Site and frequency dependence of ambient noise in the North Eastern Pacific Ocean

Abstract

One-hour averages of omnidirectional ambient noise measurements at 60 Hz and 165 Hz are analyzed for two nearby, deep ocean sites in the Northeastern Pacific during February and March 1981. Site A is a high noise site and is located near major east-west shipping lanes and near major Pacific storm paths. Site B is a lower noise site and is located approximately 450 n.m. from Site A away from major shipping lanes and Pacific storm paths. The site and frequency dependence of ambient noise is found to be highly variable with shippiing noise being totally dominant at 60 Hz at Site A and storm noise being totally dominant at 165 Hz at Site B. Both shipping and storm noise can dominate the 165 Hz Site A or 60 Hz Site B noise levels depending on weather conditions. Storm noise has a possible indirect effect on shipping noise, since very low noise periods occur in between storms, espec ially when the storm passes nearby the site. A limited number of WMO ships sampled indicate that ships slow down or stay in port during storms. Average omnidirectional noise levels at 60 Hz at both sites and at 165 Hz at Site A were 3 to 4 dB lower during a stormy week than they were during a relatively calm week. Array noise gain measurements at Site B indicate that the coherence of noise during stormy periods is much less than it is during calm periods. Generally, this implies that the increase in beam level at 165Hz, caused by storm noise, will be significantly less than corresponding increase in omnidirectional noise levels. At 60 Hz at Site B, this implies that beam levels will decrease, in general, during storms, especially if the storm system is nearby.