Underwater acoustic power measurements in reverberant fields

Abstract

Acoustic noise sources are usually characterised by source levels measured in specific directions. However for some applications the average over all directions is useful, and this can also be specified as the total acoustic output power output. Power measurements are closely related to the measurement of source levels. Whilst the source level concept is based on free field spherical spreading, some benefits of the power output rating are best seen by considering the extreme reverberant environment of a test tank. Test tanks have been shown by a variety of authors to be useful in this respect over the past 40 years. One example of the benefits is the possibly of providing a good estimate of a reverberant noise field by the summation of the noise powers of different source contributions. This technique is used in airborne acoustic problems such as the design of factory environments where it is desirable to keep the reverberant field levels below the safety limit such as 85 dBA required by statute to minimise the risks of damage to the workers hearing. There is a need to deliberately avoid the detailed directivity information in order to predict the long term overall spatial average, and the consideration of noise power and reverberation makes this simple. Also, where the average over all directions of source outputs or hydrophone sensitivities is required, reverberant test tanks can provide such data directly, in a way analogous to the use of reverberant air chambers. This technique will be discussed in detail using results taken from work over the past decade. Whereas the theory due to Sabine characterises the reverberant volume by a reverberation time the analysis described here uses an alternative technique better able to explore the limits of performance. This analysis can be applied to the prediction of the total noise power of complex machinery including ROVs and even ships. Whilst not appropriate for highly detailed work, such an approach can provide a very economic way to assist in the estimation of the effects of noise on the environment and also on the performance of some sonar systems. As has been discussed elsewhere the prediction of possible environmental impact by machinery required for a specific task (perhaps a pipe-laying operation) cannot use the detailed directional data on the possible systems to be employed. However, knowledge of the output acoustic power does allow the nature of the various sources to be compared easily, and can also be used to reassure those determining the risk, especially if it becomes clear that the risks are low.