This paper describes an active noise cancellation system utilizing computer-synthesized sound. This computer-synthesized sound is generated by using a new method to estimate the impulse response of a system involving inverse transfer function of a loudspeaker. A loudspeaker is set close to the end of a duct to radiate sound for cancellation. The loudspeaker is driven by a signal which is synthesized by convoluting the estimated impulse response and the wave form of a noise picked up by a microphone set in the duct. The system is substantially stable. In our experiment, using a duct 8.5 m in length, the sound pressure at a point outside the duct is decreased by about 3 to 8 dB over a frequency range between 50 Hz and 200 Hz.

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