A Free-Flooding Acoustical Resonator for Measurement of Bubble Size Distributions

Abstract

An instrument for the measurement of bubble size distributions is described. The sensing element exploits the free-flooding resonator design of Medwin with modifications to overcome the limitations in the original implementation, especially those due to a sensitivity to ambient pressure fluctuations in the surrounding medium. A mathematical model of the resonator provides insight into the factors affecting its performance and motivates application of appropriate signal processing algorithms. Comparison of different bubble size calculation methods shows the direct approach of Commander and MacDonald to be most successful. The stability of this new implementation of the resonator facilitates accurate measurement of the complex dispersion relation. Comparison of the real and imaginary components then leads to the definition of a measurement quality factor that may be calculated for each sample. Practical considerations are discussed for implementation of autonomous battery-powered resonator arrays for ocean deployments with real-time data processing. Examples are presented of measured bubble size distributions acquired from an autonomous array of five instruments moored in the Gulf of Mexico.