An Ambient-Noise Model That Includes Coherent Hydrophone Summation for Sonar System Performance in Shallow Water

This paper discusses the Research Ambient Noise Directionality II model (RANDI II) developed at SACLANTCEN. RANDI II was used to model ambient noise and array responses in two areas. Its special feature is the capability to carry out coherent hydrophone summations of the complex pressures of individual propagation modes, and to combine these in the processing of an arbitrary array. This is important in shallow-water situations. The modelled results for two areas, one in shallow water, were compared with estimates of ambient noise made from recent measurements with a towed array. Comparisons made on the basis of omnidirectional noise level, horizontal directionality, and the spatial statistics of the noise, show good agreement. The model could be further evaluated by carrying out simultaneous vertical and horizontal array measurements in shallow water of known bottom type in order to check its three- dimensional capability. Another aspect that needs to be investigated separately is the modelling of shallow-water wind noise and its source levels. This would require a set of measurements of wind-only noise in shallow water and various sea-states.