Low-frequency ambient ocean noise and sound transmission over a thinly sedimented rock bottom

Abstract

Measurements of ambient sea noise and sound transmission loss were made at a coastal site with a hard rock bottom, in 20 fathoms of water. A bottomed hydrophone was used; data were taken in the range from 50 to 800 Hz. A series of intermittent measurements over a period of 18 months showed the noise levels to be unusually low; spectrum levels of 40 to 78 dB re 1 μPa at 100 Hz were observed. Close correlation of noise level with wind velocity was found over the entire frequency band. CW transmission measurements at 97, 360, and 580 Hz disclosed exceptionally high loss in the area; between 1 yd and 2 mi the loss was about 100 dB. A combination of sparsity of shipping and poor transmission is believed responsible for the low noise levels. An exploratory test with a vibration sensor on the bottom indicated results similar to those of the water-borne sound experiments.