| When naval mine-hunting sonars and side-scan surveying sonars are operated in shallow coastal environments, which are characterized by high levels of suspended fine clay particles relative to open ocean, the effect of sound absorption caused by suspended particles in the water may greatly decrease these sonars¡¯ detection performance. This kind of water is also characterized as turbid seawater. In order to account for this effect in sonars performance predictions, it is necessary to calculate the absorption coefficient in particulate suspensions. However, the sound absorption properties of suspended particulate matter in natural bodies of water are not well characterized. The main difference between turbid seawater and clear seawater is that there are fine clay particles suspended in turbid seawater. In the paper, a reverberation time technique has been developed for the laboratory measurement of sound absorption at 20-60 kHz in dilute suspensions, and the measuring system has also been built up. In the test volumes employed, the effect is small. It is therefore measured by taking the difference in reverberation times of a volume of water with and without fine clay particles. This greatly reduces the effect on the measurement of the other sources of absorption. Even so, it is necessary to design the experiment to characterize and minimize acoustic losses which occur at the surfaces of the container, the hydrophones, and their cables, and losses associated with bubbles and turbulence. The pure water is taken to calibrate the energy loss of the measuring system. After that, the coefficient of sound absorption in the magnesium sulfate solutions of known concentrations has been measured and compared to theoretical predictions, so as to estimate the measurement error of the system. The measured results agree well with that predicted by theory. In clear seawater, the main cause of sound absorption at 20-60 kHz is the electrolyte magnesium sulfate. In order to characterize absorption by natural marine particles measurements in representative suspensions are required. So the solute in turbid seawater is mainly the magnesium sulfate and fine clay particles. Many turbid seawater samples of different concentrations have been measured. The results demonstrate that at a fixed frequency, the coefficient of sound absorption in turbid seawater is augmenting approximately with the increased concentrations. If the concentrations of fine clay particles in turbid seawater are below 110 mg/L, the fine clay particles don¡¯t cause additional sound absorption. However, if the concentrations of fine clay particles in turbid seawater are above 140 mg/L, the coefficient of sound absorption in turbid seawater is as twice at least as that in clear seawater. It is concluded that sound absorption measured in this paper is appropriate for estimating the sound absorption in dilute suspensions of fine clay particles. |