The measurements of the effect of storm-produced bubbles on high-frequency sound propagation

A high-frequency acoustics experiment to measure the spatial and temporal coherence of direct path signals was performed in shallow water, off the coast of Panama City, Florida. Transmission was along acoustic paths between fixed towers at middepth in 10 m of water. During the experiment there were two stormy days with strong winds, breaking waves and, nearby, high surf. It was observed that: (a) average time of arrivals increased significantly over those measured on calm days; (b) pulse to pulse variations in arrival times (over seconds) increased, showing an increase of variance of about a factor of 6, and (c) dispersion was observed. The range between the source and receivers was approximately 60 m. The only known cause for these variations is the presence of bubbles. Although no direct bubble density or distribution was measured, it is possible to construct a plausible bubble model to estimate the total average void fraction along the acoustic path and the variation necessary to cause the increase in variance. Void fractions on the order of 2.6×10−6 and fluctuations of 0.9×10−6 about this value can explain the observed data. [This work was supported by the NRL.]