Abstract
Previous studies showed that cavitation noise and the half-order subharmonic occurred at much lower thresholds than required to produce macroscopic visible bubbles in the superfluid helium II. It might be supposed that this low “audible” threshold is associated with the presence of subvisible resonant bubbles. Accordingly experiments are in progress at lower frequencies in anticipation that any such bubbles will be larger in size with consequently less distinction between audible and visible thresholds. One investigation employed a sandwich-type transducer coupled to the liquid through a velocity transformer operating at 9.93 kHz. Measurements of particle velocity were made by including the transducer as one arm of a Wheatstone bridge and monitoring the off-balance voltage. Pressure thresholds could be deduced from this information assuming progressive wave radiation. In helium II, at a temperature of about 2.0°K, pressure amplitude thresholds were found to be as follows: for subharmonic generation 0.4 mbar; for white-noise generation, 3.4 mbar; and for visible bubble generation about 30 mbar; i.e., there was still a large difference between audible and visible thresholds. A device designed to operate around 100 Hz is presently under construction.
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