240 Lecture 13 
hard-bottomed tropical waters, whereas croakers occur predominantly in Chesa- 
peake Bay and other East Coast bays of the United States. Figure 13.2 shows 
typical ambient noise levels produced by croakers and snapping shrimp. It can 
be seen that biologically produced noise does extend into the frequency range 
of interest to sonar. 
Man-made noise may be predominant in busy harbors, shipping lanes, and 
many coastal locations, particularly at lower frequencies. During the war, many 
measurements were made of ambient levels in bays and harbors in the United 
States and Great Britain and near some Pacific Islands. The results of these 
measurements are summarized in NDRC Division 6 Report No. 3 of the Survey 
of Underwater Sound. The outstanding characteristic of this coastal ambient 
noise is its great variability from place to place in the same harbor and from 
time to time at the same place. Figure 13.3 shows typical ambient noise spectra 
for both a noisy and an average location in comparison with the deep-water am- 
bient noise level for a No. 2 sea state. 
13.1.4. Rain Noise 
Falling rain, hail, and snow may be expected to increase ambient noise levels. 
Teer [7] observed an increase of from -62 to -50 db relative to 1 d/cm? in a 
1-cps band in a No, 2 sea state at 19.5 kc due to steady rain. 
Heindsmann, Smith, and Arneson [8] observed the noise changes caused by 
the passage of two heavy rainstorms over a hydrophone system located 4 ft above 
the bottom in 120 ft of water near the eastern end of Long Island. During these 
observations, a recording was made and analyzed for the fall of 0.52 in. of rain 
in 90 min; the results are shown in Fig. 13.4. 
AMBIENT NOISE DURING RAIN 
AMBIENT NOISE BEFORE AND AFTER RAIN 
PRESSURE LEVEL IN I-CPS BAND (db vs | dyne/cm*) 
-70F 
Fale 
it =] JL LILI it 1 AR ILILILIVILt 1 | es ee | 
Ol | 10 100 
FREQUENCY (kc) 
Fig. 13.4. Ambient noise levels produced by a rainstorm that precipitated 0,52 in. in 90 min. 
