Acoustic-Wave-Theory Interpretation of Surface and Bottom Reverberation

Abstract

The scattering of sound by the ocean surface or bottom is generally assumed to be proportional to sin φ, where φ, is the grazing angle at incidence and n may have a value from zero to two with the latter corresponding to Lambert's law. Wave theory indicates that the scattering form is not as simple and requires a space-directivity factor to account for the scattering from an insonified strip on a surface. Plane-wave theory is satisfactory for short pulses and also for long pulses at small grazing angles. At intermediate angles, the curvature of the wavefront for long pulses and the scattering due to random specular reflections may influence the form of the directivity space factors. However, from the limited number of generally available measurements, distribution of random specular reflectors near normal incidence with an equivalent angular aperture of about 27 degrees seen as to be the principal source of reverberation. Thus, a scattering coefficient near unity rather than the 10−3 value from empirical formulation is necessary to satisfy the observed reverberation as well as the acoustic-scattering loss that is observed on reflection at the ocean surface. Moreover, a distinction ought to be made between reverberation decay, scattering decay, and scattering strength.