Active noise reduction system

An active noise reduction system, comprising:

• a canceling sound output device configured to output a canceling sound for canceling a noise;
• an error microphone configured to generate an error signal based on the noise and the canceling sound;
• and
• a controller configured to control the canceling sound output device based on the error signal,
• wherein the controller includes:
• a control filter configured to generate a control signal for controlling the canceling sound output device;
• and
• a secondary path filter that represents an estimation value of a transfer function of a secondary path from the canceling sound output device to the error microphone,
• the active noise reduction system further comprises a reference microphone provided separately from the error microphone,
• the reference microphone is configured to generate a determination signal based on at least the noise, and
• wherein the controller is configured to:
• perform an update process for adaptively updating the secondary path filter;
• determine whether fluctuation of the secondary path filter according to an adaptive update thereof has converged;
• stop the adaptive update of the secondary path filter in a case where the fluctuation of the secondary path filter has converged;
• determine whether the transfer function of the secondary path has changed based on the error signal and the determination signal in a state where the adaptive update of the secondary path filter is stopped;
• and
• resume the adaptive update of the secondary path filter in a case where the transfer function of the secondary path has changed.