Acoustic control apparatus

1. An acoustic control apparatus, comprising:

a control filter configured to multiply a first acoustic signal by a control filter coefficient to obtain a second acoustic signal;

a first loudspeaker configured to emit the second acoustic signal; and

a second loudspeaker configured to emit the first acoustic signal,

wherein the control filter coefficient is calculated based on first head-related transfer function sets from the first loudspeaker and the second loudspeaker to target binaural positions and second head-related transfer function sets from a target virtual acoustic source to the target binaural positions in such a manner that a second spatial average of complex sound pressure ratios at the target binaural positions when the first loudspeaker and the second loudspeaker emit the second acoustic signal and the first acoustic signal is approximated to a first spatial average of complex sound pressure ratios at the target binaural positions if the target virtual acoustic source emitted the first acoustic signal, the target virtual acoustic source being different from the first loudspeaker and the second loudspeaker.

2. The acoustic control apparatus according to claim 1 , wherein a total number of the target binaural positions is two or more.

3. An acoustic control apparatus, comprising:

a first control filter configured to multiply a first acoustic signal by a first control filter coefficient to obtain a second acoustic signal;

a second control filter configured to multiply the first acoustic signal by a second control filter coefficient to obtain a third acoustic signal;

a first loudspeaker configured to emit the second acoustic signal; and

a second loudspeaker configured to emit the third acoustic signal,

wherein the first control filter coefficient is calculated based on the second control filter coefficient, first head-related transfer function sets from the first loudspeaker and the second loudspeaker to target binaural positions, and second head-related transfer function sets from a target virtual acoustic source to the target binaural positions in such a manner that a second spatial average of complex sound pressure ratios at the target binaural positions when the first loudspeaker and the second loudspeaker emit the second acoustic signal and the third acoustic signal is approximated to a first spatial average of complex sound pressure ratios at the target binaural positions if the target virtual acoustic source emitted the first acoustic signal, the target virtual acoustic source being different from the first loudspeaker and the second loudspeaker.

4. The apparatus acoustic control according to claim 3 , wherein a total number of the target binaural positions is two or more.

5. An acoustic control apparatus, comprising:

X−1 wherein X is an integer not smaller than 3, control filters configured to multiply a first acoustic signal by first, . . . , and X−1th control filter coefficients to obtain second, . . . , and Xth acoustic signals; and

X loudspeakers configured to emit the first, . . . , and Xth acoustic signals,

wherein the first, . . . , and X−1th control filter coefficients are calculated based on at least X−1 first head-related transfer function sets from the X loudspeakers to at least X−1 target binaural positions and at least one second head-related transfer function set from a target virtual acoustic source to the at least X−1 target binaural positions in such a manner that a second spatial average of at least X−1 complex sound pressure ratios at the at least X−1 target binaural positions when the X loudspeakers emit the first, . . . , and Xth acoustic signals is approximated to a first spatial average of at least X−1complex sound pressure ratios at the at least X−1 target binaural positions if the target virtual acoustic source emitted the first acoustic signal, the target virtual acoustic source being different from the X loudspeakers.

6. The acoustic control apparatus according to claim 5 , wherein a total number of the target binaural positions is X or more.

7. An acoustic control apparatus, comprising:

first, . . . , and X−1th wherein X is an integer which is not smaller than 3, control filters configured to multiply first, . . . , and Mth wherein M is an integer not smaller than 2, acoustic signals by first, . . . , and X−1th control filter coefficient sets and combine results to obtain first, . . . , and X−1th combined acoustic signals; and

X loudspeakers configured to emit the first, . . . , and X−1th combined acoustic signals and an Xth combined acoustic signal, the Xth combined acoustic signal obtained by combining the first, . . . , and Mth acoustic signals,

wherein the first, . . . , and X−1th control filter coefficient sets are calculated based on at least X−1 first head-related transfer function sets from the X loudspeakers to at least X−1 target binaural positions and at least M second head-related transfer function sets from M target virtual acoustic sources to the at least X−1 target binaural positions in such a manner that a second spatial average of at least X−1 complex sound pressure ratios at the at least X−1 target binaural positions when the X loudspeakers emit the first, . . . , and the Xth combined acoustic signals is approximated to a first spatial average of at least X−1 complex sound pressure ratios at the at least X−1 target binaural positions if the M target virtual acoustic sources emitted the first, . . . , and Mth acoustic signals, the M target virtual acoustic sources being different from the X loudspeakers.

8. The acoustic control apparatus according to claim 7 , wherein a total number of the target binaural positions is X or more.