THE EFFECT OF SUSPENDED PARTICULATE MATTER ON SOUND-ATTENUATION IN SEAWATER

Many shallow-water acoustic applications (e.g., naval mine-hunting son ars, acoustic Doppler current profilers) involve devices operating in the frequency range 50-300 kHz over path lengths up to several hundred meters. At these ranges and frequencies the thermoviscous absorption of sound energy in the boundary layers of suspended particles leads to a significant acoustic attenuation. Although the viscous absorption m echanism is well known, little appears to have been done to quantify i ts effects over the longer path lengths now of operational interest. A dditionally, scattering effects, although dominant at much higher freq uencies, contribute to the total acoustic energy loss. Preliminary cal culations of the acoustic attenuation due to scattering and viscous ab sorption by suspended particulates are presented here for particle con centrations typically encountered and at sonar frequencies of interest . This attenuation is compared with calculations of the attenuation du e to seawater alone and found to be a significant contribution to the total attenuation, possibly giving an additional attenuation of 3 dB o ver a total path length of 100 m at 100 kHz for a particle concentrati on of 0.2 kg m(-3). Concentrations of this order or even greater are c ommon in coastal and estuarine waters. The effect of distributions of particle sizes on the acoustic attenuation are also investigated. Plan s for future work are briefly described. (C) 1996 Acoustical Society o f America.