Ambient noise levels in the Northeast Pacific ocean as measured by aircraft-dropped sonobuoys

Abstract

Ambient noise measurements obtained from sonobuoys air-dropped by U.S.Navy P-3 aircraft during operations over the northeastern Pacific Ocean from January 1978 to December 1980 have been analysed for spatial and temporal trends. Annual, seasonal, and latitudinal means and variability were established as well as event analysis for depth dependence and variability at shorter time scales ranging from weeks to hours. Observations were obtained a t six frequencies (50 to 1700 Hz) and grouped in 5º latitude bands extending from 20ºN to 55ºN. Mean noise levels increased by 1 t o 2 dB from 25ºn to 45ºN, while at mid-latitude a 5 dB seasonal variation (summer louder) was noted. Variability in noise levels of 5 to 6 dB were observed for each season. Fluctuations at 50 Hz were associated with the seasonal migration of the North Pacific shipping routes. The presence of near-by shipping was also noted to severely contaminate (although unknowingly) reported ambient noise levels. The differences in level between deep and shallow buoys were ascribed to upper-ocean related differences in acoustic paths.