Low‐frequency scattering from submerged bubble clouds

Preliminary results are presented from a recent experiment carried out at the NUWC Seneca Lake test facility and designed to investigate the scattering properties of bubble clouds produced in fresh water in the absence of boundaries and under known propagation conditions. The test range consisted of both conventional and parametric sources, conventional receivers, and a transient bubbler that was submerged to 91 m. The cloud possessed an elongated teardrop shape (length≊1.4 m, diameter≊0.5 m) with a void fraction of 0.25% and a mean bubble radius of 1.5 mm, which corresponds to a single‐bubble resonance frequency of 6.6 kHz at depth. Backscatter target strengths were measured for frequencies ranging from 300 Hz to 14 kHz. These measurements revealed high target strengths (up to +1 dB) and distinctive peaks in the spectrum at 450 Hz, and 1.3, 3.0, 6.5, and 11 kHz. The low‐frequency results are consistent with calculations (based on the theory of collective bubble oscillations) of the resonance frequency and target strength of a comparably sized, spherical bubble cloud. Certain features of the high‐frequency backscatter data are consistent with single‐bubble resonance scattering. These preliminary results are consistent with the notion that, at low frequencies, the scattering characteristics of a bubble cloud are determined, not by the bubble sizes and number density, but rather by the free‐gas volume fraction and the length scales of the cloud proper