Impact and Bubble Radiation from Obliquely Incident Raindrops

Abstract

The sound generated by rainfall at sea is caused by drops of a wide range of sizes which fall at their terminal velocities and strike the water at various angles of incidence. Complete acoustical measurements have been made of the sound generated by single water drops of diameters 0.8, 0.9 and 1.0 mm striking a laboratory water surface at terminal velocities, at normal and oblique incidence. The measurements have included the total acoustic energy, peak axial pressure, frequency spectrum, polar radiation pattern, and probability of bubble creation. When normal incidence drops create bubbles, their radiated energy is much greater than from the impact. However, as the angle of incidence becomes more oblique, the probability of formation of a bubble decreases greatly. For example, a 1 mm drop, which always creates a bubble at normal incidence and terminal velocity, does so only 10% of the tests at incidence 20° away from normal. Furthermore, at increasingly oblique incidence, the bubble resonance frequency increases while the impact sound peak frequency decreases and its acoustical energy increases. These results provide specific reasons for the previously unexplained broadening, frequency increase, and magnitude decrease of the 15 kHz spectral peak of rain noise in the presence of increasing winds at sea.