count for the hydrophones appears to exist, as shown in 
figure 12; the difference between the deep and shallow 
water curves is very minor, as would be expected should 
most of the pulses heard at one hydrophone be the same as 
those heard at the other. 
DISCUSSION 
Reference 1 describes and discusses the cyclic Click 
Chorus, the Barking, and the 20 c/s Long Pulses. An 
effort will be made in the present discussion to avoid un- 
necessary repetition of material in that report. 
The listening performed in the study reported here 
has resulted in some conclusions as to the cause of certain 
diurnal variations in the minimum ambient background 
levels. Figures 2, 3, and 4 of reference 1, diurnal curves 
of (minimum) ambient levels, show the following phenomena: 
(1) In the shallow hydrophone output, increases in the 
ambient background were noted between 2000 and 0400 in 
the 125 to 1000 c/s region with sharp peaks at 2000 and 
0400 hours (particularly in the 160 and 500 c/s region). 
(2) In the deep water the nocturnal increase was limited to 
the hours 2200 to 2400 in the frequency range 80 to 250 c/s, 
particularly between 100 and 160 c/s. Figure 13 was pre- 
pared from the 19-21 July data of reference 1 to emphasize 
these effects for shallow and deep hydrophones. 
In figure 13(S), the general background spectrum at 
the shallow hydrophone before 2000 and after 0400 is shown 
as the dashed-line base, and the other curves are the 
spectra of base background level for the times indicated. 
The listening session made it abundantly clear that the 
2000 hours curve is a result of the Click Chorus, which at 
this time of day was strong and continuous without cyclic 
fluctuation with time. By 2100 hours the chorus was re- 
duced in density and intensity and was cyclic again. It 
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