Underwater Acoustic Systems

The science of acoustics involves the study and practical application of sound transmission in solid and fluid media. Although the subject is one of considerable scope, as figure 1.1 illustrates, our interest will lie particularly in applications involving sound transmission in the sea and in its underlying sediment layers and rock strata. Sound transmission is the single most effective means of directing energy transfer over long distances in seawater. Neither radio-wave nor optical propagation is effective for this purpose, since the former, at all but the lowest usable frequencies, attenuates rapidly in the conducting salt water and the latter is subject to scattering by suspended material in the sea. Underwater acoustics is thus a topic of extreme importance in military and commercial applications.

Underwater acoustic systems inevitably involve the detection of signals. The fundamental criterion which determines the effectiveness of all detection processes has to do with determining the extent to which the received signal exceeds, or is swamped by, such corrupting influences as may exist. In electromagnetic detection equipments — radio and radar systems — antenna- borne signals are of such small size that the significant corrupting influence may well be the similarly small, random, gaussian noise waveforms deriving from charge transport processes or molecular agitation in the antenna and front-end receiving circuits.

Sonar emissions and the noises which corrupt them are pressure waves travelling in a four-dimensional space-time continuum. In the past, it has often been adequate to restrict consideration of such processes only to the time-domain signals emerging from the outputs of each of the hydrophones used for signal detection. Increasing computing power, together with a rapid evolution in appreciation of the mathematics of signal analysis and its application to particular physical problems must now lead the underwater acoustician towards a more all-embracing comprehension of the spatio- temporal nature of the processes he is called upon to handle.

Sonar modelling has to do with predicting sound intensity at some point in the sea remote from a source. It provides a more detailed way of predicting performance than do the sonar equations, which would usually be used as a “first cut” and preferably “worst-case” approach to system design. Sonar modelling is of great importance in deducing the path traversed by sound as, for example, in seismics — where it is required to determine the thickness and acoustic characteristics of sea-bed sediment layers — or in military applications where range and bearing to an underwater sound source, such as an enemy submarine, must be found. A comprehensive review of modelling software currently in use is to be found in reference [4.1].

As we saw in the previous chapter, sound propagation in shallow water leads us, via the ray tracing approach, to a model of propagation wherein there will exist a multiplicity of reflected image sources. Given iso-speed conditions, the ray paths from these sources will be straight lines. In water whose depth is moderately shallow with respect to range, there may be sufficiently few bounces for the problem of estimating the summed sound intensity developed by each ray to be computationally viable. For channels which are extremely long by comparison with water depth, the problem rapidly becomes intractable, although with the increasing power of modern scientific workstations, this difficulty is less significant than it once was.

The study of acoustic noise and the reverberation of acoustic signals is of importance because one or other of these phenomena will set the limit to sonar system performance. Whilst it is true that both sources of corruption can co-exist, it is most common to find that one of the two will predominate. Notice that in a noise-limited sonar, increasing the signal power will improve the signal-to-noise ratio and hence the system performance. The same need not be true for a reverberation limited sonar, since the reverberation is, itself, directly a function of the output signal level.

Underwater sound transducers convert electrical energy into or from mechanical energy, the latter quantity being perceived as longitudinal pressure waves in water. We thus impress a voltage waveform v(t) on the terminals of a transmit transducer, or projector, and generate a sympathetic pressure fluctuation p(t) in the water. The reverse occurs with a receive transducer, or hydrophone.

In the previous chapter we examined the process of acoustic transduction and the way in which transducers might be assembled to meet particular design objectives. The common requirements in specification are:

In this chapter we examine the way in which electronics and acoustics interact to provide practical solutions to a wide range of sonar engineering problems. It is a matter of some regret that the history attaching to the development of the subject of underwater acoustics is relatively poorly documented. As history, the documentary material is of relatively recent origin and is frequently difficult to gain access to because of the explicitly military nature of much of the research which has been conducted during the past several decades. An excellent general account, of a largely nontechnical nature, has been published by Haines [9.1]. This text views the development of underwater acoustics from a British standpoint but with copious reference to contributions made elsewhere. Urick’s introduction [9.2] provides a brief historical perspective, which is nicely complemented by the first background chapter in a recent text by Burdic [9.3]. A text edited by Albers [9.4] provides a collection of benchmark papers of particular interest to the underwater acoustician.

The literature surrounding the subject of underwater acoustic communications is, in some respects, surprisingly scant. This is particularly the case if one concentrates only upon that material directly concerned with actual underwater communication systems as opposed to more general aspects of propagation and channel modelling. The major difficulties with which the communication engineer is concerned, when attempting to design underwater acoustic communication systems, revolve around the problems of reverberation and multipath transmission and high attenuation at high acoustic frequencies. An extensive bibliography on the subject has been published by the author [10.1]. A subset of that bibliography, dealing with selected specific underwater communication systems is presented here [10.2–10.13]. However, the reader is referred to the original source for more information on specific communication systems, and papers on general aspects of channel modelling and more detailed mathematical treatments than can be handled in a text of this nature.