JP3047613B2 - Super directional microphone - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a super-directional microphone.

[0002]

2. Description of the Related Art In recent years, as a super-directional microphone,
Line microphones (gun microphones), high-order sound pressure gradient type microphones, parabolic sound collectors, etc. are used. It is desired to extend the sound pickup distance.

Hereinafter, an example of the above-described conventional super-directional microphone will be described with reference to the drawings. FIG. 5 is a cross-sectional view showing a configuration of a conventional super-directional microphone. In FIG. 5, reference numeral 51 denotes a housing. Reference numeral 52 denotes a first sound wave introduction hole, which is provided at the tip of the housing 51. 53
Reference numerals 64 are second to thirteenth sound wave introduction holes provided on the side surface of the housing 51. A first acoustic resistor 65 is provided on the housing 51 from the first sound wave introduction hole 52 to the twelfth sound wave introduction hole 63.
It is provided from the inside in contact with. Reference numeral 66 denotes a second acoustic resistor, which is provided on the thirteenth sound wave introduction hole 64 in contact with the housing 51 from the inside. A microphone unit 67 includes a twelfth sound wave introduction hole 63 and a thirteenth sound wave introduction hole 64.
Between the first sound wave introduction hole 5 and the inside of the housing 51.
2 are provided. 68 is an output terminal.

The operation of the super-directional microphone configured as described above will be described below. Housing 5
The flow of the sound wave at the time of sound collection will be described with the first sound wave introduction hole 52 as the front side (θ = 0 °) of the microphone. The first sound wave introduction hole 52 to the twelfth sound wave introduction hole 6
The sound wave entering the inside of the housing 51 from the third is transmitted through the inside of the housing 51 and synthesized on the surface of the diaphragm of the microphone unit 67, and the sound wave entering the inside of the housing 51 from the thirteenth sound wave introduction hole 64. Is transmitted through the inside of the housing 51 to the back surface of the diaphragm. As a description of the formation of the qualitative directivity, as for the sound wave coming from the front direction, the sound wave entering from any of the sound wave introduction holes from the first sound wave introduction hole 52 to the twelfth sound introduction hole 63 is transmitted from the microphone unit 67. Since the distances to the sound source are equal, the sound waves are additively synthesized in phase.
On the other hand, when the sound source is located at an angle θ from the front, the distance from the microphone unit 67 to the sound source is the longest when the sound source is from the first sound wave introduction hole 52 and is the longest when the sound source is from the twelfth sound wave introduction hole 63. Is the shortest distance. Therefore, the sound wave arriving from the angle θ has a phase difference at the position of the microphone unit 67 due to the sound wave introduction hole passing therethrough, and the sensitivity is lower than the sound pressure when θ = 0 ° synthesized in the same phase. I do.

A thirteenth sound wave introducing hole 64 is provided for applying a sound pressure to the rear surface of the diaphragm when a microphone unit 67 having a unidirectional characteristic is used to obtain a directional characteristic in a low frequency range. ing.

A first acoustic resistor 65 provided between the first sound wave introduction hole 52 to the twelfth sound wave introduction hole 63,
The second acoustic resistor 66 provided in the thirteenth sound wave introduction hole 64 is provided for weighting the sound pressure of the sound wave entering from each sound wave introduction hole.

The sound pressure at the surface of the diaphragm of the microphone unit 67 of the sound wave entering from the n-th sound wave introduction hole at the angle θ is:
Assuming that the distance from the n-th sound wave introduction hole to the microphone unit 67 is xn, the following expression is obtained (for example, "Audio Engineering", by Heitaro Nakajima, Jikkyo Shuppan, pp. 210-213).

[0008]

(Equation 3)

The sound pressure applied from the thirteenth sound wave introduction hole 64 to the back surface of the diaphragm of the microphone unit 67 is given by the following equation.

[0010]

(Equation 4)

Therefore, the sound pressure applied to the diaphragm of the microphone unit 67 is

[0012]

(Equation 5)

Thus, the directional characteristics are as shown in FIG.

[0014]

However, in the above arrangement, since the wavelength is long at a low frequency, the phase of a sound wave entering from each sound wave introduction hole of a sound source located in a direction other than the front direction changes very much at the microphone unit 17. Therefore, there is a problem that the directivity is hard to be obtained in a low sound range and the directivity is too sharp in a high sound range.

The size of the hole from the first sound wave introduction hole 52 to the thirteenth sound wave introduction hole 64 and the size of the first acoustic resistor 6
It is difficult to obtain the accuracy of weighting the sound pressure from the sound wave introduction hole by the fifth or second acoustic resistor 66, and it is not possible to sufficiently secure the attenuation of the sensitivity other than in the front direction. I have.

The present invention has been made in view of the above-mentioned problems, and provides a super-directional microphone in which sharp directivity characteristics, a large amount of attenuation other than the front direction, and uniformity of directivity with respect to frequency are improved.

[0017]

In order to solve the above-mentioned problems, a super-directional microphone according to the present invention has a linear distance d.
First to Nth microphone units arranged in
A two-dimensional filter that receives signals from the first to Nth microphone units as inputs; and a frequency characteristic correction unit provided at a subsequent stage of the two-dimensional filter, and the amplitude frequency characteristic of the two-dimensional filter is a normalized time frequency f1 ( -0.5 ≦ f1 ≦ 0.5)
And on a two-dimensional frequency plane represented by a normalized spatial frequency f2 (-0.5≤f2≤0.5) a linear region represented by

[0018]

(Equation 6)

In addition, the time sampling frequency fs of the two-dimensional filter is configured to be within the range of the following equation.

[0020]

(Equation 7)

[0021]

According to the present invention, the first to the Nth
Sound waves arriving at each of the microphone units are converted into electric signals, and are converted into digital signals by first to N-th analog-to-digital conversion means provided at the subsequent stage of each microphone unit. The signal obtained here becomes two-dimensional data of a spatial sample by the arrangement of the first to Nth microphone units and a time sample by the first to Nth analog-to-digital conversion means.
The signal at time m for the microphone unit of
As (m, n), a two-dimensional frequency spectrum X (f1, f2) obtained by performing a two-dimensional Fourier transform on x (m, n) is represented by the first
Assuming that the direction in which the Nth microphone unit is viewed from the microphone unit is the front direction (θ = 0 °), the interval between the microphone units is d, the sound speed is c, and the sampling frequency is fs, the sound wave is expressed on a straight line represented by the following equation. There is a frequency spectrum (eg, "Design of Digital Filter", Miki Takebe, Tokai University Press, pp. 224-228).

[0022]

(Equation 8)

As described above, the two-dimensional frequency spectrum X (f1, f
2) is a set of sound wave spectra appearing on a straight line whose inclination changes with the incident angle θ as shown in (Equation 8).
Amplitude-frequency characteristics H (f1, f2) of a two-dimensional filter provided after the first to N-th analog-digital conversion means
In the two-dimensional filter, the processing of X (f1, f2) H (f1, f2) is performed by setting the straight line portion shown in (Equation 6) to the pass region and setting the sampling frequency fs to (Equation 7). As a result, sound waves in the front (θ = 0 °) direction pass, and sound waves from directions other than the front are blocked.

Normally, a directivity characteristic is obtained by matching the spectrum of the sound wave in the front direction with the pass region of the two-dimensional filter (corresponding to the case of b = 0). Since a phenomenon occurs in which the directivity becomes too sharp in a high-frequency range, an appropriate value is given to the constant b in (Equation 6) and (Equation 7). At this time, the spectrum of the sound wave from the front (θ = 0 °) appears on a straight line represented by the following equation when the maximum value in the range of (Equation 7) is substituted into (Equation 8).

[0025]

(Equation 9)

On the other hand, since the passing area of the two-dimensional filter is on the line of (Equation 6), the spectrum of the sound wave from the front direction has a normalized time frequency f1 of 0 as compared with (Equation 9).
(Lower range), the farther away from the passing area, the more f1 becomes 0.
The closer to 5 (treble range), the closer to the passing area. The purpose of such a setting is to achieve an acute angle of directivity and a uniform directivity with respect to the frequency. The reason for this is that the spectral position of the sound wave from the front direction is determined from the passing area of the two-dimensional filter. By removing in advance, when the arrival direction of the sound wave becomes large from θ = 0 °, the directivity becomes sharper because the spectrum of the sound wave can immediately enter the blocking region. The effect is that the effect is more pronounced in the lower frequency range and the directivity with respect to the frequency can be made uniform. The reason that the sampling frequency fs has the range of (Equation 7) is that when fs = c / d, the passing area of the two-dimensional filter which is the area represented by (Equation 6) and the sound wave in the front direction indicated by (Equation 8) Are in a parallel relationship, and the setting emphasizes sharpening of the directivity, and fs = c / ｛d (1
-2b) In the case of｝, the emphasis is on setting uniformity of directivity with respect to frequency. If fs <c / d, the spectrum of the high-frequency range does not enter the pass band or the cut-off region. As a result, microphone sensitivity in the high frequency range cannot be obtained, and when fs> c / {d (1-2b)}, the spectrum in the high frequency range passes through the pass region other than in the front direction. The maximum sensitivity appears in directions other than the front direction.

The sound waves in the front direction extracted as described above are converted into analog signals by digital-to-analog conversion means downstream of the two-dimensional filter, and are output after correcting sound pressure frequency characteristics by frequency characteristic correction means.

As described above, the directivity is improved from the low frequency range, the uniformity of the directivity with respect to the frequency is improved, the sensitivity is increased by combining the output signals of a plurality of microphone units, and the digital filter is improved. By the high-precision calculation by, the amount of attenuation of the sensitivity other than the front direction can be increased.

[0029]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A super directional microphone according to an embodiment of the present invention will be described below with reference to the drawings. FIG. 1 shows the configuration of a super-directional microphone according to an embodiment of the present invention. In FIG. 1, 1 to 3 are first to third omnidirectional microphone units, 4 is an Nth omnidirectional microphone unit, and the first omnidirectional microphone unit 1 to the Nth omnidirectional microphone unit. Up to the microphone unit 4, N omnidirectional microphone units are linearly arranged at an interval d. Hereinafter, the first to Nth omnidirectional microphone units are simply referred to as first to Nth microphone units. Reference numerals 11 to 14 denote first to N-th analog-to-digital converters, which are respectively provided at the subsequent stages of the first microphone unit 1 to the N-th microphone unit 4. 15 is a two-dimensional filter,
The output signals from the analog-to-digital converter 11 to the N-th analog-digital converter 14 are input. Numeral 16 denotes a digital-to-analog conversion means, which is provided after the two-dimensional filter 15. Reference numeral 17 denotes frequency characteristic correction means, which is provided at a stage subsequent to the digital-to-analog conversion means 16. 18 is an output terminal.

FIG. 2 shows a configuration example of the two-dimensional filter 15 of the super-directional microphone according to the first embodiment of the present invention. In FIG. 2, 151 to 154
Denotes first to Nth signal lines from the first analog-to-digital converter 11 to the N-th analog-digital converter 14, respectively. 155 to 158 are the first
To the N-th FIR filter and the first signal line 1
It is provided at the subsequent stage from the 51st to the Nth signal line 154. Fifteen
Reference numeral 9 denotes an adding unit that adds the signals from the first FIR filter 155 to the Nth FIR filter 158. An output signal line 160 is a signal line between the adding means 159 and the digital-to-analog converting means 16.

FIG. 3 is a contour display on a two-dimensional frequency plane of a passband characteristic example of the two-dimensional filter 15 of the super-directional microphone according to the first embodiment of the present invention. Here, the horizontal axis of the two-dimensional frequency plane is the time frequency f1 and the vertical axis is the spatial frequency f2. The solid line in the figure represents the passing area indicated by (Equation 6), and is obtained when b = 0.028. The dotted line indicates the spectrum of the sound wave arriving from the direction of θ = 0 (front) according to (Equation 9). Indicates the position where appears.

The operation of the super-directional microphone configured as described above will be described below with reference to FIGS. 1 to 4.

In FIG. 1, the first to Nth microphone units 1 to 4 arranged linearly at an interval d convert sound waves into electric signals. The first to Nth analog-to-digital converters 11 to 14 convert the signals into digital values, respectively. First microphone unit 1
The number of the microphone unit from the first to the Nth microphone unit 4 is represented by n, and the first analog-to-digital converter 11 to the N-th analog-digital converter 1
If the order sampled in the time direction at 4 is represented by m,
The signal at the m-th time from the n-th microphone unit is a two-dimensional signal in the form of x (m, n). The two-dimensional filter 15 performs a convolution operation on the two-dimensional signal x (m, n) as represented by the following equation.

[0034]

(Equation 10)

(FIG. 2) is an example in which (Equation 10) is constituted by a generally used FIR filter. The first FIR filter 155 to the N-th FIR filter 158 are represented by (Equation 1).
When 0) is expanded and expressed, the following equation is obtained.

[0036]

[Equation 11]

(Equation 11) is the first FI in order from the first term.
It corresponds to the R filter 155 to the N-th FIR filter 158. Thus, the configuration shown in FIG. 2 is generally used as one means for configuring the two-dimensional filter 15. According to the present invention, the passing region of the amplitude frequency characteristic of h (i, n) in (Equation 10) is on the straight line shown in (Equation 6), and sampling is performed for the microphone unit interval d and sound speed c. It is characterized in that the frequency fs is represented by (Equation 7).
Is set, the horizontal axis represents the time frequency f
On the two-dimensional frequency plane where the vertical axis is the spatial frequency f2 in FIG. 1, it appears on a straight line represented by (Expression 9), and the inclination of the straight line varies depending on the incident angle θ of the sound wave.

FIG. 3 shows (Equation 6) when b = 0.028.
Two-dimensional filter 1 having passband characteristics on the line indicated by
5 and the position of θ = 0 ° among the positions of the spectrum of the sound wave represented by (Equation 9) are indicated by dotted lines. The positional relationship of the sound wave spectrum is at the farthest position when the normalized time frequency f1 is 0, and f1 is 0.5
(High range). The purpose of such setting is to make the directivity sharper and to make the directivity uniform with respect to frequency. This is because the spectral position of the sound wave from the front direction is passed through the two-dimensional filter 15. By removing from the area in advance, the spectrum position of the sound wave immediately enters the rejection area of the two-dimensional filter 15 when the arrival direction of the sound wave increases from θ = 0 ° to θ. Because it can be sharpened. Further, by making the distance between the spectral position of the sound wave in the front direction and the passing region of the two-dimensional filter 15 longer in the lower frequency range, the above-mentioned effect can be enhanced in the lower frequency range. In this way, the directivity can be sharpened as a whole, and the uniformity of the directivity with respect to the frequency can be improved. Further, the higher the frequency, the closer the spectral position of the sound wave in the front direction is to the pass band, so that the output signal from the digital-to-analog converting means 16 is in a state of emphasizing the high frequency range. Is flattened, it is also possible to suppress the high frequency component of the noise generated from the digital-to-analog converter 16. (FIG. 4) is a directional characteristic diagram when the total length of the microphone is 50 cm, (a) when fs = c / d (1-2b), and (b) when fs = c / d.
It is time.

As described above, according to this embodiment, the first microphone unit 1 to the Nth microphone unit 4 and the first microphone unit 1 to the Nth microphone unit are linearly arranged at the interval d. A first analog-to-digital converter 11 to an N-th analog-digital converter 14 for converting the analog output signal from
A two-dimensional filter 15 to which signals from the first to N-th analog-to-digital converters 14 are input, and a digital-to-analog converter 1 provided at a subsequent stage of the two-dimensional filter 15
6 and frequency characteristic correction means 17 for correcting the frequency characteristic of the output signal from the digital-to-analog conversion means 16, wherein the normalized time frequency is f1 (-0.5≤f1≤0.5) and the normalized spatial frequency is f2 (- On the two-dimensional frequency plane where 0.5 ≦ f2 ≦ 0.5), the amplitude frequency characteristic of the two-dimensional filter 15 has a pass area in a linear area represented by (Equation 6), and
By setting the time sampling frequency fs of the two-dimensional filter 15 in the range of (Equation 7), directivity is obtained from a low frequency range, and uniformity of directivity with respect to frequency is improved.
Further, the sensitivity can be increased by synthesizing the output signals of the plurality of microphone units and the amount of attenuation of the sensitivity other than the front direction can be increased by the highly accurate calculation by the digital filter. Further, it is possible to suppress the high-frequency range portion of the noise generated by the digital-to-analog conversion means.

In the above embodiment, the first microphone unit 1 to the Nth microphone unit 4 are omnidirectional, but unidirectional or bidirectional directional microphone units may be used.

[0041]

As described above, according to the present invention, the distance d
First to Nth microphone units arranged in
A two-dimensional filter that receives signals from the first to Nth microphone units as inputs; and a frequency characteristic correction unit provided at a subsequent stage of the two-dimensional filter, and the amplitude frequency characteristic of the two-dimensional filter is a normalized time frequency f1 ( -0.5 ≦ f1 ≦ 0.5)
And a two-dimensional frequency plane represented by normalized spatial frequency f2 (-0.5≤f2≤0.5), which has a pass region in a linear region represented by (Equation 6), and a time sampling frequency fs of the two-dimensional filter. Is within the range of (Equation 7), the directivity is obtained from a low frequency range, the uniformity of directivity with respect to frequency is improved, and the sensitivity by combining output signals of a plurality of microphone units is improved. By the rise and the high-precision calculation by the digital filter, the attenuation of the sensitivity other than the front direction can be increased, and the high-frequency range portion of the noise generated by the digital-to-analog converter can be suppressed.

[Brief description of the drawings]

FIG. 1 is a configuration diagram of a super-directional microphone according to an embodiment of the present invention.

FIG. 2 is a configuration example of a two-dimensional filter according to an embodiment of the present invention.

FIG. 3 is a graph showing a contour line representing an amplitude frequency characteristic of a two-dimensional filter and a spectrum position of a sound wave in a front direction in the embodiment of the present invention.

FIG. 4 is a directional characteristic diagram of a super-directional microphone according to an embodiment of the present invention.

FIG. 5 is a configuration diagram of a conventional super-directional microphone.

FIG. 6 is a directional characteristic diagram of a conventional super-directional microphone.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 1st microphone unit 2 2nd microphone unit 3 3rd microphone unit 4 Nth microphone unit 11 1st analog-digital conversion means 12 2nd analog-digital conversion means 13 3rd analog-digital conversion means 14th N analog-digital conversion means 15 two-dimensional filter 16 digital-analog conversion means 17 frequency characteristic correction means 155 first FIR filter 156 second FIR filter 157 third FIR filter 158 N-th FIR filter 159 addition means

 ──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-3-113998 (JP, A) JP-A-2-205200 (JP, A) JP-A-56-57576 (JP, U)

Claims (1)

(57) [Claims] 1. The first to N-th linearly arranged at intervals d.
Microphone units, first to N-th analog-to-digital converters for converting analog output signals from the first to N-th microphone units into digital values, and first to N-th analog-to-digital converters. A two-dimensional filter having a signal as an input, a digital-to-analog converting means provided at a stage subsequent to the two-dimensional filter, and a frequency characteristic correcting means for correcting a frequency characteristic of an output signal from the digital-to-analog converting means; A two-dimensional frequency plane where the time frequency is f1 (-0.5≤f1≤0.5) and the normalized spatial frequency is f2 (-0.5≤f2≤0.5) is a linear shape in which the amplitude frequency characteristic of the two-dimensional filter is expressed by the following equation. It has a passing area in the area, A super directional microphone, wherein the time sampling frequency fs of the two-dimensional filter is in the range of the following expression. (Equation 2)