The experimental investigation on measuring sound absorption in the seawater containing suspended sediment particles

Shallow coastal waters are characterized by high levels of suspended sediment particles relative to the open ocean. This kind of seawater is also called turbid seawater. A cylindrical resonator technique has been developed to measure the coefficient of sound absorption in turbid seawater by the reverberation technique. The resonator is a seamless aluminum barrel with inner diameter 100 cm, inner height 55 cm, and wall thickness 4.3 mm. The aluminum barrel has been anodized, and its inner surface has been covered by lacquer. The aluminum barrel is placed coaxially in a cylindrical steel chamber and supported at four points of its lower rim by means of hard-wood chocks, so as to reduce the energy loss through sound radiation. The measurement error of the system has been estimated by 15% with the calibrated liquids. Then, the coefficient of sound absorption in turbid seawater with the concentration from 40 mg/L to 320 mg/L, in steps of 40 mg/L approximately, has been measured. The results demonstrate that if the concentration of suspended sediment particles is beyond 140 mg/L, the coefficient of sound absorption in turbid seawater is as twice as that in clear seawater. The particle size distribution of suspended sediment particles has been measured by a laser diffraction analyzer. If the coefficient of viscous sound absorption can be calculated by the median diameter of suspended sediment particles, the calculated sound absorption coefficient agrees well with that measured. It is indicated that if the density, concentration, median diameter of suspended sediment particles in turbid seawater are known, the coefficient of sound absorption in turbid seawater can be calculated roughly by the combination of the clear seawater´s sound absorption empirical formula and viscous sound absorption formula.