Underwater sound radiated by impacts and bubbles created by raindrops
Nystuen, Jeffrey A.
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The sound generated by rainfall at sea is caused by raindrops of a wide range of sizes and angles of incidence, which fall at their terminal velocities. The purpose of this laboratory research has been to make complete acoustical measurements of the sound generated by single water drops striking the water surface at their terminal velocities for normal and oblique incidence. These measurements have included the total acoustic energy, peak axial pressure, frequency spectrum and polar radiation pattern. Depending on the drop size and the angle of incidence, many drops falling at their terminal velocities create bubbles. At all angles of incidence studied here the sound radiated by an individual bubble contains more energy than the sound from an individual impact. These results, using terminal velocities and oblique trajectories, are very different from the published normal incidence, non-terminal velocity characterizations. For example bubble frequencies other than the well known 14 kHz peak are found. Also the energy of the impact sound increases significantly for larger drop sizes and for larger deviations from normal trajectories. Furthermore, drops of diameter 0.8 mm to I mm, which always produce bubbles at normal incidence, create bubbles only about 10% of the impacts at oblique incidence. These observations provide specific reasons for the previously unexplained broadening, shifting and reduction in magnitude of the14 kHz spectral peak of the rain noise in the presence of winds at sea.
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