Impact and bubble sound from raindrops at normal and oblique incidence

Abstract

The sound generated by rainfall at sea is caused by drops of a wide range of sizes that fall at their terminal velocities and strike the water at various angles of incidence. 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. The measurements have included the total acoustic energy, peak axial pressure, frequency spectrum, polar radiation pattern, and probability of bubble creation. When bubbles are created, they radiate more energy than the impact. However, as the angle of incidence becomes more oblique, the probability of formation of a bubble drops rapidly. For example, a 1-mm drop that always creates a bubble at normal incidence and terminal velocity does so only 10% of the tests at incidence 20° away from normal. These results provide specific reasons for the previously unexplained broadening, frequency increase, and magnitude reduction of the 15-kHz spectral peak of rain noise in the presence of winds at sea.