Low-frequency ambient sound in the North Pacific: Long time series observations

Long-term statistics of ambient sound in an ocean basin have been derived from 2 years of data collected on 13 widely distributed receivers in the North Pacific. The data consist of single hydrophone spectra (1-500 Hz in l-Hz bands) averaged over 170 s and recorded at 5-min intervals. Cumulative probability distributions of the ambient sound level show that for the open-ocean arrays at 75 Hz, sound levels are 3 dB higher than the median:level 10% of the time and 6 dB higher 1% of the time. For the coastal arrays, sound levels are 7 dB higher than the median level 10% of the time and 15 dB higher 1% of the time. The dearest feature in many of the spectrograms is a strong annual cycle in the 15-22 Hz band with peak signal levels up to 25 dB above the sound floor; this cycle is attributed to the presence and migration of blue and fin whales. On average, whales are detected 43% of the time. Ships are heard 31%-85% of the time on the coastal receivers and 19%-87% of the time on the open-ocean receivers, depending on the receiver. On average, ships are detected 55% of the time. The correlation coefficient between the sound level in the 200-400 Hz band and wind speed, determined from satellite and global meteorological analysis, is on average 0.56 for the coastal receivers and 0.79 for the open-ocean receivers. For some receivers, the sound level in the 12-15 Hz band is correlated with the sound level in the 200-400 Hz band, with a correlation coefficient of 0.5. (C) 1999 Acoustical Society, of America. [S0001-4966(99)02512-6].