Effect of longitude on vertical distribution of ambient noise due to wind and coastal shipping

Using multielement vertical arrays deployed from FLIP, measurements of the vertical distribution of ambient noise have been made at a latitude of 32N at three stations, 124W, 136W, and 150W, the latter station being about 1700 miles from the coast. The arrays used were deployed at the sound channel axis and were uniformly spaced at half‐wavelength either at 200 or 300 Hz. From 75 to 300 Hz, the change in the vertical distribution of noise shifts, especially at higher frequencies, with increasing range from coastal shipping was in a manner consistent with the effect of chemical absorption on low‐angle noise due to coastal shipping. Whereas a shipping noise pedestal at low angles is observed at all frequencies at short range, at long range the absorption effect makes the vertical distribution of ambient noise more isotropic at higher frequencies than that at low frequencies. Effects of wind speed on the vertical distribution of noise were also measured. As wind speed increases, ambient noise at angles greater than 15° from the horizontal (that is, noise of local origin) increases and approaches the levels of the pedestal at 124° W and presumably exceeds them at high wind speeds. Our results are orthogonal to those obtained by Kibblewhite et al., ARL/UT, with their deployment at 150 W of three Vedabs bouys on a north‐south line to measure low‐frequency absorption using northern shipping as a sound source. This conflict suggests a simultaneous north‐south and east‐west experiment as well as the utility of using a multielement vertical DIFAR array. [Work supported by ONR.]