Active noise control apparatus

An active noise control apparatus having a feedback control filter which simulates a characteristic of a feedback path leading to a noise detecting microphone connected to an input of a noise control filter for synthesizing a secondary noise, through a loud speaker which transmits the secondary noise having a same amplitude as and an opposite phase to a primary noise for suppression comprising:

• an inverse filter composing circuit which forms a filter having a characteristic including a transfer function opposite to a closed circuit starting from the noise control filter and returning to the noise control filter again through both of the feedback path and the feedback control filter, the transfer function expressing a generation process of the primary noise, and
• a system identification circuit for providing two pairs of different fixed coefficients either to the feedback control filter or the noise control filter to update a coefficient of an adaptive filter in order that a difference becomes a minimum between an output of a first filter forming a numerator of a solution obtained by eliminating a transfer function component expressing the generation process of the primary noise, from simultaneous equations based on two transfer functions of an inverse filter formed within the inverse filter composing circuit for the two pairs of coefficients and an output of the adaptive filter connected to a second filter in cascade forming a denominator of the solution,
• a coefficient of the feedback control filter being updated by using the coefficient of the adaptive filter obtained by operating the system identification circuit.

An active noise control apparatus of a feedback type having a feedback control filter which simulates a characteristic of a feedback path leading to a noise detecting microphone connected to an input of a noise control filter for synthesizing a secondary noise, through a loud speaker which transmits the secondary noise having a same amplitude as and an opposite phase to a primary noise for suppression comprising:

• an inverse filter composing circuit which forms a filter having a characteristic including a transfer function opposite to a closed circuit starting from the noise control filter and returning to the noise control filter again through both of the feedback path and the feedback control filter, the transfer function expressing a generation process of the primary noise, and
• a system identification circuit for providing two pairs of different fixed coefficients either to the noise control filter or the feedback control filter to update a coefficient of an adaptive filter in order that a difference becomes a minimum between an output of a first filter forming a numerator of a solution obtained by eliminating the transfer function component expressing the generation process of the primary noise, from simultaneous equations based on two transfer functions of an inverse filter formed within the inverse filter composing circuit for the two pairs of coefficients and an output of the adaptive filter connected to a second filter in cascade forming a denominator of the solution,
• the system identification circuit further being repeatedly used for calculating the coefficient of the noise control filter optimally canceling the primary noise as the coefficient of the adaptive filter by making a numerator, which forms a ratio of the transfer function expressing the generation process of the primary noise and the transfer function of the feedback path previously identified on the system identification circuit, the first filter and making a denominator the second filter.