Efficiency of parametric ultrasound generation in relaxing media for very shallow-water echo sounders

Abstract

Parametric acoustic generation provides the possibility to obtain narrow and nearly side-lobe free acoustic beams for low frequencies. In this way, the nonlinear interaction of two intense beams with close frequencies generates combinations of the spectral components: high frequency harmonics, difference-frequency harmonics and sum harmonics. The most used in underwater acoustics to obtain the advantages above mentioned is the difference-frequency harmonic for its lower attenuation. Moreover, sea water ultrasound absorption presents not only thermo viscous, but also ionic relaxation losses, both at the primary beam (due to magnesium sulfate presence), and at lower frequencies (boric acid ions). In this work, a computational finite differences method for nonlinear acoustical propagation with relaxation losses is employed to study the efficiency of the parametric sound generation in sea water. Thereby, we present the design and development of parametric echo sounders for very shallow water (<20m), with typical wider beam apertures than commercially available scientific or parametrical echo sounders. This kind of devices, with low frequency narrow beams, provides the possibility to go through a fish school without screening effects due to the proximity between the targets. Beam spatial properties has been studied both numerically and experimentally.