Experimental evaluation of a dedicated underwater loudhailer for diver warning

To improve port security capability there is a need for systems for detection, warning and deterrence of unwanted divers in harbour environments. In a complex environment such as a harbour, these topics present challenging problems and as such are subject to research and development. Already a few dedicated systems are commercially available, whether for underwater intruder detection, warning or deterrence. However, due to the complex nature of underwater environments and the difficulties of testing underwater, there is still a gap in knowledge on the performance of each of these systems in the actual operational environments of interest, particularly warning and deterrence systems. A commercially available underwater loudhailer has been tested by order of the NL MoD in the naval harbour of Den Helder, the Netherlands. The main purpose of the test was to assess the underwater speech intelligibility of the loudhailer for various distances in a realistic port environment. Furthermore, apart from range, the main dependence of the intelligibility on the environment has been investigated. There are many types of standard tests available to determine speech intelligibility of loudhailing systems. However, these tests are developed for airborne sound. The specific underwater environment imposes many constraints that make the majority of the standard tests of limited use. Therefore, the tests have been carefully selected and made suitable for underwater utilization. During first experiments with the loudhailer performed in December 2009 a fifty percent sentence intelligibility was obtained up to a distance of 150 meter, which means that on average each message needs to be repeated once. Intelligibility scores were highly variable, also for fixed ranges. Unlike what could be expected based on knowledge from airborne sounds, the correlation between the results of various tests was very poor. All this can probably be attributed to the noisy underwater environment and the temporal variability of the background noise levels.