Simulation of bio-acoustic absorption and its effect on volume reverberation

A significant density of pelagic fish and fish schools in underwater environments can lead to higher acoustic reverberation, thus affecting applications, such as environmental remote sensing or fish abundance estimation. Fish aggregations can also increase absorptivity in the water column and increase transmission loss. This phenomenon is described by bio-alpha, i.e., the attenuation coefficient within the scattering layer. Theoretical models and historical databases exist which describe volume scattering strength in underwater environments of interest. Additionally, effects of bio-alpha on transmission loss have been studied [for example, see Diachok, J. Acoust. Soc. Am. 105, 2107–2128 (1999)]. However, less consideration has been given to quantifying the effect of bio-alpha on volume reverberation. Presented here is a simple volume reverberation model which directly accounts for biological absorptivity due to bio-alpha in the scattering layer. Representative cases for different frequencies and environments are presented. Based on the known diurnal behavior of many species of pelagic fish, it is shown in nighttime conditions that reverberation is high but decays quickly whereas during daytime reverberation is relatively lower with a relatively smaller rate of decay. [Work supported by the ARL:UT Internal Research and Development Program.]