THE SCIENCE AND APPLICATIONS OF ACOUSTICS - H. H. Arnold ...

428 15. Underwater AcousticsIn our mathematical exposition of the sonar principle, we subscribe to the assumptionthat target strength TS is a function of the source and echo levels, respectively,as well as the transmission loss TL that occurs in the echo-ranging process.The assigned function of the sonar may be the detection of an underwater target, orit can be the homing of an acoustic torpedo at the instant when it begins to ascertainits target. Of a total signal energy received by a sensor, a portion may be desired,and is considered to be the signal. The balance of the acoustic energy is undesiredand is termed the background. The background consists of noise, the basicallysteady state portion that is not attributable to the echo-ranging, and reverberationwhich represents the slowly decaying portion of the background caused by thereturn of the original acoustic output from scatterers dispersed in the sea. In thedesign of a sonar system, it is the objective to find ways of increasing the overallresponse of the system to the signal and to decrease the response of the system tothe background, in other words, to increase the signal-to-background ratio.A sonar system serves a practical purpose such as detection, classification (establishingthe character of the target), torpedo hunting, communication, or fishfinding. In each of these tasks there will be a specific signal-to-background ratioand a level of performance in successfully detecting targets with a minimum of“false alarms” that erroneously indicate the presence of a target when no target ispresent. If the signal increases sufficiently to equal the level of the background,the desired purpose will be achieved when the signal level equals the level of thebackground, which just masks it, i.e.Signal level = Background masking level (15.14)Masking does not mean that all of the background interferes with the signal. Onlythe portion that lies in the frequency band of the signal will cause masking, just asin psychoacoustics, where a broadband noise masks out a pure tone or narrow-bandsignal presented to the human ear.The equality of (15.14) constitutes the one instant of the time when a targetapproaches or recedes from a receiver. At short ranges, the signal level from atarget should handily exceed the background masking level, while the reverse willoccur at long ranges. But it is at the instant of (15.14) when the sonar system justbegins to perform its function, which is of greatest interest to the sonar designer.The source level, SL, which is defined in terms of intensity at 1 m (formerly1 yard), was derived from physical concepts in order to express separately theeffects on the signal strength of the echo, namely, (i) the size, shape, and orientationof the target; (ii) the intensity of the source; and (iii) the range of the target. Atlong ranges, only the transmission loss TL depends on the range. At shorter ranges,target strength TS depends on the range as well as the size, shape, and orientationof the target. If the source is quite close to the target, different parts of the targetare struck by sound of different intensities, or if the receiver is so close that thespreading of the sound reflected from the target to it is not the same as the spreadingfrom a point source, the target strength term will depend on the range.