Marine Mammals and Low-Frequency Sound: Progress Since 1994
Final rept. May 1999- Mar 2000
NATIONAL ACADEMY OF SCIENCES-NATIONAL RESEARCH COUNCIL WASHINGTON DC
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Sound has become a major tool for studying the ocean. Although the ocean is relatively opaque to light, it is relatively transparent to sound. Sound having frequencies below 1,000 Hertz Hz is often defined as low-frequency sound. The speed of sound is proportional to the temperature of the water through which it passes. Therefore, sound speed can be used to infer the average temperature of the water volume through which sound waves have passed. The relationship between water temperature and the speed of sound is the basis for the Acoustic Thermometry of Ocean Climate ATOC experiment. The ATOC experiment is designed to monitor the travel time of sound between sources off the coasts of Hawaii and California and several receivers around the Pacific Ocean in order to detect trends in ocean temperature and for other research and monitoring purposes ATOC Consortium, 1998. The ATOC transmissions are centered at a frequency of 75 Hz, with peak source levels of 180 decibels dB re 1 microPa 1m at this frequency and 195 dB for its broadband source level. Based on well-tested models of signal loss over distance in deep water, the source level should decrease to 155 dB within 100 m from the source and to 135 dB at 1 km from the source.