Periodic Variations of the Underwater Ambient Noise Level of Biological Origin off Southern California.
NAVY ELECTRONICS LAB SAN DIEGO CALIF
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Two distinct periodic variations of the underwater noise level have been observed in the coastal waters off San Diego. The first, called the sunset chorus, appeared diurnally before sunset, and faded away within 3 hours. This variation was observed in the 100- to 1000-cycles per second cps band and raised the sound-pressure level 5 to 20 decibels dB above the normal background. During the winter months, the variation had a shorter period and was less intense than in summer. The effect was sometimes observed at dawn, although with reduced duration and amplitude. Two members of the croaker family, spotfin, Roncador stearnsi, and yellow-fin, Umbrina roncador, are believed to be responsible for the sunset chorus, as Johnson concluded. The second periodic variation is called the cycling sound. It is observed only at night in the late spring and summer months. Its amplitude is commonly from 3 to 6 dB, but maximums of 16 dB have been noted. It is observed above the background in the 300- to 800-cps band with peak amplitude occurring at about 450 cps. Author