Measurement of the complex impedance of a bubbly fluid in the vicinity of the bubble resonance frequency

Abstract

The acoustic impedance of bubbly fluids has been investigated and results for mixtures above and below resonance are common in the literature [e.g., A. B. Wood, A Textbook of Sound (Macmillan, New York, 1941)]. Because of the high attenuation encountered at frequencies near individual bubble resonance, traditional standing-wave and pulse propagation methods fail and little information has appeared in the literature for this regime. A new technique was developed to overcome this difficulty. Laboratory measurements of the complex plane-wave impedance of mixtures containing monodispersed air bubbles and water have been obtained for frequencies spanning the individual bubble resonance frequency. The experiments were conducted in a thick-walled water-filled tube, piston driven at one end and terminated with a bubbly fluid at the other end. This arrangement allows for operation near the bubble resonance frequency, since high attenuation is not encountered inside the waveguide itself. In addition, the standard transfer function method [J. Y. Chung and D. A. Blaser, J. Acoust. Soc. Am. 68, 907–921 (1980)] can be utilized, which results in broadband complex-valued impedance measurements. Results over a range of frequencies (low kHz) and void fractions with known size distributions will be presented and compared to existing theory. [Work supported by ONR.]