Abstract
Noise recorded with 16-element vertical line arrays moored at two sites near 80-m water depth has been analyzed. One site is south of New England (Shelfbreak PRIMER, June 1996), the other south of China in the South China Sea (ASIAEX, May 2001). Both sites exhibited downward refracting conditions, so that a noise notch is realizable, and both exhibit internal gravity waves. The waves were more plentiful and of larger amplitude at the South China Sea site. Sound in a 40-Hz band centered at 135 Hz was extracted and separated into vertical beams using delay and sum beamforming. At each location, noise lower than the median level exhibited a peak near zero degrees for each data set (near-horizontal arrival). Noise higher than the median level in the New England data set exhibited a 5-dB noise notch, with peak levels at about ±15 deg. Noise higher than the median level in the South China Sea data set did not show the notch and was essentially flat between 10 and −10 deg. The results are consistent with observed fluctuating propagation between fixed sources and re- ceivers at the sites, and with differences in coupled-mode propagation through the two internal-wave fields.
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