Abstract:
The characteristics of the sound interaction with a bubble cloud produced by a series of hypodermic needles are studied both experimentally and theoretically. It has been observed that the sound-pressure level is magnified when the incoming waves are at resonance with the first collective mode (breathing mode) of the cloud. The magnification, as high as 20 dB, depends on the void fraction. A simple theory based upon a forced mechanical oscillator is developed. This theory suggests that when the incident wave frequency is close to the fundamental frequency of the cloud, the system is at resonance. As a result, a significant sound magnification occurs. The theory and the experimental observations are in a good agreement. In this paper, the resonance magnification is discussed in great detail and some practical applications of this phenomenon are offered. [Work supported by the Office of Naval Research.]