Abstract
Interest in measuring underground water flow has motivated synthesis of encapsulated microbubbles for use as contrast agents. The large acoustic attenuation in earth prohibits use of the high frequencies required to exploit resonant scattering. Instead, contrast enhancement must rely on the reduction of acoustic impedance due to higher compressibility of the microbubbles. Bubble compressibility is measured at the kilohertz frequencies of interest using a resonance tube filled with water and observing the change in tube resonance frequency due to the presence of bubbles for different void fractions [Wilson and Dunton, J. Acoust. Soc. Am. 125, 1951 (2009)]. Buoyancy makes it difficult to maintain a uniform distribution of bubbles throughout the tube in order to relate sound speed to resonance frequency. Therefore, the bubbles were restrained with acoustically transparent barriers to form discrete layers within the water column. A model was developed to investigate the effect on the tube resonance frequency due to different spatial distributions of the bubble layers, and the predictions were compared with measurements. Good agreement with the known compressibility of air was obtained experimentally with only three or four layers. [Work supported by Advanced Energy Consortium.]
Published Version
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