Medium-induced low-frequency fluctuations in acoustic transmission loss: examples from measurements in selected geographical areas

Abstract

The ocean is a complex, highly variable acoustic nedium. A propagating acoustic signal is affected by a host of phenomena, including the sea-surf ace and bottom, volume inhomogenei ties, internal waves and tides, and non-stationary water masses. These effects cause fluctuations in the ampli tude and phase of an acoustic signal and an accompanying loss in its coherence properties. The responsible nechanisms, and hence the acoustic effects, cover a wide range of temporal and spatial scales and, in general, can be understood only in terms of deterministic and random forces acting in concert. Although the listing of these nechanisms is generally easier than their isolation in realistic situations, it has been possible to correlate fluctuations in acoustic transmission loss with envi ronmental variabi 11 ty, especially for very low acoustic frequencies. In particular, neasurements conducted by SACLANTCEN in diverse geographical areas have identified semi-diurnal effects (tidal as well as heating), internal waves, inertial oscillations, and moving water masses as significant contributors to low-frequency acoustic fluctuations.