Simultaneous Measurements of Surface Generated Noise and Attenuation at the Fixed Acoustic Shallow Water Range “Nordsee”

Abstract

The influence of air bubble induced sound attenuation on surface generated noise has been investigated experimentally by long term statistics and by a wind jump from breeze to storm. The attenuation due to suspended air, increasing by the n’th power of the wind speed (n between 3 1/2 and 4 1/2) and by about the first power of the frequency, rises to higher values than by any other mechanism, eventually screening the high frequency noise beyond about 10 kHz under storm conditions. However, at frequencies up to several kHz, the bubble suspensions remain transparent enough to leave the second power relation between noise intensity and wind speed up to the hurricane regime unimpaired. The strong high frequency loss transient induced by a rain/hail shower may be also due to air bubbles. The measurements which include propagation conditions of lowest loss confirm that the surface generated shallow water noise depends on the local agitation only, unaffected by remote irradiation, at least beyond about 0.5 kHz. Even at an approaching storm front, a preceding noise increase at frequencies of lowest loss could not be observed; instead the noise increase of the lowest octave measured (25–50 Hz) appears after the passage of the front overhead of the receiver as well as the high frequency noise screening.