Ambient noise environments in shallow tropical seas and the implications for acoustic sensing John R. Potter , Lim Tze Wei & Mandar Chitre Acoustic Research Laboratory, c/o Dept. Physics, NUS, 10 Kent Ridge Crescent, Singapore 119260. Abstract Oceanographic in-situ measurements are becoming increasingly expensive as remote sensing tools concurrently enjoy a continuing expansion of capability. While in-situ observations will never be replaced completely, the current trend is clearly to make as much use of inferential and remotely-garnered observations as possible. In the ocean, acoustics is one of the foremost tools to use for remote interrogation of the environment. It represents the only mechanism for quickly propagating energy (and hence also information) over large distances. As a direct result, a large array of acoustic tools have been developed for communication, navigation, sonar, current profiling and so forth. All of these tools, passive more than active, are impacted by the natural background noise of the region in which they are deployed. In warm, shallow waters, the ambient noise environment is very different from colder and/or deeper waters, being dominated for some 80% of the usable bandwidth by snapping shrimp. The noise levels in the range 2-250 kHz can easily be 20 dB above that expected in similar conditions in deeper or cooler water. This is predominantly the case for SE Asian coastal regions, and even for considerable distance offshore where shallow water persists. In order to optimise the design and operable performance of acoustic systems, we need to understand the spatial and temporal statistics of this noise. Ambient noise results are presented from several tropical sites and some robust statistical features are identified. Physical models for these attributes are developed, and some strategies suggested for improving acoustic system performance in the face of this cacophony. Introduction The ambient noise environment of Singapore is substantially different from many sites for which data are available, being tropical, shallow over large areas and dominated by an unusually large collection of merchant vessels. These qualities are matched by many SE Asian coastal regions, so that results obtained here are likely to be typical. Little is known about the natural soundscape of these waters, so the ARL began a project to acquire a suitable acoustic data collection capability and to begin making measurements. A data acquisition system has been built which is compact, flexible and suitable for operation from small vessels of opportunity. We are currently collecting data from a number of representative sites around Singapore and elsewhere. The soundscape at these sites has been examined, and some preliminary results are now available which provide the first elements of an acoustic description of shallow waters in the region. It has been confirmed that snapping shrimp (Alpheus & Synalpheus) dominate most of the usable passive acoustic bandwidth for all geographical locations investigated so far, as anticipated for warm and shallow water environments. Although these animals have long been recognised as major contributors to ambient noise (Johnson et. al., 1947) the statistical behaviour of shrimp colonies has received scant attention to date. Ahn et. al (1993a and b) have investigated the individual noises made by these animals, as has Au more recently in Kaneohe Bay, Hawaii (1996). It is known that they produce substantial energy up to frequencies well above 200 kHz (Cato and Bell, 1991). Yet their collective and spatial behaviour is still largely unknown.