NUMERICAL-CALCULATION OF THE UNDERWATER NOISE OF RAIN

When raindrops with a diameter of the order of 1 mm hit a plane water surface they entrain air bubbles that radiate noise in the course of volume oscillations. The paper presents a model of the underwater noise of rain produced by this process. The depth of submergence, radius, and initial energy of the entrained bubbles are obtained numerically for a number of drop sizes. The bubbles are assumed to radiate as dipoles, and the total underwater noise is calculated by integrating over the size of the entraining rain drops. The results are compared both with laboratory experiments of single-drop impacts and field data of rain noise. It is found that the model gives somewhat larger bubbles than are observed experimentally. As a consequence, the characteristic spectral peak of rain is predicted to occur at a somewhat lower frequency than found in experiment. However the level of the peak is in reasonable agreement with data. The amount of noise due to the process of drop impact itself is also estimated and found to be several orders of magnitude lower than the data. Therefore, in spite of some deficiencies of the model and of the computational results, the proposed mechanism for the underwater noise of rain is strongly supported by this study.