A Study of the Infrasonic Wave in the Atmosphere

Abstract

Continuous observation of the infrasonic wave has been recently started at Aichi University of Education in Kariya City. Major interest is in the infrasounds frequently radiated into the atmosphere by the summit explosion of the volcano Sakura-jima.In detection of the infrasonic signal in the turbulent boundary layer, it is often indispensable to reduce the irregular noise caused by the turbulent fluctuation of the wind. In the present study, a new type of wind noise reducer has been designed. It is a multi-pipe line microphone, having a number of branches of pipes of equal diameter. The function of it is to take the equally-weighted spatial average of the pressure field along a straight line, and is essentially the same as the Daniels' line microphone (1959). However, the manufacturing of the multi-pipe line microphone is practically easier than the Daniels' type. Another merit of the new device is in that it is as easy to take the two-dimensional average if necessary.From several field tests, it has been shown that the wind noise is remarkably attenuated by the use of multi-pipe line microphone, although the measured noise attenuation has been smaller than the theoretical estimation based on the turbulence model given by Priestley(1965) by about 5dB in average.