Location of the ambient noise study site off southern California, west of San Nicolas Island. Bathymetric contours (2000 and ) delineate the continental shelf. A nearby weather buoy gives detailed information on wind and wave conditions.
Mean pressure spectrum levels ( bins) at the San Nicolas South site for November 2003 to March 2004, compared to January 1964 to June 1966. Band level averages were reported by Wenz (1968a, 1968b) as corrected to units of pressure spectrum level.
Sound pressure spectrum levels ( bins) for the month of February 2004 compared to February 1965 and 1966. During February blue whales are absent from this site and the peak near in 2004 is from fin whale calls.
Pressure spectrum level for November 2003 compared to November 1964–1965 in which blue whale calls are prominent near in 2003 and near in 1964–1965, illustrating a more than 30% the shift in call frequency over four decades.
Monthly averaged pressure spectrum levels at , plotted vs time of day (in GMT). The average band levels from 1964–1965 (solid line) are compared to 2003–2004 (dashed line) with band levels scaled as equivalent pressure spectrum levels. The hours from sunset to sunrise are shaded. Nightly chorusing, presumably from fish, is observed in 1964–1965 and is absent in 2003–2004.
(a) December 2003 cumulative distribution function for pressure spectrum levels at the San Nicolas South site. (b) December 1965 at the San Nicolas South site, after Wenz (1968a). Note the 50th percentile line does not closely track the mean because the mean was computed at as many frequencies as the 50th percentile, each over a wider bandwidth.
Sound pressure level at for the San Nicolas South site vs hourly mean wind speed during November, 2003 at Tanner Bank (NDBC buoy 46047). The influence of wind driven wave noise is apparent above . The Ross (1976) predicted mean ambient spectrum level at for sea surface agitation corresponding to wind speed is shown.
North Pacific ambient noise measurements at low frequency show an overall increase of about per decade, based on repeat measurements at (●) San Nicolas South (this study) and (∎) Point Sur (after Andrew et al., 2002).
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