Active noise suppressor

1. An active noise suppressor having a fundamental sound source which generates a fundamental waveform having a predetermined frequency, and suppresses a frequency component of noise which corresponds to the predetermined frequency by generating a control sound from a signal obtained by multiplying the fundamental waveform by an adaptive filter coefficient, comprising:

a phase detecting unit that detects a phase of a current control sound and a phase of a previous control sound respectively based on a current adaptive filter coefficient and a previous adaptive filter coefficient;

a change amount detecting unit that detects an amount of a phase change between the phase of the current control sound and the phase of the previous control sound; and

a frequency adjusting unit that increases or decreases, by a predetermined amount, the frequency of the fundamental waveform output from said fundamental sound source, if the amount of the phase change of the control sound is larger than a predetermined threshold value.

2. An active noise suppressor according to claim 1 , wherein the adaptive filter coefficient comprises a first filter coefficient and a second filter coefficient, and the control sound is represented as a synthetic waveform of a cosine wave multiplied by the first filter coefficient and a sine wave multiplied by the second filter coefficient.

3. An active noise suppressor according to claim 1 , wherein

said change amount detecting unit further detects a direction of the phase change of the control sound, and

said frequency adjusting unit determines, in accordance with the detected direction, whether to increase or decrease the frequency of the fundamental waveform.

4. An active noise suppressor according to claim 3 , further comprising:

an error signal obtaining unit that obtains, as an error signal, the frequency component of the noise after the control sound is applied;

a reference signal generating unit that generates a reference signal from the fundamental waveform and a transfer function of a premeasured system; and

a filter coefficient calculating unit that calculates and updates the adaptive filter coefficient on the basis of an adaptive algorithm by using the error signal and the reference signal.

5. An active noise suppressor according to claim 4 , further comprising:

a peak frequency detecting unit that detects peak frequencies of the noise; and

a frequency setting unit that sets a predetermined number of peak frequencies in order from a peak frequency having a maximum peak of the detected peak frequencies, as output frequencies of said fundamental sound source.

6. An active noise suppressor according to claim 5 , further comprising:

a virtual noise generating unit that generates virtual noise from the control signal, the transfer function, and the error signal;

a frequency difference detecting unit that detects a difference between a presently set peak frequency and the peak frequency detected by said peak frequency detecting unit; and

a frequency resetting unit that changes, if the difference exceeds a predetermined value, the output frequency of said fundamental sound source, which generates a fundamental waveform corresponding to the presently set peak frequency, to the peak frequency detected by said peak frequency detecting unit.

7. An active noise suppressor according to claim 6 , wherein said virtual noise generating unit generates, from the control sound and the transfer function of the system, a signal when the control sound has reached said error signal obtaining unit, and generates the virtual noise by subtracting the signal from the error signal.