1. Field of the Invention
The present invention relates to a method of determining the sound transfer characteristic of an active noise control system usable with various electronic equipment such as computers.
2. Description of the Related Art
A conventional active noise control system is installed for, for example, a computer room. The computers in the computer room accommodate computer circuit boards that generate heat. The circuit boards are cooled by cooling fans. The exhaust from the fans is guided through a duct. The moving air, the cooling fans, etc., cause noise. To detect and cancel the noise, the active noise control system has a noise-detection microphone, a speaker for generating noise-cancelling sound, an error detecting microphone for detecting a cancellation error, and an adaptive filter whose parameters are controlled to minimize the output of the error detecting microphone. The sound from the speaker is propagated towards a noise-source and enters the noise detection microphone, to cause feedback sound signal. It is necessary, therefore, to provide an active noise control system that is capable of preventing such feedback.
The active noise control system having a prevention function must determine the sound-transfer characteristic in the system, to deal with the sound propagating in the exhaust duct. Since the sound transfer characteristic is dependent on the length of the duct and the operating conditions of the system, it is very difficult to correctly determine the sound transfer characteristic even using a plurality of microphones arranged in the duct, transfer characteristic estimating algorithms, or FFT (Fast Fourier Transform) analyzer. Namely, there are no conventional methods for correctly determining the sound-transfer characteristic in the active noise-control system.