MODELING THE ENVIRONMENTAL INFLUENCE ON THE VERTICAL DIRECTIONALITY OF AMBIENT NOISE IN SHALLOW-WATER

The vertical directionality of acoustic ambient noise has been a subje ct of much interest in the past. It is a well-defined physical quantit y that can be measured experimentally with a vertical array. It posses ses certain deterministic features that can be modeled theoretically w ith environmental acoustic and source data. Ambient noise in shallow w aters, including its vertical directionality, is not very well known a nd is also difficult to model/predict. This is because the acoustic en vironment varies with time and is location dependent. Thus arises the question, how does the vertical directionality of the ambient noise de pend on the acoustic environments (found in typical coastal waters)? D ue to the shallow water depth, it is noted that sound (noise) propagat ion can be significantly influenced by the bottom. The degree of botto m-interaction will depend on the sound-speed profile in the water colu mn: whether it is downward refractive or not. Bottom attenuation will in turn determine how far the sound will propagate in the water column . Using a modal representation, a closed-form expression is obtained w hich can be used to interpret and predict the distant noise vertical d irectionality as a function of the environmental acoustic parameters. The nearby (overhead) noise is separately modeled and is found less se nsitive to environmental changes as the propagation distance is short. As the deterministic features of the noise vertical directionality is controlled by the sound propagation in the channel, it could be used as an acoustic indicator of the acoustic environment in the area. Time variations of the noise directionality will also be discussed.