296 



DEEP-WATER REVERBERATION 



RANGE 100 YARDS 



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UNE JOINING QUARTILES 



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SEA STATE 



FiGUBE 22. Dependence of standard reverberation level on sea state. 



the same reverberation runs as the data in Figure 20, 

 with the exception that the levels shown were meas- 

 ured at 1,500 yd rather than at 100 yd. No significant 

 wind-strength dependence is observed. It seems 

 justifiable to conclude from these data that surface 

 reverberation, which is frequently dominant at 

 100 yd, has little effect at 1,500 yd ; in other words, at 

 1,500 yd the observed reverberation usually arises 

 from volume scattering. Figure 22 shows the de- 

 pendence of reverberation on sea state at ranges of 

 100 and 1,500 yd. The 100-yd levels depend on sea 

 state while the 1,500-yd levels apparently do not; 

 thus the quaUtative dependence in Figure 22 is the 

 same as that in Figures 20 and 21. The relation be- 

 tween wind force and sea state is given in the NDRC 

 survey report on ambient noise.* 



Figure 23 shows the reverberation levels as a func- 

 tion of range, for high and low wind speeds. In this 

 illustration are given the median reverberation levels 

 and the upper and lower quartiles at each range for 

 wind speeds less than 8 mph and greater than 20 

 mph. It appears from Figure 23 that the reverbera- 

 tion is entirely independent of wind speed at ranges 

 greater than 1,500 yd. Actually, the quartiles at 

 ranges greater than 1,500 yd are not precisely the 



same for wind speeds less than 8 mph and wind speeds 

 greater than 20 mph (see Figure 5 of reference 4), but 

 these differences are not thought to be significant. 

 Consequently, in Figure 23, data for all wind speeds 

 are used to determine the range dependence of the 

 reverberation at ranges of 1,500 yd or more. 



From Figure 23, for wind speeds greater than 20 

 mph, the median reverberation level drops 46 db be- 

 tween 100 and 1,000 yd. Thus, for high wind speeds, 

 the reverberation intensities usually drop off nearly 

 as the fifth power of the range, rather than as the 

 third power predicted in Chapter 12 on the assump- 

 tion that 10 log m' is independent of range. (At 100 

 yd or more, with the JK at a depth of only 16 ft, the 

 variation of J, {6) with range can be neglected.) Even 

 faster rates of decay than the fifth power are fre- 

 quently observed. For example, Figure 24 shows a 

 plot of the reverberation levels obtained on the Point 

 Conception cruise, on March 2, 1944. The rate of 

 decay in this figure is approximately as the sixth 

 power of the range; that is, R decreases approxi- 

 mately as —60 log r. The wind speed on this run was 

 37 mph. 



Figure 25 shows the reverberation level at 1,500 yd 

 plotted against date and area. The data fall into five 



