A Preliminary Study Of High-frequency Acoustic Attenuation Of Coastal Turbid Waters

Offshore acoustic becomes a research hotspot after the Cold War. China coastal waters are very turbid with the river discharge(include lots of suspended particles), tide, wind, ocean current and human activities. The suspended particles of turbid seawater are the dominant factors influencing the acoustic attenuation of coastal waters. Thus the sound transmission loss would be increased and the detection performance of sonar be deteriorated. These would be more serious for the various offshore detecting and anti-submarine sonar in high frequency. So, it would be of practical significance to study on high-frequency acoustical attenuation of coastal waters.In this paper, firstly, the acoustic attenuation of coastal waters in high-frequency had been studied both theoretically and experimentally. Theoretically, based on the discussion of acoustic attenuation mechanism of coastal waters, an acoustic attenuation prediction model was given, and the acoustic attenuations of coastal waters in high-frequency were calculated. experimentally, based on reverberation method, an experimental measured system of acoustic attenuation of turbid water in high-frequency had been designed. The acoustic attenuations of simulation coastal waters in high-frequecy were then measured. The measured results had been compared with those of predictions. Finally, with the improvement of the observation techniques and survey means, the prospect of the studies on acoustic attenuation of coastal waters were further discussed. From this paper we can conclude:(1) The viscous absorption of suspended particles at 100 kHz may exceed 1 dB for 100 meter when the volume concentration of the suspended particles reaches to 0.1 kg/m~3 magnitude. The acoustic attenuation of suspended particles for coastal turbid waters should not be ignored. Viscous absorption of suspended particles is the dominant factor in the gross acoustic attenuation of coastal waters. The clean seawater absorption and viscous absorption of suspended particles were the two terms needed to be considered when the acoustic attenuation of coastal turbid waters was calculated.(2) The viscous absorption of suspended particles is affected by suspended particle size. In this paper, the measurement results were more consistent with the prediction results using particle size distributions. By comparison of the prediction results using particle size distributions instead of using average particle radius, the discrepancy is 2.3～2.6dB every 100 meters for 1.0kg/m~3 of sediment turbid seawater within the scope of experimental frequency. It was suggested that the particle size distributions be used in the predictions of viscous absorption in coastal turbid waters.