Experimental Study on the Underwater Rain-Drop Noise

• Main Authors: Wu Han-Chieh, 吳漢傑
• Other Authors: Huang Ching-Jer
• Format: Others
• Language: zh-TW
• Published: 1999
• Online Access: http://ndltd.ncl.edu.tw/handle/43696474984500475983

碩士 === 國立成功大學 === 水利及海洋工程學系 === 87 === ABSTRACT The underwater sound and the bubble formation caused by rain drops were studied experimentally in the present paper. A CCD camera was set up to photograph the drop impact and the process of bubble formation. Underwater noise was received with a hydrophone connected to a charge amplifier and a FFT analyzer for capturing the main characteristics. The experimental results showed that two types of bubbles are generated as a single rain drop strikes the free surface. The first one is the formation of a single bubble, radiating sound of frequencies ranging from 3 to 5 kHz. This type of bubble has been accepted by many researchers as the main source of rain drop noise. The second one is that as the rain drop impacts the free surface, a downward jet is formed which develops into a vortex ring. Several bubbles are generated along this ring. The latter bubbles are smaller than the former one, hence radiating sounds of higher frequencies, ranging from 8 to 20 kHz, with a peak sound pressure level at about 14 kHz. The first mode of bubble generation happens to rain drops of lower impact velocities. However, as the impact velocities increase, both modes of bubble generation are possible. These two different types of bubbles and the radiated sounds can be used to explain the double peak pressure of the real rain drop noise, one locates at 3-5kHz, the other one at about 14 kHz. Finally, the sound spectra of different rain rates were also studied in this paper.