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

15.13 Noise, Echo, and Reverberation Levels 431used—i.e., the source and the receiver are separate, and the two transmissionlosses to and from the target are not generally equal. In some sonars it is virtuallyimpossible to resolve the receiving directivity index DI and detection thresholdDT, so it becomes legitimate to combine these two terms as DI − DT into a singleparameter describing the increase in signal-to-background ratio produced by theentire receiving system which includes the transducer, processing electronics, andthe display.What happens when the background consists of reverberation rather than noise?Instead of DI, which was defined in terms of an isotropic background and is nowinappropriate, the (NL − DI) term in Equation (15.16) is replaced by an equivalentplane-wave reverberation level RL observed at the hydrophone terminals.Equation (15.16) then becomesSL − 2TL + TS = RL + DT (15.17)The parameter DT will possess a value for reverberation that is different from theDT for noise.In the passive or “listening” situation, the target itself produces the signal thatis detected, and one-way transmission rather than two-way transmission is entailed.Target strength TS now becomes irrelevant and the passive sonar equationbecomesSL − TL = NL − DI + DT (15.18)Table 15.3 lists the parameters and their definitions in brief, while Table 15.4provides the terminology of commonly used terms for describing sonar parameters.A number of the parameters in Table 15.3, namely, SL, TL, TS, and the scatteringstrength (which determines RL) use 1 yard as the reference distance. To convertto 1-m reference, these quantities should be reduced by 0.78 = 20 log [1 m ×(39.37 in./m) × (1 yard/36 in.)]. The attenuation coefficient commonly expressedin dB/kiloyard should be multiplied by 1.094 to convert the coefficient to dB/km. Inthe United States, it is generally more convenient to find the range first in kiloyardsand then to divide by the factor 1.094 in order to express the range in kilometers.15.13 Noise, Echo, and Reverberation LevelsThe above sonar equations constitute a statement of equality between the signal,which is the desired portion (the echo or noise from the target) of the acoustic field,and the undesired portion, i.e., the background of noise and reverberation. Thisequality holds true at only one range; at all the other ranges, the equality will nolonger exist. This fact is demonstrated in Figure 15.10 in which the curves of theecho level, noise-masking level, and the reverberation-masking level are displayedas functions of range. The echo and reverberation levels drop off with increasingrange, but the noise remains fairly constant. The echo level curve falls off morerapidly with range than does the reverberation-masking curve, and intersects it atthe reverberation-limited range r r . This curve will also meet the noise-masking