible to compensate for variations in the characteristics of the line
microphone at a lower cost.
[0020]
Hereinafter, a superdirective microphone according to an embodiment of the present invention
will be described with reference to the drawings.
[0021]
FIG. 1 is a block diagram of a superdirective microphone according to an embodiment of the
present invention.
In FIG. 1, 11 is an acoustic tube, 12a to 12d are slit-like openings provided in the acoustic tube
11, 13 is an acoustic resistance provided to cover the openings 12a to 12d, and 14 is a first nondirectional Microphone unit 10 is a line microphone composed of an acoustic tube 11, apertures
12a to 12d, an acoustic resistor 13 and a nondirectional microphone unit 14, and 15 is centered
behind the line microphone 10 on substantially the same central axis A second omnidirectional
microphone unit disposed with an axis, 16 is a low pass filter having a first-order cutoff
characteristic, and 17 is a subtractor.
[0022]
The operation of the superdirective microphone of FIG. 1 configured as described above will be
described below.
[0023]
The line microphone 10 composed of the acoustic tube 11, the openings 12a to 12d, the acoustic
resistance 13 and the first nondirectional microphone unit 14 has the same configuration as that
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of the conventional example shown in FIG. It shows exactly the same characteristics of FIG.
[0024]
The output of the line microphone 10, that is, the output of the first nondirectional microphone
unit 14 is input to a subtractor 17.
On the other hand, the output of the second nondirectional microphone unit 15 having the same
characteristics as the first nondirectional microphone unit 14 is subjected to low-pass component
extraction and delay processing by the low-pass filter 16 having first-order cutoff characteristics.
And are input to the subtractor 17.
The subtractor 17 subtracts the output of the low pass filter 16 from the output of the line
microphone 10 and outputs the result to the outside.
[0025]
The low-pass component of the second nondirectional microphone unit 15 extracted by the low
pass filter 16 having the first-order blocking characteristic is subjected to delay processing
optimum for forming unidirectionality by the low pass filter 16, By subtracting this signal from
the output of the line microphone 10 by the subtractor 17, unidirectivity is introduced in the low
band without deteriorating the high band directivity (FIG. 5) of the line microphone 10.
The directivity characteristic of the superdirective microphone according to the configuration of
the present embodiment can obtain directivity characteristics as shown in FIG. 6 as in the
conventional example described above.
Only a low-pass filter having a first-order cutoff characteristic can provide an appropriate
amount of delay.
The reason is described below.
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[0026]
FIG. 7 shows the sensitivity characteristic of the line microphone 10 with respect to the incident
sound wave from the 180 ° direction shown in FIG.
FIG. 6A shows the amplitude characteristic, and FIG. 6B shows the phase characteristic as the
phase difference with the second nondirectional microphone 15.
The phase characteristic of FIG. 7B corresponds to representing the delay of space propagation
due to the arrival of the sound wave to the line microphone 10 later than the second
nondirectional microphone 15 as a numerical value of phase difference.
Further, FIG. 8 shows the transfer characteristic of the low pass filter 16 having a first-order
cutoff characteristic.
The figure (a) has shown the amplitude characteristic and (b) has shown the phase characteristic.
Comparing FIG. 7 with FIG. 8, it can be seen that the characteristics of the two closely
approximate each other in the low frequency band of 1 kHz or less.
[0027]
The fact that the phase characteristic of FIG. 7 and the phase characteristic of FIG. 8 approximate
to each other indicates that the low-pass filter 16 having a first-order cutoff characteristic has
sufficient performance as a delay for compensating for the delay due to space propagation It is
shown that. Furthermore, the fact that the amplitude characteristic of FIG. 7 and the amplitude
characteristic of FIG. 8 approximate is a line simply by simply subtracting the outputs of the two,
without performing the complex filtering process as in the conventional example. It is shown that
unidirectivity can be introduced only in the low band without disturbing the high band
characteristic of the microphone 10.
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[0028]
Therefore, by subtracting the output of the second nondirectional microphone unit 15 processed
by the low-pass filter 16 having the first-order cutoff characteristic from the output of the line
microphone 10, the unidirectionality in which the output in the 180 ° direction becomes zero It
can be seen that the directivity characteristic of can be introduced only in the low band.
[0029]
According to the configuration of the present invention, unidirectionality is introduced in the low
band simply by processing the output of the second omnidirectional microphone 15 with the
first-order lowpass filter 16 and subtracting it from the output of the line microphone 10
Although the configuration is different from that of the conventional example shown in FIG. 4,
the directivity characteristic of FIG. 6 is obtained as in the conventional example.
This is because the first-order low-pass filter 16 works just as well as extracting low frequency
components and also as a delay for introducing single directivity. The first-order low-pass filter is
used, and the cutoff frequency is adjusted to near the frequency where the directivity of the line
microphone 10 begins to appear (400 to 800 Hz in this embodiment using a 10 cm-long acoustic
tube). With the simple configuration as in the embodiment of the present invention, the same
effect as the configuration of the conventional example of FIG. 4 can be obtained.
[0030]
As described above, according to this embodiment, the output of the line microphone 10, the
second nondirectional microphone unit 15 provided behind the line microphone 10, and the
second nondirectional microphone unit 15 is subjected to filtering processing And a subtractor
17 for subtracting the output of the low-pass filter 16 from the output of the line microphone 10
and outputting the result externally to constitute a superdirective microphone. A superdirective
microphone having excellent directivity over a wide frequency band can be realized
inexpensively with less variation in characteristics, without using a unidirectional microphone
unit or a high pass filter.
[0031]
Next, a second embodiment will be described.
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FIG. 2 is a block diagram showing a superdirective microphone in a second embodiment of the
present invention. In FIG. 2, reference numeral 21 denotes a first-order low-pass filter having a
cutoff frequency adjustment function, and is shown in FIG. 1 except that the low-pass filter 16 of
FIG. The configuration is the same as that of the first embodiment, and the same applies to the
operation.
[0032]
In this embodiment, a change in the characteristics of the line microphone 10 due to the
variation of the acoustic resistance 13 is to be absorbed by the adjustment of the cut-off
frequency of the low pass filter 21. Specifically, the above-mentioned adjustment is easily
possible by varying the resistance value of the variable resistor 22 provided in the low pass filter
21. When the resistance value of the acoustic resistance 13 is large and the directivity of the line
microphone 10 is obtained to a relatively low range, the cutoff frequency is low, and the
resistance value of the acoustic resistance 13 is small, the directivity in the low range If it is not
obtained, it is clear from experiments that it is possible to compensate for the characteristic
deterioration due to the variation of the characteristics of the line microphone 10 by setting the
cutoff frequency high, and the above compensation is made by the configuration of the present
embodiment. While properly applied, directivity characteristics as shown in FIG. 6 can be
obtained.
[0033]
In order to manufacture the line microphone 10 with uniform characteristics, the acoustic
resistance 13 needs to be strictly sorted, which is extremely expensive. Therefore, by adopting a
configuration in which the cutoff frequency of the low pass filter 21 is adjusted in accordance
with the characteristics of the line microphone 10 as in this embodiment, an inexpensive line
microphone manufactured in a rough process can be used. Thus, the superdirective microphone
can be realized at low cost.
[0034]
The third embodiment will be described below. FIG. 3 is a block diagram showing a
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superdirective microphone in a third embodiment of the present invention. In FIG. 3, reference
numeral 31 denotes a rod attached to the acoustic tube 11, and reference numeral 32 denotes a
fixture installed on the second nondirectional microphone twill unit 15, except that the rod 31
and the fixture 32 are newly provided. This is the same as the first embodiment shown in FIG.
[0035]
In this embodiment, the variation in the characteristics of the line microphone 10 is to be
absorbed by the movement of the second nondirectional microphone unit 15. When the
resistance value of the acoustic resistance 13 is large and the directivity of the line microphone
10 is obtained to a relatively low frequency range, the fixing tool 32 is slid relative to the rod 31,
and the second nondirectional microphone unit When the distance 15 is away from the line
microphone 10 and the resistance value of the acoustic resistance 13 is small and directivity in
the low band is not obtained, the fixture 32 is slid relative to the rod 31 to make the second It
has been clarified by experiments that by installing the directional microphone unit 15 close to
the line microphone 10, it is possible to compensate for the characteristic deterioration due to
the variation of the characteristic of the line microphone 10. As described above, by adopting the
configuration of this embodiment, directivity characteristics as shown in FIG. 6 can be obtained
while appropriately performing the above-mentioned compensation.
[0036]
As described above, in the embodiment shown in FIG. 3, since it is not necessary to use the
variable resistor 22 provided in the low pass filter 21 as in the embodiment of FIG. 2, the cost is
lower than in the embodiment of FIG. Can realize a super-directional micro-phone.
[0037]
The present invention is not limited to the configuration of each of the above-described
embodiments. For example, the configurations of the second and third embodiments can be
combined.
[0038]
As described above, according to the present invention, a line microphone, a second
nondirectional microphone unit, a low pass filter having first-order cutoff characteristics, and a
subtractor are used. Thus, the superdirective microphone having excellent directivity over a wide
frequency band can be supplied inexpensively and with less variation in characteristics.
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[0039]
Furthermore, by providing the low-pass filter with the function of adjusting the cutoff frequency,
it is possible to compensate for variations in the characteristics of the line microphone, and a
superdirectional microphone with less variation in characteristics is realized.
[0040]
Furthermore, by providing the position adjustment function in the second nondirectional
microphone unit, it is possible to compensate for variations in the characteristics of the line
microphone at a lower cost.
[0041]
Brief description of the drawings
[0042]
1 is a block diagram of the superdirective microphone according to the first embodiment of the
present invention
[0043]
2 is a block diagram of the superdirective microphone according to the second embodiment of
the present invention
[0044]
3 is a block diagram of the superdirective microphone according to the third embodiment of the
present invention
[0045]
Fig. 4 Block diagram of the conventional superdirective microphone
[0046]
Fig. 5 Directivity characteristics of the line microphone according to the prior art and the present
invention
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[0047]
Fig. 6 Directivity characteristics of the superdirective microphone according to the prior art and
the present invention
[0048]
Fig. 7 Sensitivity characteristics of the line microphone according to the conventional example
and the present invention to the sound wave incident from the 180 ° direction
[0049]
Fig. 8 Transfer characteristic diagram of the low-pass filter having the first-order cutoff
characteristic according to the embodiment of the present invention
[0050]
Explanation of sign
[0051]
DESCRIPTION OF SYMBOLS 11 Acoustic pipe 12a-12d Slit-shaped opening part 13 Acoustic
resistance 14 1st nondirectional microphone unit 15 2nd nondirectional microphone unit 16 low
pass filter 17 Subtractor 21 low pass filter 22 variable resistor 31 rod 32 fixation Tool
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