The effect of suspended particulate matter on sound attenuation in seawater

Abstract

Many shallow-water acoustic applications (e.g., naval mine-hunting sonars, 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 mechanism is well known, little appears to have been done to quantify its effects over the longer path lengths now of operational interest. Additionally, scattering effects, although dominant at much higher frequencies, contribute to the total acoustic energy loss. Preliminary calculations of the acoustic attenuation due to scattering and viscous absorption by suspended particulates are presented here for particle concentrations typically encountered and at sonar frequencies of interest. This attenuation is compared with calculations of the attenuation due to seawater alone and found to be a significant contribution to the total attenuation, possibly giving an additional attenuation of 3 dB over a total path length of 100 m at 100 kHz for a particle concentration of 0.2 kg m−3. Concentrations of this order or even greater are common in coastal and estuarine waters. The effect of distributions of particle sizes on the acoustic attenuation are also investigated. Plans for future work are briefly described.