Low frequency sound sources: statement of problem and some possible solutions

The main stumbling block in a project to measure the backscattering cross-section (target strength) of commercial fish at frequencies in the band 300 Hz to 3000 Hz has been that of providing a suitable sound source. The spectrum level requirement of this source is + 70 dB relative to 1 ¿bar (0.1 N/m2) at 1 m per Hz across the band, or about 100 W peak power during a 10 ms pulse at any frequency. The general problem of radiating this power level into water at these frequencies is considered. Several sources employing various principles of transduction including sparkers, boomers, explosives, hydroacoustic, electrodynamic, piezoelectric and magnetostrictive sources, are considered in terms of a common parameter¿the maximum output spectrum level¿as well as efficiency.