Abstract:

The nonlinear acoustic properties of sediments have been studied using a novel experimental approach. A small amount (10--100 cm[sup 3]) of sediments is suspended in a water column in a sound transparent vessel. The vessel is placed inside an experimental tank and exposed to harmonic acoustic pressure. Nonlinearity of sediments causes the generation of higher harmonics which are radiated into the surrounding water and measured with a hydrophone. The nonlinear response for various frequencies (0.1--10 kHz) of the excitation is studied varying both the grain-size distribution of the sediment and the amount of trapped air. A very significant level of nonlinear distortion is observed even for a relatively low amplitude of the excitation signal (under 200 Pa). In some cases (coarse sand with a 0.01--0.03 gas void friction) up to 20 harmonics are observed, with the second harmonic attaining 30% (-10 dB) of the fundamental. Such a high level is 120 dB greater than the estimated amplitude of the second harmonic generated by the same volume of pure water and more than 40 dB greater than could be generated by water having the same gas void friction. The theoretical model of such a considerable nonlinear behavior based on the interaction of grains in the presence of gas voids is discussed. [Work supported by ONR.]