Measurements of ambient noise spectra in the south Norwegian Sea

Measurements have been made of ambient sea noise north of the Shetland Islands in the southern part of the Norwegian Sea. Shore-terminated, omnidirectional hydrophones, bottomed in 240 and 350 fathoms, were used. Twenty minute samples of single hydrophone outputs were recorded during daylight hours about every 3 days over a 4-year period from the summer of 1957 to 1961. Amplitude levels were measured at selected frequencies in the band 30 cps to 1 kc. Matching observations of wind and weather were coordinated with the noise data. The noise spectrum was found to be relatively flat compared with the spectrum measured in deep ocean areas of the western North Atlantic. Also, the pressure levels at frequencies above 100 cps are considerably higher. Seasonal changes were observed in the spectrum below 400 cps. The annual excursion in level from winter high to midsummer low was about 5 db at 50 cps and 3.5 db at 100 cps. Although no seasonal changes occurred in the upper spectrum, day-to-day variations in level at the higher frequencies generally followed patterns of change in local weather. There was good correlation between wind velocity and ambient noise at 800 and at 1000 cps. The character of sea-noise in the area seems to be shaped to two factors. First, a combination of thermal structure and a very gradual bottom slope result in high propagation losses. This in turn restricts the acoustic access at a bottomed hydrophone to a localized field. Second, except for a few summer calms, weather in the islands is severe, and strong winds and high sea states are sustained throughout the year.