Abstract
The sizing of mm size bubbles at ranges of 1–2 m using the double‐frequency technique [V. L. Newhouse and P.M. Shankar, J. Acoust. Soc. Am. 74, 1473–1477 (1984)] is described. The experiment was performed in a small swimming pool installed in the laboratory. Bubbles generated, using a number of pipettes connected to a compressed air supply, were insonified continuously by a pump field in the range of resonant frequency of the bubbles and an imaging field at 450 kHz. The imaging field was pulsed with a Gaussian envelope, generated using an IBM PC. The echo from the bubbles at the sum and difference frequency was received by a third transducer forming a crossed‐beam geometry and displayed on an HP 3585 spectrum analyzer. The sizes of the bubbles were obtained from the peaks at the sum and difference frequencies. The passive oscillations of the bubbles were also recorded at short ranges so that the size estimations can be compared. The double‐frequency technique was capable of measuring the sizes of bubbles with diameters 1 mm and above at ranges of up to 2 m. The sum and difference frequency signal that was used for the measurement was also demodulated using a 450‐kHz carrier, recovering the resonant pump signal. This signal recovery should be useful for off‐shore measurements of bubble sizes since it is easy to record and store low‐frequency information on tapes. [Work supported by ONR.]
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