The Journal of the Marine Acoustics Society of Japan

Sound speed and absorption constant of saturated marine sediment samples of Setouchi (the inland sea of Japan) and Hekinan Coast were measured by use of a tube method in a frequency range from 6.6 kHz to 11.0 kHz. Grain diameter distribution, porosity and density of sediments were also measured for each sample. And sound speed and absorption constant of the same sediment samples were measured in a frequency range from 40 kHz to 250 kHz by free sound field method. The sound speed and the attenuation coefficient were theoretically estimated based on the generalized Biot's theory on the acoustic wave propagation through porous media. A validity of the Biot's theory to apply the saturated marine sediment is confirmed by a comparison between the theoretical estimation and experimental measurements.

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In the case of the passive sonar, the LOFAR-Grams of received signals using frequency analysis and variable density visualization technique is widely used for target signal detection. The auto-spectrum analysis method is a signal processing means for the LOFAR-Grams.
As the means to improve the ratio of signal to noise levels (S/N), there is a process of averaging (cumulative processing) analyzed signals used in the LOFAR-Grams.
The discrimination signals smoothed cumulatively in the auto-spectrum method are apt to be difficult due to the nature of noise components, and this auto-spectrum method is not used in this case.
We proposed a new method here, and analyzed experimentally several types of signals of the passive sonar utilizing this method for the LOFAR-Grams.
The results showed that the process gain by cumulative method was obtained in accordance with calculation, and the delay type cross-spectrum method gave processing gain 3 dB higher than the auto-spectrum method.
It was considered that this technique was to be applied extensively to the passive sonar in the future.

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