Field experiments on ANC for infrasound by using pneumatic powered sound source

摘要

In Japan, a serious environmental noise problem such as the window rattling has been caused by infrasound. It is difficult to reduce exposure of infrasound by passive mechanisms such as noise barriers, porous materials, acoustic resonators, and so on. On the other hand, active noise control (ANC) technique has not been applied to the infrasound, since it is not easy to generate the infrasound at high sound pressure level. We have developed transportable low frequency sound generators (LFSGs) driven by pneumatic servo actuators. LFSGs can reproduce the arbitrary waveform, in which the frequency range is 5 to 20 Hz. For pure tones, sound pressure level of generated sound reaches over 100 dB at a front of the source in the free-field. To investigate the applicability of ANC technique to the infrasound, two field experiments were performed. One is an experiment that ANC technique was applied to reducing pure tones of 5 Hz by using LFSGs. As the result, it was confirmed that sound pressure level at an observation point was reduced by over 25 dB. The other is a field experiment to apply ANC to an actual sound was performed. Using LFSG, window displacement due to a helicopter passing could be reduced by one-third.