Bathymetry of the Southern California continental margin showing the ambient noise study site off the west coast of San Clemente Island, CA.
The locations of the sites occupied in 1958 (circle), 1963 (square), and 2005 (triangle) are shown with bathymetric contours in meters.
The 2005–2006 pressure spectrum level distribution at the San Clemente site including local shipping. A total of 62 504 spectra, each in duration, spanning was used.
The 2005–2006 pressure spectrum levels at the San Clemente site excluding local shipping are compared to the spectrum levels from representative one-third octave averages of November-December 1958 (◻) and 1963 (⦿) at for the same site with local ship noise excluded and the 2005-2006 mean values with ships included. The horizontal bars on the 1958 and 1963 values indicate the frequency ranges of the one-third octave bands used to compute the values.
The four most prominent biological sounds are (a) blue whale song, (b) fin whale countercalling, (c) fin whale song, and (d) a fish chorus referred to as the “motorboat,” each spectrogram computed with a FFT from the 2005–2006 data. Representative spectra are shown for each sound type, computed as a average over a time period when the given biological sound dominates. The diel variation shown here is the spectrum level of the most distinctive spectral peaks (a) 47.3 and (d) for each sound for the 319 spectral averages.
In the absence of local shipping traffic, surface wind/wave noise is seen across the entire study band when winds exceed about . The generalized wind speed/noise curves of Ross (1976) are included for comparison.
The lowest one percentile ambient noise curve is shown with noise sources identified. The dashed line is suggested as the minimum noise level in the absence of biological sources. The wind is presumed to be calm at this site for the lowest 1% of noise levels.
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