Measuring the Acoustic Parameters of Fish Schools

date April 25, 2014
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Friday, April 25, 2014 4:00 p.m. ETC 4.150

Craig Dolder
Department of Mechanical Engineering
Applied Research Laboratories
The University of Texas at Austin

While the literature contains extensive in situ measurements of scattering by fish schools, significant uncertainties exist with respect to characterizing the size, quantity, and distribution of fish within the schools that confound accurate measurement-model comparison. Measurements of the sound speed through collections of live fish (Danio rerio) were conducted in a laboratory setting. The sound speed was investigated using a resonator technique which yielded inferences of the phase speed within the fish school though measurements of the resonances of a one-dimensional waveguide. Fish densities were investigated ranging from 8.6 to 1.7 fish lengths per mean free path. Measurements agree well with a predictive model that is based on shell-free spherical bubbles, which indicates that the phase speed is not significantly affected by the fish flesh or swimbladder morphology for the species studied. The variation in phase speed due to individual fish motion within the model school was measured to be up to ±5.6%. This indicates that precise knowledge of the fish position is required to achieve greater model accuracy. To complement the phase speed measurements, the attenuation through a cloud of encapsulated bubbles was evaluated through insertion loss measurements. Multiple arrangements of balloons of radius 4.68 cm were used to surround a projector. The insertion loss measurements indicated an amplification of around 10 dB at frequencies below the individual balloon resonance frequency and an insertion loss of around 40 dB above the individual balloon resonance frequency. Analytical modeling of the bubble collection predicted both the amplification and loss effect, but failed to accurately predict the level of amplification and insertion loss. Finally, effective medium models and full scattering models (requiring knowledge of bubble size and position) were evaluated for a model fish school. The two models agree for forward scattering for all frequencies except those immediately around the individual bubble resonance frequency. Back scattered results agree at low frequencies, however as soon as the wavelength becomes smaller than four mean free paths between fish the models diverge. Ramifications of these findings and potential future research directions are discussed.