Sound field temporal fluctuations due to mode coupling in shallow water with internal soliton

Abstract

Sound field temporal fluctuations due to mode coupling, resulting from traveling internal solitons (IS) approximately along the acoustic track is addressed. The spectrum of received signal amplitude fluctuations in the range of very low frequency (∼ F = 0.001–0.01 Hz) is obtained. Maximums in this spectrum, correspond to the times which IS passes the sound field ray cycles. A relationship between F‐spectrum and acoustic frequency is illustrated by showing Frequency‐frequency (F−f) diagram. It is shown that depending on the sound field frequency (f), different mode pairs give the most pronounced contribution to the sound intensity fluctuations. At the same time invariant frequency, F, of maximal fluctuations remains approximately the same for all sound frequency (f) bands. For example in the case of the SWARM95 experiment, F is about 0.0015 Hz for sound frequency, f in the range of 30–150 Hz. Generally, F depends on waveguide parameters and velocity of IS. In this paper, the physical meaning of the invariant frequency and its relationship to the sound frequency is discussed. In addition, the experimental setup for observation of F in future shallow‐water acoustic experiments is proposed. [Work supported by CRDF, PFBR and ONR‐321OA.]