Abstract
A common technique for measuring the phase velocity of an acoustic wave under free‐field conditions has been a determination of the phase shift between transmitted and received signals. We report here a modification of this scheme involving the addition of a feedback loop which increases the precision of the absolute velocity measurement. In addition, this technique provides automatic tracking of relative velocity variations with high precision and noise immunity. Measurements illustrating this technique for acoustic waves in thin superfluid helium films are presented with precisions of better than 80 ppm obtained.
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