Low-frequency surface interference effects in long-range sound propagation

Abstract

Characteristics of surface decoupling loss are discussed for acoustic frequencies below 250 Hz. It is shown that large losses will occur at frequencies below 100 Hz and for source or receiver depths less than 25 m. A significant dependence of decoupling loss on the average angle of surface-incident energy is indicated. It is this property which gives rise to two major effects in long-range propagation. Firstly, experimental data are presented from a selection of bottom-limited enviroments, which show a range dependence in decoupling loss for 18-m source depths. At 25 Hz these losses correspond to a loss coefficient of 0.013 dB/km and at 50 Hz to 0.008 dB/km. At these frequencies this is greater than the water column attenuation coefficient. The effect is caused by successive bottom interactions reducing the average angle of incidence at the surface consequently increasing surface decoupling loss. A second effect described is also due to bottom interactions. In this case, encounters with an upward-sloping bottom increase the average angle and reduce the decoupling loss. An apparent level enhancement results. Both experimental data and models are used to describe and interpret these effects.