Patent Description:
Conventionally, a microphone array installed in a conference room or the like is known. In the conventional microphone array disclosed in <CIT>, a plurality of microphones are provided on a plurality of concentric circles.

<CIT> discloses circularly symmetric, zero redundancy, planar array antenna. Planar arrays having broad frequency range applications for source location, source imaging or target illumination with projected beams are circularly symmetric and made up of a plurality of sensing and/or transmitting elements (<NUM>) arranged in circles (<NUM>) along the arms (<NUM>) of a set of logarithmic spirals so as to substantially eliminate grating lobes for a broad range of frequencies. Signals received from or transmitted to the elements are appropriately phased to control the beam of the array.

<CIT> discloses a method involves arranging three microphones (<NUM>-<NUM>) on a circle (<NUM>) in an area level. An intended rotational body, whose rotational axis (<NUM>) stands in center of the circle, perpendicularly on the area level, is formed. The microphones are positioned along a sectional line, from the section of the rotational body with the level, in which the rotational axis and the positioned microphone are situated, so that maximum three of the microphones are located in an space level and the microphones do not form polyhedron. An independent claim is also included for a microphone array with four microphones.

<CIT> discloses a two-dimensional array of a plurality of transducers comprising a first plurality of like sub-arrays (<NUM>, 11a, 11b) of transducers (<NUM>) in a circularly symmetric arrangement around a common centre (C), where the transducers in each sub-array of the first plurality have individual distances from the common centre that form a progressive series of distances with a first lower limit and a first upper limit. Each sub-array in the first plurality of sub-arrays comprises at least three transducers arranged on a first straight line (<NUM>), and the first straight line is offset laterally a first distance (d) from the common centre. The number of sub-arrays is odd, and the sub-arrays may be separate units that can be selectively assembled to form the two-dimensional array and selectively disassembled.

An arrangement of the microphones in the conventional microphone array is determined by the experience and intuition of a designer. Therefore, a difference between a main lobe and a side lobe in directional characteristics of the microphone array is insufficient, and it has been required to improve a directivity.

This invention focuses on this point, and an object of the invention is to improve the directivity of the microphone array.

A method for determining microphone position according to a first aspect of the present invention is a method for determining positions of a plurality of microphones in a microphone array having the plurality of microphones arranged in a plurality of concentric circles.

The method for determining microphone position includes a constraint condition acquiring step of acquiring constraint conditions including the maximum number of the plurality of microphones; and a selecting step of selecting, from among a plurality of combinations of (i) the number of microphones included in each of the plurality of concentric circles and (ii) the radius of each of the plurality of concentric circles, a combination indicating directional characteristics with the smallest difference from a target value of the directional characteristics of the microphone array, where the plurality of combinations satisfy the constraint conditions.

The selecting step may include selecting the combination of the number of microphones included in each of the plurality of concentric circles and the radius of each of the plurality of concentric circles that indicates the directional characteristics with the smallest difference from the target value by using a variable vector including the number of the microphones included in each of the plurality of concentric circles and the radius of each of the plurality of concentric circles, as a mutant vector used in a differential evolution algorithm.

The constraint condition acquiring step may include acquiring the number of a plurality of sound source localization microphones used for specifying a direction of a sound source, as one of the constraint conditions. The constraint condition acquiring step may include acquiring a radius of an outermost concentric circle of the plurality of concentric circles, as one of the constraint conditions. The constraint condition acquiring step includes acquiring the number of microphones included in each of the plurality of concentric circles being three or more, as one of the constraint conditions.

The constraint condition acquiring step may include acquiring a target value of the directional characteristics corresponding to a difference between the magnitude of a main lobe and the magnitude of a side lobe of the sensitivity to input sound signals, as one of the constraint conditions.

The selecting step may include: setting a vector including, as variables, the number of the plurality of concentric circles in which the plurality of microphones are arranged, the radius of each of the plurality of concentric circles, and the number of the microphones arranged in each of the plurality of concentric circles to an initial variable vector; calculating an initial objective function value which is a value indicating an error between an ideal value of the directional characteristics of the microphone array and the directional characteristics of the microphone array calculated using the initial variable vector; determining a plurality of updated variable vectors different from the initial variable vector; calculating a plurality of update objective function values, which are values indicating an error between the ideal value of the directional characteristics of the microphone array and the directional characteristics of the microphone array calculated using the plurality of updated variable vectors, and selecting, from among the initial objective function value and the plurality of update objective function values, a combination of positions of the plurality of microphones corresponding to the minimum objective function value.

A microphone system according to a second aspect of the present invention is a microphone array having a plurality of microphones arranged on a plurality of concentric circles, wherein a variation amount of a difference between the radii of two concentric circles adjacent to each other among the plurality of concentric circles does not increase monotonically according to a distance from the center position of the plurality of concentric circles, and an attenuation amount of a side lobe relative to a main lobe in the directional characteristics is equal to or greater than <NUM> dB.

The microphone system may include: a plurality of localization microphones provided at the center position and at a plurality of positions on the innermost concentric circle, which is the closest to the center position of the plurality of concentric circles, and used for specifying a direction of a sound source; and a plurality of beamforming microphones provided on the plurality of concentric circles and used for collecting a sound generated from the sound source specified by the plurality of localization microphones.

Three or six of the localization microphones are arranged at uniform intervals on the innermost concentric circle. A distance between two localization microphones adj acent to each other among the plurality of localization microphones may be less than or equal to half of the minimum wavelength of a sound in a frequency band used to specify the direction of the sound source. The distance between the two localization microphones may be <NUM> or less.

Some microphones among the plurality of microphones may be provided at a plurality of intersections where at least one straight line passing through the center of the plurality of concentric circles intersects each of the plurality of concentric circles.

The above-mentioned microphone system may further include an audio processing part for processing a sound signal output from the microphone array, wherein the audio processing part may include: a direction specification part that specifies a direction of a sound source, on the basis of a plurality of the sound signals input from the plurality of localization microphones; and a sound output part that outputs sounds synthesized by weighting each of a plurality of sounds input to the plurality of beamforming microphones on the basis of the direction of the sound source specified by the direction specification part.

According to the present invention, an effect of making a microphone array less likely to collect unnecessary sounds is achieved.

<FIG> each illustrate an outline of a microphone system S. <FIG> shows a configuration of a microphone array <NUM>. The microphone system S includes the microphone array <NUM> and an audio processing part <NUM> and is a system for collecting voices generated by a plurality of speakers H (speakers H-<NUM> to H-<NUM> in <FIG>) in a space such as a conference room or a hall. The microphone system S does not need to include the audio processing part <NUM>, and may be connected to a computer that performs audio processing.

As shown by black circles in <FIG>, the microphone array <NUM> includes a plurality of microphones <NUM> and is installed on a ceiling, a wall surface, or a floor surface of the space where the speakers H stay. The microphone array <NUM> inputs, to the audio processing part <NUM>, a plurality of sound signals based on the voices input to the plurality of microphones <NUM>.

The audio processing part <NUM> is a device that processes the sound signals output from the microphone array <NUM> (that is, the plurality of sound signals output from the plurality of microphones <NUM>). The audio processing part <NUM> specifies a direction to a position where a speaker H who has spoken (i.e., a sound source) is located, by analyzing the sound signals input from the microphone array <NUM>. Further, the audio processing part <NUM> executes a beamforming process by adjusting weight coefficients of the plurality of sound signals corresponding to the plurality of microphones <NUM> on the basis of the direction toward a specified speaker H and makes sensitivity to the voice generated by this speaker H higher than sensitivity to sounds coming from directions other than the direction toward this speaker H.

<FIG> shows a state where the speaker H-<NUM> is speaking. <FIG> shows a state where the speaker H-<NUM> is speaking. In the state shown in <FIG>, the audio processing part <NUM> performs the beamforming process such that a main lobe in directional characteristics of the microphone array <NUM> is directed toward the speaker H-<NUM>. In this case, the audio processing part <NUM> synthesizes the plurality of sound signals, for example, by assigning a greater weight to the sound signal output from the microphone <NUM> at a position near the speaker H-<NUM> than to sound signals output from the other microphones <NUM>. In the state shown in <FIG>, the audio processing part <NUM> performs the beamforming process such that the main lobe in the directional characteristics of the microphone array <NUM> is directed toward the speaker H-<NUM>. In this case, the audio processing part <NUM> synthesizes the plurality of sound signals, for example, by assigning a greater weight to the sound signal output from the microphone <NUM> at a position near the speaker H-<NUM> than to sound signals output from the other microphones <NUM>.

In the microphone array <NUM>, the plurality of the microphones <NUM> are arranged such that a difference in the directional characteristics between the main lobe and a side lobe is equal to or greater than <NUM> dB due to the audio processing part <NUM> performing the beamforming process. Next, a configuration of the microphone array <NUM> and a method for determining an arrangement of the plurality of the microphones <NUM> will be described in detail.

As shown with the black circles in <FIG>, the microphone array <NUM> includes the plurality of microphones <NUM> that are arranged on a plurality of (for example, four or more) concentric circles. In the microphone array <NUM>, the plurality of the microphones <NUM> are provided for each of four concentric circles: C1, C2, C3, and C4. The concentric circle C1 is the innermost concentric circle, and three microphones <NUM> are provided on the concentric circle C1. Those three microphones 11b (11b-<NUM>, 11b-<NUM>, and 11b-<NUM>) provided on the concentric circle C1 function as (i) sound source localization microphones <NUM> for specifying the directions to positions where speakers H who are sound sources are located and (ii) beamforming microphones <NUM> for collecting the voices generated by the speakers H.

The concentric circle C2 is the second inner concentric circle, and four microphones 11c are arranged on the concentric circle C2. The concentric circle C3 is the third inner concentric circle, and seven microphones 11d are arranged on the concentric circle C3. The concentric circle C4 is the outermost concentric circle. On the concentric circle C4, seventeen microphones 11e are arranged. The microphones <NUM> arranged on the concentric circles C2, C3 and C4 function as the beamforming microphones <NUM>. It should be noted that, in <FIG>, among the plurality of microphones 11c, 11d, and 11e, the reference numerals are denoted only for the microphones <NUM> arranged on a straight line L.

As will be described in detail below, the radii of the four concentric circles C1, C2, C3 and C4, as well as the number and positions of the microphones <NUM> included in each concentric circle, are determined by searching for optimal directional characteristics. As a result, a variation amount of a difference between the radii of two concentric circles adjacent to each other among the four concentric circles C1, C2, C3, and C4 is determined such that the variation amount does not increase monotonically according to a distance from the center position of the plurality of concentric circles.

Specifically, in the microphone array <NUM> shown in <FIG>, the radius of the concentric circle C1 is <NUM> [m], the radius of the concentric circle C2 is <NUM> [m], the radius of the concentric circle C3 is <NUM> [m], and the radius of the concentric circle C4 is <NUM> [m]. A difference between the radii of the concentric circles C1 and C2 is <NUM> [m], a difference between the radii of the concentric circles C2 and C3 is <NUM> [m], and a difference between the radii of the concentric circles C3 and C4 is <NUM> [m], and these differences do not increase monotonically according to the distance from the central position of the concentric circles. Also, an attenuation amount of the side lobe with respect to the main lobe in the directional characteristics of the microphone array <NUM> is -<NUM> dB, and sufficient directivity is realized. The microphone array <NUM> has such good directional characteristics because the arrangement of the plurality of microphones <NUM> is determined by using an algorithm for searching for an optimal arrangement of the plurality of microphones <NUM>, as will be described in detail below.

Among the plurality of microphones <NUM> included in the microphone array <NUM>, both (i) a microphone 11a arranged at the central position of the plurality of concentric circles and (ii) three microphones 11b (11b-<NUM>, 11b-<NUM>, and 11b-<NUM>) provided at uniform intervals on the innermost concentric circle C1, which is the closest to the central position, function as a plurality of sound source localization microphones <NUM> used for specifying positions of the sound sources. The other microphones <NUM> included in the microphone array <NUM> function as a plurality of beamforming microphones <NUM> used for collecting sounds generated from the sound sources whose positions are specified by the sound source localization microphones <NUM>. The microphone 11a and the microphones 11b-<NUM> to 11b-<NUM> may further function as the beamforming microphones <NUM>. In other words, the microphone 11a and the microphones 11b-<NUM> to 11b-<NUM> may be used for two purposes: for the sound source localization and for beamforming.

A distance between two sound source localization microphones <NUM> adjacent to each other among the plurality of microphones <NUM> that function as the sound source localization microphones <NUM> is less than or equal to half of the minimum wavelength of a sound in a frequency band used to specify the direction to the position where the speaker H, who is the sound source, is located. Since aliasing does not occur when the distance between the two sound source localization microphones <NUM> is set in this manner, the accuracy of estimating the direction toward the speaker H improves.

When a frequency range that includes main frequency components of the voice of an assumed speaker H is equal to or above <NUM> and equal to or below <NUM>, a distance D between the two sound source localization microphones <NUM> adjacent to each other is preferably <NUM> or less, since the wavelength of a sound with a frequency of <NUM> is <NUM>. When the frequency range that includes the main frequency components of the voice of the assumed speaker H is equal to or above <NUM> and equal to or below <NUM>, the distance D is preferably <NUM> or less since the wavelength of a sound with a frequency of <NUM> is <NUM>. It should be noted that if the distance D is too small, a difference in sounds entering each of the sound source localization microphones <NUM> becomes too small, and for this reason, the distance D is preferably, for example, <NUM> or more and <NUM> or less.

Also, some of the microphones <NUM> are provided at a plurality of intersections where at least one straight line L passing through the center of the plurality of concentric circles C1, C2, C3, and C4 intersects with the respective concentric circles C1, C2, C3, and C4. In an example shown in <FIG>, the microphones 11a, 11b-<NUM>, 11c, 11d, and 11e are arranged on the same straight line L. That is, one of the microphones <NUM> arranged on the concentric circle C1, one of the microphones <NUM> arranged on the concentric circle C2, one of the microphones <NUM> arranged on the concentric circle C3, and one of the microphones <NUM> arranged on the concentric circle C4 are arranged on the same straight line L as one of the microphones <NUM> arranged on the other concentric circles.

Because the microphone array <NUM> is configured in this manner, the accuracy of performing audio processing to enhance the directivity of the direction toward the speaker H is improved, and the load of the audio processing is reduced. Also, since a positional relationship of the plurality of microphones <NUM> becomes clearer, the accuracy of specifying the direction toward the speaker H is improved.

<FIG> shows a configuration of the audio processing part <NUM>. The audio processing part <NUM> includes an AD converter <NUM>, an AD converter <NUM>, a direction specification part <NUM>, and a sound output part <NUM>.

The AD converter <NUM> converts a plurality of sound signals based on sounds that entered the plurality of sound source localization microphones <NUM> into a plurality of pieces of sound source localization digital data. The AD converter <NUM> inputs the converted sound source localization digital data to the direction specification part <NUM>. The AD converter <NUM> converts a plurality of sound signals based on sounds that enter the plurality of beamforming microphones <NUM> ("BF" in <FIG>) into a plurality of pieces of beamforming digital data. The AD converter <NUM> inputs the converted beamforming digital data to the sound output part <NUM>. The AD converter <NUM> and the AD converter <NUM> may be configured by a plurality of devices or may be configured by a single device.

The direction specification part <NUM> specifies the direction to the position where the speaker H who is the sound source is located, on the basis of the plurality of sound signals input from the plurality of sound source localization microphones <NUM>. Specifically, the direction specification part <NUM> specifies the direction toward the speaker H on the basis of a plurality of pieces of sound source localization digital data input from the AD converter <NUM>. The direction specification part <NUM> specifies the direction toward the speaker H, for example, on the basis of a relationship between the loudness of sounds which each of the plurality of sound source localization digital data indicates. The direction specification part <NUM> notifies the sound output part <NUM> of the direction toward the specified speaker H.

The sound output part <NUM> outputs sounds synthesized by weighting each of the plurality of sounds input to the beamforming microphones <NUM> on the basis of the direction toward the speaker H, specified by the direction specification part <NUM>. Specifically, the sound output part <NUM> outputs the synthesized sounds by generating a plurality of multiplied values by multiplying a weight coefficient, which is determined on the basis of a direction to a position where the speaker H who is speaking is located, to each of the plurality of beamforming digital data corresponding to each microphone <NUM>, and by adding the generated plurality of multiplied values. For example, an absolute value of a weight coefficient for the microphone <NUM> at a position corresponding to the direction toward the speaker H is set to a value greater than an absolute value of a weight coefficient for a microphone <NUM> at the other position. Due to the direction specification part <NUM> and the sound output part <NUM> operating in this manner, reproducibility of the sounds generated by the speakers H is improved regardless of the directions to the positions where the speakers H are located.

Since the directional characteristics of the microphone array <NUM> are different according to the arrangement of the plurality of microphones <NUM>, the quality of the sounds synthesized by the sound output part <NUM> is affected by the arrangement of the plurality of microphones <NUM>. Next, a method for determining the arrangement of the plurality of microphones <NUM> for improving the quality of the sounds synthesized by the sound output part <NUM> will be described in detail.

<FIG> is a flowchart showing an outline of a method for determining the arrangement of the plurality of microphones <NUM>. As an example, an arrangement search device has a computer and determines the arrangement of the plurality of microphones <NUM> by a method for determining microphone position shown in the flowchart of <FIG> by executing programs. The arrangement search device determines the optimal arrangement for the plurality of microphones <NUM> when a sound source is in a particular direction, by executing the method shown in the flowchart of <FIG>. The arrangement search device changes a direction of the sound source (i.e., a direction to a position where the sound source is located) to a plurality of different directions in order to determine the optimal arrangement of the plurality of microphones <NUM> for the respective directions. The arrangement search device determines the arrangement of the plurality of microphones <NUM> that is as suitable as possible for each of the directions in which the plurality of sound sources are located, for example, by using the least squares method.

Hereinafter, the process in which the arrangement search device determines the arrangement of the plurality of microphones <NUM> will be described with reference to <FIG>. The arrangement search device determines the arrangement of the plurality of microphones <NUM> using, for example, a differential evolution (DE) method, which is a differential evolution algorithm, or a JADE method which is an improved DE method.

In order to determine the arrangement of the plurality of microphones <NUM>, the arrangement search device first acquires constraint conditions (step S1). For example, the arrangement search device displays a screen for inputting the constraint conditions on a display, and acquires the constraint conditions input on the screen.

The arrangement search device acquires, for example, the maximum number of the plurality of microphones <NUM>, as one of the constraint conditions. The arrangement search device may acquire the number of the sound source localization microphones <NUM> and the radius of the outermost concentric circle of the plurality of concentric circles, as one of the constraint conditions. Due to the arrangement search device acquiring these constraint conditions, the time for determining the arrangement of a plurality of microphones <NUM> that satisfy the size and cost requirements of the microphone array <NUM> can be reduced. The arrangement search device may acquire the number of microphones <NUM> included in each of the plurality of concentric circles to be three or more, as one of the constraint conditions. By having three or more microphones <NUM> in one concentric circle, it is possible to reduce the variability of the directional characteristics due to the direction of the sound source.

Subsequently, the arrangement search device acquires a target value of the directional characteristics of the microphone array <NUM> (step S2). The directional characteristics of the microphone array <NUM> are represented by a value corresponding to a difference between (i) the magnitude of a main lobe of sensitivity to the input sound signals and (ii) the magnitude of a side lobe of the sensitivity to the input sound signals. For example, the directional characteristics of the microphone array <NUM> are expressed as an attenuation amount of the side lobe relative to the main lobe when a predetermined sound is input to the microphone array <NUM>. For example, the arrangement search device displays a screen for inputting the target value on the display, and acquires the target value inputted on the screen.

Next, the arrangement search device determines an initial variable vector for starting a search for the optimal arrangement of the plurality of microphones <NUM> by using the JADE method (step S3). For example, the arrangement search device sets a vector including, as a variable, the number of concentric circles in which the microphones <NUM> are arranged, the radius of each concentric circle, and the number of microphones <NUM> in each concentric circle to the initial variable vector.

Subsequently, the arrangement search device calculates an objective function value (i.e., an initial objective function value) when the determined initial variable vector is used (step S4), and temporarily stores the calculated objective function value as a reference function value in association with the initial variable vector (step S5). The objective function value is a value indicating an error between an ideal value of the directional characteristics of the microphone array <NUM> and the directional characteristics of the microphone array <NUM> calculated using the initial variable vector. The smaller the objective function value, the better the directional characteristics.

Next, the arrangement search device determines an updated variable vector (step S6). The updated variable vector is a variable vector in which at least one variable included in the initial variable vector is changed. The arrangement search device determines the updated variable vector by setting at least one of (i) the number of concentric circles in which the microphones <NUM> are arranged, (ii) the radius of each concentric circle, and (iii) the number of microphones <NUM> in each concentric circle to a value different from the initial variable vector. The arrangement search device uses, for example, the differential evolution algorithm in determining the updated variable vector.

The arrangement search device uses a variable vector including, for example, the number of microphones <NUM> included in each of the plurality of concentric circles and the radius of each of the plurality of concentric circles, as the updated variable vector which is a mutant vector used in the differential evolution algorithm. The arrangement search device selects, from among a plurality of combinations of (i) the number of microphones <NUM> included in each of the plurality of concentric circles and (ii) the radius of each of the plurality of concentric circles, a combination indicating directional characteristics with the smallest difference from the target value of the directional characteristics, where the plurality of combinations satisfy the constraint conditions.

Specifically, the arrangement search device first calculates the objective function value when the updated variable vector is used (step S7). The arrangement search device compares the calculated objective function value with the objective function value stored in step S5 (step S8). When the calculated objective function value is equal to or greater than the stored reference function value (YES in step S8), the arrangement search device advances the arrangement determination process to step S10. When the calculated objective function value is less than the stored objective function value (NO in step S8), the arrangement search device stores the calculated objective function value (i.e., the updated objective function value) as a new reference function value in association with the updated variable vector (step S9).

Next, the arrangement search device determines whether or not the objective function value has been calculated a predetermined number of times (step S10). That is, the arrangement search device determines whether or not the objective function value has been calculated for a predetermined number of variable vectors. The predetermined number of times is, for example, a number set by a designer of the microphone array <NUM>. When the object function value has been calculated the predetermined number of times (YES in step S10), the arrangement search device determines the arrangement indicated by the variable vector stored in association with the reference function value as the arrangement of the plurality of microphones <NUM>, and ends the process.

If the number of times that the calculation of the objective function value has been performed has not reached the predetermined number of times (NO in step S10), the arrangement search device returns the arrangement determination process to step S6. By executing a selection step of steps S7 to S10 in this manner, the arrangement search device selects, from among a plurality of combinations of positions of the microphones <NUM>, an optimal combination indicating the directional characteristics with the smallest difference from the target value of the directional characteristics, where the plurality of combinations satisfy the constraint conditions (step S11). That is, the arrangement search device selects, from among the initial objective function value and a plurality of updated objective function values, a combination of positions of the plurality of microphones <NUM> corresponding to the minimum objective function value.

Hereinafter, an example that shows searching for an optimal arrangement of the plurality of microphones <NUM> using the JADE method is described. The following designing process is performed by executing the programs with the arrangement search device, which executes the flowchart of <FIG>. In the JADE method, an algorithm with enhanced global searchability of the DE method is used to automatically adjust parameters for each problem. Therefore, even for a problem in which a multimodal objective function exists, such as when determining the arrangement of the plurality of microphones <NUM>, the arrangement search device can realize a good search by using the JADE method.

<FIG> shows a model used in the present search example. As shown in <FIG>, in a space where a position is defined by an x-axis, a y-axis, and a z-axis, a sound source, which is a premise of searching for the optimal arrangement of the plurality of microphones <NUM>, is at an angle of θ from the x-axis in an xy-plane and at an angle of Φ from the xy-plane to the z-axis. That is, the arrangement search device searches for the arrangement of the plurality of microphones <NUM> whose directivity becomes optimal when the microphone array <NUM> receives a sound from the sound source oriented in (θ, Φ) with respect to the origin.

It is supposed that a total number of concentric circles is P, the radius of each concentric circle is rp, and the number of microphones <NUM> arranged in each concentric circle is Mp (p = <NUM>, <NUM>,. If a distance between a sound source and the microphone array <NUM> is sufficiently large with respect to the radius rP of the largest concentric circle, a sound signal generated by the sound source is considered to be a plane wave in the vicinity of the microphone array <NUM>. In this case, a sound receiving signal zpm(n) of the m-th microphone <NUM> on a certain concentric circle p can be expressed by the following equations using an arrival time difference τpm(θ, Φ) based on a sound receiving signal zp,xaxis(n) of the microphones <NUM> on the x-axis of each concentric circle. <MAT> <MAT> <MAT>.

Here, c is the speed of sound. In this case, a directivity G(θ, Φ, ωk) corresponding to the size of the main lobe of the microphone array <NUM> can be expressed by the following equation.

A weight coefficient w*pm,k of a delay-sum beamformer can be expressed by the following equation.

A design problem relevant to the optimal arrangement of the plurality of microphones <NUM> can be replaced by a problem of searching for the arrangement of the microphones <NUM> which can obtain a directivity G(θ, Φ, ωk), which is close to a desired directivity D(θ, Φ, ωk), serving as the target value. The error E(θ, Φ, ωk) used in the search can be expressed by the following equation.

The optimal placement can be specified by obtaining a variable vector that minimizes the maximum error in an approximate band, as shown in the following equation.

Here, in order to obtain the variable vector that minimizes the maximum error by using the JADE method, the arrangement search device first initializes N solution populations Xi (i=<NUM>, <NUM>,. , N) using a uniform random number for within a domain range of a search space, and calculates the objective function value of each individual. The arrangement search device generates differential mutant individuals, child individuals, and evolution individuals up to the maximum generation number I, and searches for the minimal solution of the objective function.

In order to apply the JADE method to a microphone arrangement design problem, a variable vector x is defined as follows: <MAT>.

Here, to make sure that the arrangement will not be determined to be an arrangement that is impossible to realize, the constraint conditions for keeping the number of microphones <NUM> within the maximum number Mmax that can be realized are defined as follows: <MAT>.

In the microphone system S, a sound source localization process is performed prior to the beamforming process. Therefore, when determining the arrangement of the plurality of microphones <NUM>, an arrangement of the sound source localization microphones <NUM> must also be considered. To arrange one concentric circle at the central position of the concentric circles and three or six sound source localization microphones <NUM> in the innermost concentric circle C1, as shown in <FIG>, the following constraint conditions are added: <MAT>.

When the maximum radius of the outermost concentric circle is Rmax, the constraint conditions on the radius rp of each concentric circle are as follows: <MAT>.

In this case, a variable vector x' to be obtained is expressed as follows: <MAT>.

Therefore, the design problem of arranging the plurality of microphones <NUM> is formulated as a mixed integer programming problem, as shown below: <MAT> <MAT> <MAT>.

Here, θs and Φs (s = <NUM>,. , S) represent discrete directions, and δ represents the maximum error in the approximate band in Equation <NUM>. In the search for the optimal arrangement by the JADE method, the following magnification objective function f(x') using this δ is used.

Here, λu(x') (u = <NUM>,. , <NUM>) represents a penalty function. λ<NUM>(x') is a penalty function for limiting the maximum number of microphones <NUM>. <MAT> <MAT>.

The λ<NUM>(x') is a penalty function for the number of sound source localization microphones <NUM>.

λ<NUM>(x') is a penalty function for preventing the number of microphones <NUM> arranged in each concentric circle from being <NUM> or less.

λ<NUM>(x') is a penalty function for arranging the radii in ascending order. α > <NUM> is a constant for preventing the difference between the radii of the adjacent concentric circles from being <NUM>.

In the present search example, ΦL = <NUM>[rad], for simplicity. A desired directivity D(θ, ωk) is set as shown in the following equation.

Here, θS1 and θS2 are the directions of the borders of the main lobe. In the present search example, θS1 = -π/<NUM>[rad], θS2 = π/<NUM>[rad], a sound source direction θL = <NUM>[rad], and the sound speed c = <NUM>[m/s]. In the JADE method, the initial values of µF and µCR are <NUM>, and Pbest is <NUM>.

As a result of determining the arrangement of the plurality of microphones <NUM> with the JADE method using a computer as the arrangement search device under the above conditions, the microphone array <NUM> shown in <FIG> was designed. In the microphone array <NUM>, the radius of each concentric circle and the number of microphones <NUM> in each concentric circle are shown in Table <NUM>.

<FIG> shows directional characteristics of the microphone array <NUM> (i.e., the microphone array <NUM> shown in <FIG>) of a first search example. <FIG> shows the directional characteristics for a sound of each frequency: <NUM>, <NUM>, <NUM>, <NUM>, and <NUM>. In <FIG>, the maximum value of the main lobe is indicated as <NUM> dB.

As a comparative example, the radius of each concentric circle and the number of the microphones <NUM> for each concentric circle of a microphone array, in which the microphones <NUM> are arranged without using the JADE method, are shown in Table <NUM>. <FIG> shows directional characteristics of the microphone array of the comparative example.

By comparing <FIG> and <FIG>, the directional characteristics shown in <FIG> are confirmed to have stronger directivity than the directional characteristics shown in <FIG>. Specifically, in the directional characteristics shown in <FIG>, the minimum value of the attenuation amount of the side lobe relative to the main lobe is <NUM> dB, whereas in the directional characteristics shown in <FIG>, the minimum value of the attenuation amount of the side lobe relative to the main lobe is <NUM> dB. From this, it was confirmed that it is effective to determine the arrangement of the plurality of microphones <NUM> using the JADE method.

The radius of each concentric circle and the number of microphones <NUM> in each concentric circle determined using the JADE method under the condition that the number of microphones <NUM> is <NUM> and the maximum radius of the concentric circle is <NUM> [m] is shown in Table <NUM>.

<FIG> shows directional characteristics of the microphone array <NUM> of a second search example. In the directional characteristics shown in <FIG>, the minimum value of the attenuation amount of the side lobe relative to the main lobe is <NUM> dB. The directional characteristics shown in <FIG> are also confirmed to have stronger directivity than the directional characteristics shown in <FIG>.

The radius of each concentric circle and the number of microphones <NUM> in each concentric circle determined by using the JADE method under the condition that the number of microphones <NUM> is <NUM> and the maximum radius of the concentric circle is <NUM> [m] is shown in Table <NUM>.

<FIG> shows directional characteristics of the microphone array <NUM> of a third search example. In the directional characteristics shown in <FIG>, the minimum value of the attenuation amount of the side lobe relative to the main lobe is <NUM> dB. The directional characteristics shown in <FIG> are also confirmed to have stronger directivity than the directional characteristics shown in <FIG>.

The microphone arrays <NUM> designed by using the JADE method have the following common features:.

An example where three sound source localization microphones <NUM> are arranged at uniform intervals on the innermost concentric circle C1 has been shown above, but six sound source localization microphones <NUM> may be arranged at uniform intervals on the innermost concentric circle C1.

Claim 1:
A method for determining positions of a plurality of microphones (<NUM>) in a microphone array (<NUM>) having the plurality of microphones (<NUM>) arranged in a plurality of concentric circles, wherein the plurality of microphones (<NUM>) includes a microphone (11a) provided at a center position of the plurality of concentric circles, a plurality of localization microphones (11b) provided at a plurality of positions on the innermost concentric circle, which is the closest to the center position of the plurality of concentric circles, and used for specifying a direction of a sound source and a plurality of beamforming microphones (11c, 11d, 11e) provided on the plurality of concentric circles and used for generating synthesized sounds by collecting sounds generated from the sound source after the direction of the sound source is specified by using the plurality of localization microphones (11b), the method comprising:
a constraint condition acquiring step (S1) of acquiring constraint conditions including (i) a maximum number of the plurality of microphones (<NUM>) and (ii) a condition that the number of the plurality of localization microphones (11b) and the plurality of beamforming microphones (11c, 11d, 11e) provided in each of the plurality of concentric circles is three or more and at least three of the plurality of localization microphones (11b) are arranged at uniform intervals and (iii) the number of a plurality of sound source localization microphones (11a, 11b) used for specifying a direction of a sound source; and
a selecting step (S11) of selecting, from among a plurality of combinations of (i) the number of microphones (<NUM>) included in each of the plurality of concentric circles and (ii) the radius of each of the plurality of concentric circles, a combination indicating directional characteristics with the smallest difference from a target value of the directional characteristics of the microphone array (<NUM>), where the plurality of combinations satisfy the constraint conditions.