1985 年 41 巻 5 号 p. 316-321
An active noise attenuation system, which is a compact low frequency noise silencer, was developed. The system in this paper controls repetitive low frequency noise in a pipe, such as the intake noise of air compressors. The conventional low frequency noise absorbers like resonance type became very large, because of the wavelength of low frequency sound. But active technique made a low frequency attenuator compact. In this system, a synchronous signal of a machine is fetched to measure the fundamental frequency of a noise source. And the sound waveform at the pipe end is analyzed with discrete Fourier expansion by a micro-computer. Then the system drives an additional loudspeaker which produces a canceling waveform. Within one second after the system started operating it reduced the sound generated by a primary loudspeaker up to 15 dB. And it finally achieved over 20 dB of attenuation. The system also attenuated an intake noise of a real mini-compressor by 18 dB in experiment.