Abstract
Measurements of the threshold amplitude of the pump signal for parametric excitation of ultrasonic standing waves in a water-filled container are described in the paper of Teklu et al. [(2006). J. Acoust. Soc. Am. 120, 657-660]. The threshold is reported to increase with increasing pump frequency from 2 to 7 MHz, in apparent disagreement with earlier measurements made in 1970 over the same frequency range in similar apparatus showing a decrease in parametric threshold with increasing frequency. It is shown that the very theory presented in both papers may explain the frequency dependence observed with the new 2006 data rather than the old 1970 data if proper account is taken of the variation in the acoustic absorption coefficient of water with frequency. However, other questions raised by data in both papers indicate the possibility that the phenomenon they describe was not parametric excitation.
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