The ambient noise fluctuation and directionality study in South China Sea

This study is based upon the vertical line array (VLA) in South China Sea experiment of the Asian Seas International Acoustic Experiment (ASIAEX), dated from May 3, 2001 to May 16, 2001, in which the ambient noise in shallow water was measured. We present two analyses of ambient noise, the fluctuation and directionality. The fluctuation of ambient noise level is measured by using Fourier analysis, and we did the similar analysis on static water pressure and temperature variation, the acoustic data has shown obvious K1 and M2 tidal periods. Typhoon Cimaron passed the experimental area during the experiment, which increased ambient noise levels at higher frequency (above 400 Hz) and affected the tidal fluctuations. Accordingly, the threshold frequency of the ambient noise levels affected by the wind wave is about 400 Hz. On the other hand, we use beamforming method to calculate the vertical directionality of shallow water ambient noise. The results show that distant shipping noise was observed near the horizontal angles, and surface noise occurred at high grazing angles. We also noted the phenomena of "noise notch" appeared at some duration, and the environmental effects (especially sound speed profile) on the notch were discussed in the end part of this paper. The boundary effects of shallow water have significant impacts on sound propagation, thus strong time variation of ambient noise directionality was recorded.