288 



DEEP-WATER REVERBERATION 



f 500 



-170 -150 -130 



REVERBERATION LEVEL R', IN OB 



Figure 8. Observed volume reverberation levels versus scattering depth; GB units; sound beam vertical. 



is a slight increase in 10 log M with frequency. This 

 systematic increase is small compared to the irregular 

 variation from position to position, but according to 

 reference 2, the observed trend is considerably larger 

 than the sampling error of the measurements, and 

 also somewhat larger than the errors which could be 

 introduced by calibration. The straight line shown in 

 Figure 13 was fitted by least squares; its slope indi- 

 cates that Af « wi increases as the 0.9 power of the 

 frequency. It seems safe to say that the results of 

 reference 2 do not exclude the possibility that on the 

 average the scattering coefficient is independent of 

 frequency. They admit the possibility also that m 

 may vary as the second power of the frequency but 

 not that it varies as the fourth power of the frequency. 

 The lines 



m = kp (3) 



are drawn in Figures 12 and 13 for comparison. This 

 fourth-power dependence of the scattering coefficient 



on frequency is known as Rayleigh's scattering law 

 and is true for scattering from particles whose di- 

 mensions are small compared to the wavelength of 

 the scattered soimd.' 



14.2 



TRANSDUCER HORIZONTAL 



That short-range reverberation with horizontal 

 pings is often due primarily to scattering from the 

 surface of the sea has been amply demonstrated by 

 experiment. Reverberation intensity has been meas- 

 ured first with the sound beam directed horizontally, 

 and then with it directed vertically downward. In the 

 first transducer position, surface scatterers are ir- 

 radiated by much of the central portion of the beam; 

 in the second position, they are strongly discrimi- 

 nated against by the directivity of the transducer. 

 When the experiment is performed in a choppy sea 

 with whitecaps, the horizontal reverberation is many 

 decibels higher than the vertical reverberation at 



