34 
THIRD-OCTAVE BAND LEVEL, RELATIVE DB 
2000 
—-— 0400 
eC0an000000 MIDNIGHT 
——-—- <2000, >0400 
RHYTHMIC GRUNT 
50) 
<== <AND> 2300 
20 
Pat WUE Tia loa T 
10 100 1000 10,000 
FREQUENCY, C/S 
Figure 13. Relative ambient noise spectra during 
nighttime hours at the shallow-water hydrophone 
(S) and at the deep-water hydrophone (D). The 
curves are based upon data shown in figures 25 35 
and 4 of reference 1 for the period 1200 on 
19 July to 1200 on 21 Julye The Rhythmic Grunt 
spectrum is based on one sample (from the shallow 
hydrophone, 1900, 22 July) and is the same one 
shown in figure 2 of the present reporte The 
curve labeled 2300 is for the diurnal deep-water 
peak that extends roughly from 2200 to 2400. The 
base ambient noise level curve represents the 
general background level before and after this 
peak. 
