JP4797539B2 - Acoustic system - Google Patents

Description

  The present invention relates to an acoustic system that measures the position of a listener and forms a sound field based on the measured position.

Conventionally, there has been disclosed an acoustic system that measures the position of a listener and forms a sound field based on the measured position (see, for example, Patent Document 1).
In Patent Document 1, in order to measure the difference in propagation time when the left and right speakers reach the remote controller and the volume difference when each tone signal reaches the remote controller 4, a predetermined measuring means is provided in the remote controller, and the measured propagation time difference is measured. It is disclosed that the delay and the volume adjustment of the reproduction signal supplied from the sound field reproduction device 1 to the left and right speakers are performed based on the volume difference.

By measuring this distance, the listener's position can be measured correctly, so the volume balance and appropriate model head-related transfer function (transfer function simulating spatial propagation characteristics from the speaker to the ear) can be selected, and a speaker array can be used. Since the directivity can be controlled at the listening position, a free sound field can be formed.
JP 2004-356958 A

  However, the acoustic system disclosed in Patent Document 1 has a problem that it is sensitive to ambient noise when a test sound is output from a speaker and a distance from the speaker to a listener is measured. Such a measurement is to measure the distance to the listener by emitting a predetermined sound from a speaker, collecting the sound with a microphone at hand, and measuring the delay of the sound. Therefore, if there is ambient noise when a predetermined sound is emitted from the speaker, the distance measurement may be erroneously performed and an inappropriate sound field may be formed. Conventionally, such a measurement error may occur even if the test sound is played at a considerable volume. When such distance measurement is mistaken, reproduction of the sound system is started and there is a problem that it is understood that the distance measurement is incorrect only after the listener notices an abnormality.

  Therefore, the present invention is not appropriate by detecting and correcting a measurement error in an acoustic system that uses a result of measuring a distance between a listener and a speaker in advance for forming a sound field. An object of the present invention is to provide an acoustic system that prevents the formation of a sound field in advance.

In the present invention, means for solving the above-described problems are configured as follows.

(1) The present invention
Multiple speakers,
A test sound generating unit that sequentially inputs a test signal to each of the plurality of speakers to generate a test sound;
A remote control device operated by a user, comprising: a receiving means for receiving the test sound; and a function of returning a reception confirmation signal as an infrared signal when received,
A remote control receiver for receiving the infrared signal of the remote control device;
Measuring means for measuring a transmission time until the remote control receiving unit receives a reception confirmation signal after generating the test sound;
Position calculating means for calculating the position of the remote control device based on the propagation time from each speaker to the remote control device;
An acoustic system comprising:
The position calculating means retries to generate the test sound when the positional relationship between each speaker and the remote control device does not satisfy a condition for establishing a triangle .

  If comprised in this way, the said position calculation means can measure the distance of the said speaker and remote control by multiplying the propagation speed measured by the said test sound generation means and the measurement means by a sound speed. At that time, if the measured distance does not satisfy a predetermined mathematical condition, the position detecting means can determine that the measured propagation time is not appropriate. In that case, the position calculation means will retry the generation of the test sound. Therefore, in the acoustic system of the present invention, it is possible to prevent in advance the formation of an invalid sound field based on erroneous distance measurement.

  In addition, “a condition for mathematically determining one position”, that is, a condition on whether the distance measurement is appropriate is, for example, the distance between the speaker and the remote controller as in the configuration of (2) described later. It can be set as a condition for establishing a triangle formed by a listener and two speakers.

(2) The present invention
The conditions for establishing the triangle of the position calculating means are:
Absolute value, said predetermined distance is smaller than the distance (X0 + alpha) which is the sum of the alpha and spacing X0 of the two speakers of the difference between the distance X1, X2 position was calculated for the remote control device of the loudspeaker The first condition,
A second condition that a sum of the distances X1 and X2 is larger than a distance (X0 + β) that is a sum of an interval X0 between the two speakers and a predetermined distance β;
Alternatively, at least one of the third condition and the fourth condition in which the first condition and the second condition are replaced with the propagation time condition, respectively.

  The first condition and the second condition represent conditions for forming a triangle formed by a listener and two speakers. In addition, the third condition and the fourth condition correspond to those obtained by dividing the respective comparison targets by the sound velocity V in the first condition and the second condition, and the first condition, the second condition, It represents an equivalent condition. If any one of the first to fourth conditions is used, it can be easily determined that the distance measured by the measuring unit is not appropriate. Then, when the measured distance becomes an inappropriate value, it is possible to prevent the formation of an invalid sound field in advance by performing the measurement again.

  According to the present invention, it is possible to determine that the sound propagation time from the speaker to the listener obtained by calculation is not appropriate. Therefore, it is possible to prevent the sound system of the present invention from forming an invalid sound field in advance.

  The configuration of the acoustic system of this embodiment will be described with reference to FIGS. 1 and 2. FIG. 1 is a conceptual diagram showing an outline of the acoustic system of the present embodiment. FIG. 2 is an internal configuration diagram of the acoustic system of the present embodiment. As shown in FIG. 1, the acoustic system of this embodiment includes a remote controller 10 and a speaker array 20 in which a plurality of speakers SP1 to SP5 are linearly arranged.

  First, the configuration of the remote controller 10 will be described. The remote controller 10 is an infrared remote controller that instructs the speaker array 20 to select a source, adjust the volume, and the like, and has the following configuration and functions in addition to the normal functions. As shown in FIG. 1, a position detection start button 101 is provided on the upper surface of the remote controller 10 in addition to a normal function operator (not shown in FIG. 1, see FIG. 2). In addition, a microphone 12 and an infrared light emitting unit 13 are provided on the front surface of the remote controller 10. As shown in FIG. 2, the remote controller 10 accepts inputs of a plurality of operating elements 101-1xx (xx is an integer), generates a signal corresponding to the inputs, and sends the signal to the infrared light emitting unit 13. A control circuit 11 and an infrared light emitting unit 13 that generates infrared light based on a signal generated by the remote control circuit 11 are included. Similarly, as shown in FIG. 2, the microphone 12 and the waveform determination circuit 14 are provided. Each configuration will be described below.

The microphone 12 is a microphone that converts a test sound (details will be described later) output from the speaker array 20 into an electrical signal, and is a small size that can be mounted on the remote controller 10.
The waveform determination circuit 14 includes a filter that passes only the frequency of the test sound, a detection circuit that detects an envelope by DC detection of the passed signal, and a quantization circuit that quantizes the envelope into an I / O signal. . Only the frequency of the test sound is input to the detection circuit to detect that the test sound is input. The quantization circuit transmits a signal indicating that a test sound has been input as 1 and 0.
Operators 102 to 1xx are operation buttons including a position detection start button 101, and send instructions to the remote control circuit 11.
The remote controller control circuit 11 generates an infrared signal pattern to be transmitted to the system controller 22 in accordance with instructions from the operators 102 to 1xx.
The infrared light emitting unit 13 is configured by, for example, a light emitting diode. The control current of the remote control circuit 11 is input, and various signals are transmitted by emitting light as infrared rays.

  Next, the configuration of the speaker array 20 will be described. As shown in FIG. 1, on the front surface of the speaker array 20, an infrared light receiving unit 21 that receives infrared rays emitted from five speakers SP1 to SP5 and an infrared light emitting unit 13 attached to the remote controller 10 (details will be described later). It has. As shown in FIG. 2, the speaker array 20 includes a circuit unit that outputs the input from the input interface 24 to the speakers SP1 to SP5. That is, the audio input control unit 25, the equalizer circuit 26 that performs sound field formation, the speaker control circuit 23, and the amplifiers A1 to A5 that amplify the signals generated by the speaker control circuit 23 and output the signals to the speakers SP1 to SP5, respectively. I have. In addition, a system controller 22 that receives the input of the infrared light receiving unit 21 and controls the overall operation of the speaker array 20 is provided. Each configuration will be described below.

The speakers SP1 to SP5 shown in FIG. 1 can be configured by dynamic speakers or the like, and are fixed to the acoustic system 1 as an array 20 as a speaker array 20.
The infrared light receiving unit 21 is configured by an infrared light receiving element such as a photodiode.
The audio input control unit 25 in FIG. 2 includes a selector for selecting various audio input systems.
The equalizer circuit 26 performs a convolution operation of the FIR filter and adjusts the sound field of the voice input obtained from the audio input control unit 25.

  The speaker control circuit 23 shown in FIG. 2 can be composed of a ring buffer memory, an IC for controlling the reading, a buffer for storing the control parameters of the IC, and the like, and adjusting the input timing (delay) of each of the speakers SP1 to SP5. And adjust the volume. A D / A converter is also included, and the digital audio signal processed inside the speaker control circuit 23 is converted into an analog signal.

  The amplifiers A1 to A5 in FIG. 2 include an FET amplification stage and the like, and amplify the analog signal obtained by the audio input control unit 25. Alternatively, a so-called digital amplifier in which the audio input control unit 25 outputs a digital signal, amplifies the digital amplitude before converting it to a D / A converter, and then removes the high frequency to obtain an analog signal. You may comprise.

  As described above, in the configuration described with reference to FIGS. 1 and 2, at least two or more independent audio signal systems among the speakers SP1 to SP5 are provided. This is because, as will be described later, in the apparatus of the present embodiment, sound is output from two of SP1 to SP5 that are spaced apart from each other, and the positional relationship between the remote controller 10 and the speaker array 20 is measured. In the apparatus according to the present embodiment, the D / A converter and the amplifiers A1 to A5 such as a signal processing system in the signal processing speaker control circuit 23 can output sound independently.

  The system controller 22 controls the audio input control unit 25 and the speaker control circuit 23. That is, the system controller 22 receives an instruction via the infrared light receiving unit 21 based on the operation of the operators 101 to 1xx of the remote controller 10 and controls the above-described units. For example, the operations shown in FIGS. 3 and 4 described later are performed.

In the system of the present embodiment, the speakers SP1 to SP5 are provided in the speaker array 20, but the number of speakers is not limited to five. Further, the present invention can be applied to any speaker system that can output two or more independently without using a speaker array. The infrared light receiving unit 21 is installed in the speaker array 20, but if the instruction can be transmitted to the system controller 22, the installation location can be freely determined.
Further, the speaker array 20 shown in FIG. 1 needs to contain the speakers SP1 to SP5, but it does not need to contain all of the internal configuration shown in FIG. Also good.

  Next, the function of the acoustic system 1 of the present embodiment will be described using FIG. 1 and FIG. In the acoustic system 1, the audio input obtained from the input interface 24 is equalized by the equalizer circuit 26 under the control of the system controller 22, and the timing and volume between the speakers are adjusted by the speaker control circuit 23. To output to speakers SP1 to SP5. At this time, various sound field feelings can be given to the listener by timing and equalization. In addition to such basic functions, the acoustic system 1 according to the present embodiment detects the position of the remote controller held by the listener, and estimates the position of the listener, that is, the listening point based on the position information. In addition, a sound field formation function is provided by providing sound field information balanced at the listening point. Here, various functions for forming the sound field have been proposed. Therefore, the position detecting means of the remote controller among the functions will be described in detail below.

  When the position detection start button 101 of the remote control 10 shown in FIG. 1 is pressed, the remote control control circuit 11 shown in FIG. 2 transmits a specific information for the infrared light emitting unit 13 to transmit that the position detection start button 101 has been pressed. Send an instruction to emit infrared light in a pattern. When the infrared light emitting unit 13 emits infrared light, the infrared light receiving unit 21 receives a signal indicating that the position detection start button 101 has been pressed, and this signal is transmitted to the system controller 22. Then, the system controller 22 in FIG. 2 controls the speaker control circuit 23 to output a position detection pulse sound (hereinafter referred to as “test sound”) having a preset frequency to the speaker SP1 via the amplifier A1. When a test sound is emitted from the speaker SP1, the microphone 12 built in the remote controller 10 converts the test sound into an audio signal and outputs it to the waveform determination circuit 14. The waveform determination circuit 14 extracts a signal having a specific frequency from the sound signal of the test sound, and transmits a signal indicating that the test sound has been transmitted to the remote control circuit 11. The remote controller control circuit 11 outputs a detection signal to the infrared light emitting unit 13 when “1” meaning a test sound is detected from the quantization circuit of the waveform determination circuit 14. The infrared light emitting unit 13 transmits this detection signal as an infrared signal from the infrared light emitting unit 13. This infrared signal is received by the infrared light receiver 21 of the speaker array 20 and transmitted to the system controller 22. The system controller 22 gives a time T1 [s from when the remote controller 10 receives the test sound until the detection signal is transmitted to the system controller 22 via the infrared light receiver 21 after instructing the output of the test sound. ] Is measured. This measurement is performed based on a timer that is started when an instruction to output a pulse sound is given.

  When this measurement is completed, the system controller 22 controls the speaker control circuit 23 to output a test sound to the speaker SP5 via the amplifier A5. Similarly to the case where the test sound is emitted from the speaker SP1, the system controller 22 instructs the output of the test sound, and then the remote controller 10 receives the test sound, and the detection signal is transmitted via the infrared light receiving unit 21. Time T2 [s] until it is transmitted to the system controller 22 is measured. This measurement is performed based on a timer that is started when an instruction to output a pulse sound is given.

  From each of T1 and T2 [s], the system controller 22 instructs the output of the position detection pulse sound to the output to the speaker array 20, the delay time generated by the remote controller 10, and the infrared light reception. By subtracting the delay time until the signal is transmitted from the unit 21 to the system controller 22, it is possible to measure the times t1 and t2 from the speaker array 20 to the remote controller 10 (however, from the infrared light emitting unit 13 to the infrared light receiving unit 21) The propagation time is ignored because the propagation speed is the speed of light and the order of the propagation time is relatively small relative to the sound speed.) A value obtained by integrating the sound speed V [m / s] at the times t1 and t2 [s] is distances X1 and X2 from the speakers SP1 and SP5. Therefore, the position of the listener with respect to the speakers SP1 and SP5 can be obtained from the distances X1 and X2 from the speakers SP1 and SP5 to the remote controller.

  However, in reality, the noise collected by the microphone 12 of the remote controller 10 from the surroundings may include sound having the same frequency as the test sound. Then, the system controller 22 reacts to this noise, and there is a possibility that the wrong remote control position (that is, listening position) is calculated by measuring the wrong times t1 and t2. For this reason, the system according to the present embodiment has a function of preventing the formation of a sound field based on an erroneous listening position by determining whether the measurement result is appropriate. This function uses the predetermined distances α and β to determine that the measurement is incorrect depending on whether or not the following conditional expression is satisfied.

| X1-X2 | <X0 + α (inequality 1)
X1 + X2> X0 + β (inequality 2)
This condition uses a condition for establishing a triangle formed by the distance X0 between the speakers SP1 and SP5 and the distances X1 and X2 shown in FIG. 1, and when the condition is not satisfied, X1 and X2 are not valid.
Here, α and β are not necessary if they are ideal conditions, and α = 0 and β = 0 may be set. However, depending on the trigger timing of the CPU and the acoustic conditions, it is difficult to accurately measure the distance. It is a constant required as a margin.

  Although both sides of the inequalities 1 and 2 may be calculated, in practice, it is more computationally efficient to convert the conditional expression into a conditional expression of t1 and t2. This is because it is not necessary to calculate the distance by multiplying by the sound velocity V [m / s], and it is possible to determine whether the measured result is appropriate. Therefore, the previous conditional expression can be converted into the expressions t1 and t2 as follows using the above-described sound velocity V [m / s].

| T1-t2 | <(X0 + α) / V (inequality 3)
t1 + t2> (X0 + β) / V (inequality 4)
In the system of this embodiment, since the interval of the speaker array 20 is fixed, (X0 + α) / V and (X0 + β) / V are fixed values. If this fixed value is stored in advance, it is not necessary to obtain the distances X1 and X2, and it is possible to determine whether the values are appropriate using t1 and t2. This is because X0 is often fixed. That is, the position of the listener can vary, but the distance between the speakers often does not move frequently, so X0 + α and X0 + β are fixed values. By using this conditional expression, it can be easily determined whether or not the system controller 22 has calculated an incorrect remote control position (ie, listening position).

  Next, the flow of the distance measurement process in the system of the present embodiment described above will be described with reference to FIG. FIG. 3 is a diagram illustrating a flow of the distance measurement process. 3 is based on the premise that the position detection start button 101 in FIG. 1 has been pressed and an instruction has been given to the system controller 22 in FIG. 2, and the control subject is the system controller 22.

  In S1 of FIG. 3, the speaker control circuit 23 is controlled to output a test sound from SP1. The speaker control circuit 23 reads the digital data of the test sound from a memory outside the figure, converts it to an analog signal by a D / A converter (not shown) in the speaker control circuit 23, and performs a test from the speaker SP1 via the amplifier A1. Output sound.

  In S2 of FIG. 3, the microphone 12 incorporated in the remote controller 10 receives the test sound output in S1 and converts it into an audio signal. Whether the test signal is input to the audio signal by the waveform determination circuit 14 or not? The quantization circuit therein is sent to the remote control circuit 11 through a signal indicating that a test sound has been input. The remote controller control circuit 11 determines that the test sound has been received, and transmits an infrared signal to that effect from the infrared light emitting unit 13. The infrared light receiver 21 transmits an infrared signal to the system controller 22 indicating that the remote controller 10 has received the test sound.

  In S3 of FIG. 3, a time t1 from when the test sound is output from the speaker SP1 until reaching the microphone 12 is calculated. The method is as follows. That is, the system controller 22 receives the infrared signal from the remote controller 10 after instructing the output of the test sound, and the time from when the detection signal is transmitted to the system controller 22 via the infrared light receiver 21 (see FIG. 2) as described above. 2, the delay time from the time difference T1 to the output from the system controller 22 to the speaker SP1, the output from the microphone 12 to the infrared light emitting unit 13, and the system controller from the infrared light receiving unit 21. A delay time until transmission to the terminal 22 is calculated in advance, and a time t1 obtained by subtracting these delay times from T1 is calculated. Also, before outputting a new test sound from SP5 in S4 described later, output of the test sound output from SP1 is stopped.

S4 to S6 of FIG. 3 correspond to S1 to S3, and the steps of S4 to S6 can be adapted to the steps of S1 to S3 as SP1 → SP5, T1 → T2, and t1 → t2. Therefore, the description of S4 to S6 in FIG. 3 is omitted.
In S7 of FIG. 3, it is determined whether t1 and t2 satisfy the following expression.

| T1-t2 | <(X0 + α) / V (inequality 3)
t1 + t2> (X0 + β) / V (inequality 4)
That is, both sides of this inequality are calculated.
In S8 of FIG. 3, when the inequality of S7 is satisfied, Y is determined and the process moves to S9. If not, the process moves to the flow of FIG. 4 to be described later.
In S9 of FIG. 3, t1 and t2 are multiplied by the sound velocity V, respectively, and distances X1 and X2 (shown in FIG. 1) from the speakers SP1 and SP5 to the position of the remote controller 10, that is, the listener are measured. Thereafter, the flow of FIG. 3 ends.

  The flow of FIG. 3 is summarized as follows. That is, the time t1 from when the test sound is output from SP1 until it reaches the microphone 12 of the remote controller 10 is measured (S1 to S3). Similarly, the time t2 from when the test sound is output at SP5 until it reaches the microphone 12 of the remote controller 10 is measured (S4 to S6). In S7, it is determined whether the conditional expression described in S7 is satisfied using t1 and t2. If the conditional expression of S7 is satisfied (Y of S8), X1 and X2 are calculated in S9. If the conditional expression of S7 is not satisfied (N of S8), the flow of FIG. 4 described later is executed.

  When the flow of FIG. 3 is normally completed after the step of S9, the positions of the speakers SP1 to SP5 and the remote controller 10, that is, the positional relationship between the listeners are calculated based on X1 and X2 calculated in S9 of FIG. Based on this, the system controller 22 instructs the speaker control circuit 23 to create a sound field environment that is effective at the calculated remote control position (ie, listening position). In response to this, the speaker control circuit 23 determines a frequency equalizer parameter for creating an appropriate sound field environment or a delay parameter for directivity control from the given information of X1 and X2. A memory table (not shown) in the speaker control circuit 23 stores a data table of a frequency gain for creating a sound field corresponding to the parameter, for example, data of a model head-related transfer function. Read out and set in the equalizer circuit 26.

Note that SP1 to SP5 in FIG. 1 correspond to any of “a plurality of speakers” of the present invention. SP1 and SP5 in FIG. 1 correspond to speakers used for “generating test sound” in the present invention. In addition, the speaker used for “generating a test sound” in the present invention may not be a combination of SP1 and SP5, but may be another combination. Further, in the flowchart of FIG. 3, S1 to S3 are performed using SP1 before S4 to S6 using SP5, but the order may be reversed. Further, if possible, the order shown in FIG. 3 may be different.
Further, not satisfying the above inequality 1 corresponds to not satisfying the “first condition” of the present invention. The relationships between the inequalities 2 to 4 and the “second condition” to “fourth condition” of the present invention are also the same.
In S7 of FIG. 3, the first condition and the second condition described above may be used instead of the third condition and the fourth condition used in S7.
Furthermore, the processing of FIG. 3 may be performed simultaneously when, for example, the activation of the disk device is instructed by the operation element 1xx of the remote controller 10. If this is done, the distance measurement in FIG. 3 will be completed in about one second, while starting the disk device takes several seconds for disk selection, tilt setting, etc., so that it is possible to overlap each other.

Next, processing when the measurement result of FIG. 3 is not valid will be described with reference to FIG. FIG. 4 is a flowchart showing the processing when it is not valid, and the processing when N is performed in S8 of FIG. 3 is performed. Note that the control subject of this processing is the system controller 22.
In S11 of FIG. 4, the number of times that the distance measurement is determined to be invalid as a result of the determination in S7 and S8 of FIG. 3, that is, the number of times of measurement failure is counted.
In S12 of FIG. 4, it is determined whether the number of times counted in S11 has reached a predetermined number N. When it has reached, it becomes Y and moves to S13. If the number counted in S11 has not reached the predetermined number N, the result is N, and the process moves to S15 to detect an error indicating that distance measurement is to be performed again. In S15 of FIG. 4, when the measurement result of FIG. 3 is not valid, a warning that the distance measurement is to be performed again is generated by a predetermined method. For example, a warning sound is emitted or a warning is displayed on a display (not shown) of the speaker array 20. Thereafter, the process returns to S1 in FIG. 3 to repeat the distance measurement (S16).
The branch to S13 in FIG. 4 occurs when the measurement data is not valid even after the measurement of FIG. 3 is performed a predetermined number of times (N times). Therefore, in order to stop at a predetermined (N times) predetermined, a notification that measurement is impossible is performed in S13 of FIG. For example, a warning sound is emitted or a warning is displayed on a display (not shown) of the speaker array 20. Note that the warning in S11 may be omitted, and another warning may be performed together with S13.
In S14 of FIG. 4, it is estimated that the position of the remote control is in a default position that is a predetermined distance away from the center position of the speaker array 20, and the speaker control circuit 23 is effective so as to exert the effect at that position. Sets the equalizer circuit 26. Thereafter, the flow of FIG. 4 ends.

It is a conceptual diagram showing the outline | summary of the acoustic system of this embodiment. It is an internal block diagram of the acoustic system of this embodiment. It is a figure showing the flow of the process of the distance measurement in the acoustic system of this embodiment. It is a figure showing the flow when the process of the distance measurement in the acoustic system of this embodiment is inappropriate.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1- Acoustic system 10- Remote control 11- Remote control control circuit 12- Microphone 13- Infrared light emission part 14- Waveform determination circuit 101- Position detection start button 102-1xx- Operating element 20- Speaker array 21- Infrared light receiving part 22- System controller 23-speaker control circuit 24-input interface 25-audio input control unit 26-equalizer circuit SP1-SP5-speaker A1-A5-amplifier X0-interval X1, X2-distance

Claims (2)

Multiple speakers,
A test sound generating unit that sequentially inputs a test signal to each of the plurality of speakers to generate a test sound;
A remote control device operated by a user, comprising: a receiving means for receiving the test sound; and a function of returning a reception confirmation signal as an infrared signal when received,
A remote control receiver for receiving the infrared signal of the remote control device;
Measuring means for measuring a transmission time until the remote control receiving unit receives a reception confirmation signal after generating the test sound;
Position calculating means for calculating the position of the remote control device based on the propagation time from each speaker to the remote control device;
An acoustic system comprising:
The position calculation means is an acoustic system that re-attempts the generation of the test sound when the positional relationship between each speaker and the remote control device does not satisfy a condition for establishing a triangle . The conditions for establishing the triangle of the position calculating means are:
Absolute value, said predetermined distance is smaller than the distance (X0 + alpha) which is the sum of the alpha and spacing X0 of the two speakers of the difference between the distance X1, X2 position was calculated for the remote control device of the loudspeaker The first condition,
A second condition that a sum of the distances X1 and X2 is larger than a distance (X0 + β) that is a sum of an interval X0 between the two speakers and a predetermined distance β;
Alternatively, the acoustic system according to claim 1, wherein the acoustic system is at least one of a third condition and a fourth condition in which the first condition and the second condition are replaced with the propagation time condition, respectively.

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