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High-frequency ambient noise in warm shallow waters

In deep water, the dominant sources of natural ocean noise are often events occurring at the sea/air interface. In shallow water, particularly if it is warm, this is often no longer the case. Where wind-related noise is less pronounced, snapping shrimp alone can easily dominate the soundscape over two decades of bandwidth. Particularly for ambient noise imaging, the high frequency content of these sources (up to at least 250 kHz) provides good angular resolution, at the cost of short range due to absorption. Despite the central role that these animals can play in shallow-water ambient noise, very little is known about their spatial and temporal ensemble acoustic behaviour. Some data collected in Singapore and elsewhere are presented, and the temporal statistics discussed. It is found that some features of ensemble snapping shrimp noise appear to be quite robust, suggesting that knowledge of these properties can be used to improve algorithms for ambient noise oceanography. Two physical models are proposed which are consistent with our acoustic observations.