Abstract
Underwater noise produced by rainfall is an important part of underwater ambient noise. The bubbles produced by raindrops are the main noise source of underwater noise. Generally, the sound pressure signal of individual bubbles is easily contaminated by tank reverberation, hydrodynamic flow, and laboratory electrical noise. In order to solve this problem, this study proposes a method for calculating the acoustic energy of the bubble produced by a raindrop when the latter falls onto a plane water surface. For this purpose, a series of experiments was conducted in a 15 m × 9 m × 6 m reverberation tank filled with tap water. The bubble produced by a raindrop behaves as a simple exponentially damped sinusoidal oscillator. Based on the dipole radiation pattern, a formula was derived to predict the sound energy of these bubbles. The damping coefficient of the bubble formed by raindrops is found to differ appreciably from the empirical value of the bubble formed by other mechanisms. The resonance frequency of the bubbles is found to decrease with time. It is due to the rapid increase in the distance between the bubble and the interface. Then, the formula is optimized by using these two improved variables. The experimental results agree well with the theoretical derivation.
Highlights
Wind-generated and rain-generated ambient sound from the ocean surface represents the background baseline of ocean noise
E bubbles produced by raindrops are the main noise source of underwater noise due to rain. e dynamics of underwater bubbles have long been of considerable interest because of their importance in various physical and engineering problems, including cavitation on ship propellers [11,12,13], underwater explosions [14,15,16], and ultrasonic cleaning [17,18,19]
10 20 30 40 50 60 Number of bubbles produced by each size of drop in the present experiments is fitted using equation (3). e actual damping coefficient and empirical damping coefficient are shown in Figure 6. e actual damping coefficients are different from the empirical values. e reason is that the bubbles produced by rainfall are different in mechanism from those produced by bubble generators
Summary
Wind-generated and rain-generated ambient sound from the ocean surface represents the background baseline of ocean noise. In frame 10, a bubble is trapped underwater as the downward-moving jet penetrates the crater bottom Bubbles formed by this mechanism are defined as being of type II [31,32,33]. In order to eliminate the contamination, this study proposes a method for predicting the acoustic energy of the bubble produced by a raindrop Using this method, the sound energy can be accurately calculated by measuring the direct sound pressure amplitude and initial resonance frequency of the bubble
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