292 



DEEP-WATER REVERBERATION 



it is clear that in these experiments Js{d) was inde- 

 pendent of range. With neghgible A and constant m' 

 and Js{d), the theoretical equation for surface rever- 

 beration, equation (43), Chapter 12, leads to a 

 straight line with a slope corresponding to inverse 

 third-power decay. This simple reverberation decay 

 is indicated by the solid line in Figure 15. The points 

 in Figure 15 are the averages of 36 pings each 8 msec 

 long and agree fairly well with the theoretical 

 straight line. 



m 



o 



z 

 o 



-70 



-90 



-110 



RANGE IN YARDS 

 40 80 



400 800 



K -130- 



O 

 K 

 bl 



> 

 bJ 

 K 



-150 



0.01 



0.05 0.1 

 TIME IN SECONDS 



0.5 



Figure 15. Surface reverberation levels showing sim- 

 ple inverse cube dependence. Projector and receiver 

 effectively nondirectional in the vertical plane. 



800 yd the observed points in Figures 15 and 16 are 

 in close agreement. This agreement was not to be 

 expected if the scattering coefficient of the surface 

 did not change with time; it is easily verified that 

 the values of Je{d) are quite different for the hori- 

 zontal and vertical orientations of the QCH-3.'' Thus 

 the agreement at ranges greater than 80 yd between 

 the observed points in Figm-es 15 and 16 means that 

 the value of the surface scattering coefficient must 

 have changed during the interval between the two 

 experiments. The value of 10 log m'/2 estimated 

 from Figure 15 is about 5 db greater than the value 

 estimated from Figure 16. 



OS 



a 



RANGE IN YARDS 

 40 80 



400 800 



-100 



-120 



< 



111 



m 

 a. 



Ml 



> 

 bl 



-140 



0.01 



0.05 0.1 



TIME IN SECONDS 



0.5 



Figure 16. Surface reverberation levels showing agree- 

 ment with theoretical curve. Projector and receiver 

 directional in vertical plane. 



On the same day (May 8, 1942) surface-reverbera- 

 tion measiu^ements were also carried out with the 

 QCH-3 transducers oriented differently. In these ex- 

 periments the transducers were placed at a depth of 

 20 ft with the transducer axes parallel to the ocean 

 surface, as before; but the long dimensions of the 

 transducers were vertical instead of horizontal. With 

 this transducer orientation, the correction factor 

 h{e — i,Q)h'{d - ^,0)/cos e cannot be neglected. The 

 values of the correction factor as a function of range 

 were calculated from the known directivity pattern 

 of the QCH-3. A theoretical reverberation curve was 

 then obtained, using equation (43) of Chapter 12, 

 assuming 10 log w'/2 independent of range, and 

 neglecting the term 2A in that equation. This curve 

 is plotted as the solid line in Figure 16. It can be 

 compared with the points which show the actual 

 reverberation levels observed in this experiment; 

 each point represents the average reverberation from 

 30 pings each of length 8 msec. Evidently the agree- 

 ment between theory and experiment is quite good. 

 It may be remarked that at ranges between 80 and 



Evidently if surface reverberation arises from scat- 

 tering in a thin layer near the ocean surface, a drop 

 in reverberation is to be expected when the sound 

 beam is bent away from the surface, provided, of 

 course, that the surface reverberation is not masked 

 by volume reverberation at the range where the beam 

 leaves the surface. Under conditions of sharp down- 

 ward refraction such sudden drops have been ob- 

 served. For example. Figures 17 and 18 show rever- 

 beration levels obtained with the QB transducer on 

 July 24, 1942. On this day ground swells were almost 

 absent, but the ocean surface was scuffed up with a 

 few whitecaps forming. The wind speed was 12 mph. 

 The QB transducer was placed with its axis parallel 

 to the surface at depths of 20 and 60 ft. Twelve con- 

 secutive pings, each 11 msec long, were averaged at 

 each depth to give the points shown in Figures 17 



■> The values of J,{B) for both horizontal and vertical orien- 

 tations of the QCH-3 can be obtained by using the QCH-3 

 directivity patterns given in Section II of reference I, in 

 equation (42) of Chapter 12. 



