Frequency-selective attenuation of sound propagation and reverberation in Shallow Water

In the summer of 1991 studies of sound propagation and reverberation were carried out in the Huanghai (Yellow) Sea with a strong thermocline by using explosive sound sources.Experimental results of both the sound transmission loss along a certain course and the average reverberation intensity showed an abnormally large attenuation over a frequency range from 1 000 to 2 000 Hz when both the source and the receiver were located above the thermocline.An interesting point is that the anomalous attenuation of the average reverberation intensity obtained by using omnidirectional source and receiver and that of the transmission loss along the certain course exhibit a resonance-like or frequency-selective characteristic with the same centre frequency, similar bandwidth and comparable extra attenuation values.Obviously,this anomalous attenuation cannot be explained by any anisotropic mechanism (e.g.,internal waves,regular fluctuations of sea surface or bottom, etc.).On the basis of the depth dependences of the transmission loss and reverberation intensity obtained by the authors and some indirect evidences,it is believed that the observed frequency-selective attenuation is due to fish with swimbladders (probably, anchovies) living dispersedly above the thermocline.