WAVE CHARACTER OF SOUND 



351 



signal length as well as the signal intensity, since the 

 reflected energy will be less for a short signal than a 

 long signal and therefore — as long as the echo length 

 remains constant — the echo intensity will be re- 

 duced. 



For short pulses, then, the echo intensity and 

 therefore the target strength will depend on the pulse 

 length. In practice, however, fluctuations in the 

 course of each echo, and from echo to echo, tend to 

 obscure this relationship for any individual echo. For 

 long pulses, the echo very closely reproduces the 

 signal envelope, which is usually square-topped. For 

 short pulses, however, fluctuations in echo intensity 

 result in a very irregular hashed structure where a 

 sharp peak or group of peaks stands out clearly 

 against a background which is sometimes 10 db lower. 



The peak echo intensity, which is usually used in 

 computing target strengths, is, in general, different 

 from the average echo intensity. Therefore, for short 

 pulses the peak echo intensity maj-^ be considerably 

 different from the average echo intensity and may 

 vary in a different way with signal length. Peak and 

 average echo intensities, and how they vary with 

 signal length, are discussed in Sections 21.6.4 and 

 23.5.1. 



19.4 WAVE CHARACTER OF SOUND 



Ray acoustics has been used exclusivelj'' through- 

 out this chapter in defining target strength, in de- 

 riving the target strength of a sphere, and in dis- 

 cussing target strength as a function of experimental 

 variables, just as ray acoustics was employed in 



Chapter 3 of this volume in treating the transmission 

 of sound through sea water. Experience shows, how- 

 ever, that sound does not always travel in straight 

 lines, and that, for many purposes, ray acoustics is 

 inadequate in explaining and interpreting underwater 

 sound phenomena. An alternative approach in terms 

 of wave acoustics becomes necessary. 



Throughout this chapter it has been tacitly as- 

 sumed that sound is propagated along straight lines 

 as sound rays, and that reflection is wholly specular, 

 in other words, that the angle of reflection always 

 equals the angle of incidence. Many modifications 

 must be introduced if allowance is made for the 

 various wave phenomena affecting echo ranging. 

 Sound is diffracted when it strikes a target or parts 

 of a target whose dimensions approximate its wave 

 length. Thus, the previous discussion applies only to 

 targets considerably larger than the wave length. 

 For the same reason, these results apply only to 

 pulses whose length in the water is at least several 

 wave lengths. In addition, sound reflected from one 

 part of a target may interfere with sound reflected 

 from other parts. Much of the fluctuation commonly 

 encountered in analyzing echoes from underwater 

 targets is attributable to interference. The results in 

 the preceding section on echoes from extended tar- 

 gets are valid only if the interference effects arising 

 from constructive or destructive interference can be 

 eliminated by averaging over several successive 

 echoes. The effect of the wave length of sound on 

 target strength for both specular and nonspecular 

 reflection is discussed in greater detail in Sections 

 20.4 and 20.6. 



