Active noise-reduction apparatus and method

1. An active noise-reduction apparatus for reducing a target sound having periodicity, the apparatus comprising:

an error microphone which converts a sound including the target sound into a first error signal;

a reference signal generator configured to generate a reference signal;

a control filter configured to convert, in accordance with a control characteristic, the reference signal into a control signal used to cancel the target sound;

a control loudspeaker which emits a control sound based on the control signal;

a first control effect estimation filter configured to convert the control signal into a first signal in accordance with an estimated secondary path characteristic, the estimated secondary path characteristic being generated based on a result of identifying a secondary path characteristic from the control loudspeaker to the error microphone in advance;

an estimated noise signal generator configured to generate an estimated noise signal by subtracting the first signal from the first error signal;

a second control effect estimation filter configured to convert the control signal into a second signal in accordance with a processed secondary path characteristic, the processed secondary path characteristic being obtained by shortening a delay included in the estimated secondary path characteristic by a time, the time corresponding to a period of the target sound multiplied by a constant; and

an updating unit configured to update the control characteristic so that a second error signal which is a sum of the estimated noise signal and the second signal is minimized,

wherein letting T be the period, a be the delay, and m be the constant, the constant is a positive integer satisfying T×m≦a.

2. The apparatus according to claim 1 , wherein the constant is a maximum integer satisfying T×m≦a.

3. The apparatus according to claim 1 , wherein the reference signal generator comprises a reference microphone which converts the target sound into the reference signal.

4. The apparatus according to claim 1 , wherein the reference signal generator comprises the estimated noise signal generator, and the reference signal is the estimated noise signal.

5. The apparatus according to claim 1 , further comprising:

a period calculation unit configured to calculate the period based on the reference signal; and

a determination unit configured to determine the processed secondary path characteristic based on the calculated period.

6. An active noise-reduction apparatus for reducing a target sound, the apparatus comprising:

an error microphone which converts a sound including the target sound into an error signal;

a reference signal generator configured to generate a reference signal;

a control filter configured to convert, in accordance with a control characteristic, the reference signal into a control signal used to cancel the target sound;

a control loudspeaker which emits a control sound based on the control signal;

a control effect estimation filter configured to convert the control signal into a first signal in accordance with an estimated secondary path characteristic, the estimated secondary path characteristic generated based on a result of identifying a secondary path characteristic from the control loudspeaker to the error microphone in advance;

a secondary path filter configured to convert the reference signal into a first auxiliary signal in accordance with the estimated secondary path characteristic;

a virtual control effect estimation filter configured to convert the first auxiliary signal into a second signal in accordance with the control characteristic; and

an updating unit configured to update the control characteristic so that an evaluation function based on the error signal and a second auxiliary signal is minimized, the second auxiliary signal being a difference between the second signal and the first signal.

7. The apparatus according to claim 6 , wherein the reference signal generator comprises a reference microphone which converts the target sound into the reference signal.

8. The apparatus according to claim 6 , wherein the reference signal generator generates the reference signal by subtracting the first signal from the error signal.

9. An active noise-reduction method for reducing a target sound having periodicity, the method comprising:

providing an error microphone which converts a sound including the target sound into a first error signal;

generating a reference signal;

converting, in accordance with a control characteristic, the reference signal into a control signal used to cancel the target sound;

providing a control loudspeaker which emits a control sound based on the control signal;

converting the control signal into a first signal in accordance with an estimated secondary path characteristic, the estimated secondary path characteristic being generated based on a result of identifying a secondary path characteristic from the control loudspeaker to the error microphone in advance;

generating an estimated noise signal by subtracting the first signal from the first error signal;

converting the control signal into a second signal in accordance with a processed secondary path characteristic, the processed secondary path characteristic being obtained by shortening a delay included in the estimated secondary path characteristic by a time, the time corresponding to a period of the target sound multiplied by a constant; and

updating the control characteristic so that a second error signal which is a sum of the estimated noise signal and the second signal is minimized,

wherein letting T be the period, a be the delay, and m be the constant, the constant is a positive integer satisfying T×m≦a.

10. The method according to claim 9 , wherein the constant is a maximum integer satisfying T×m≦a.

11. The method according to claim 9 , wherein the generating the reference signal comprises providing a reference microphone which converts the target sound into the reference signal.

12. The method according to claim 9 , wherein the reference signal is the estimated noise signal.

13. The method according to claim 9 , further comprising:

calculating the period based on the reference signal; and

determining the processed secondary path characteristic based on the calculated period.

14. An active noise-reduction method for reducing a target sound, the method comprising:

providing an error microphone which converts a sound including the target sound into an error signal;

generating a reference signal;

converting, in accordance with a control characteristic, the reference signal into a control signal used to cancel the target sound;

providing a control loudspeaker which emits a control sound based on the control signal;

converting the control signal into a first signal in accordance with an estimated secondary path characteristic, the estimated secondary path characteristic generated based on a result of identifying a secondary path characteristic from the control loudspeaker to the error microphone in advance;

converting the reference signal into a first auxiliary signal in accordance with the estimated secondary path characteristic;

converting the first auxiliary signal into a second signal in accordance with the control characteristic; and

updating the control characteristic so that an evaluation function based on the error signal and a second auxiliary signal is minimized, the second auxiliary signal being a difference between the second signal and the first signal.

15. The method according to claim 14 , wherein the generating the reference signal comprises providing a reference microphone which converts the target sound into the reference signal.

16. The method according to claim 14 , wherein the generating the reference signal comprises generating the reference signal by subtracting the first signal from the error signal.