The maximum ultrasonic power density Υmax which has been experimentally attained with a piezoelectric quartz crystal up to the present time is considerably less than the value computed from the electrical and mechanical characteristics of quartz. Failure of such systems is attributed to dielectric breakdown in the surrounding insulating fluid, and a simple theory is developed in which the maximum power density is predicted from the value of the dielectric strength. If the breakdown voltage gradient for the medium (V/d)max is independent of thickness, the maximum power density does not change with frequency; but if (V/d)max decreases with thickness as is experimentally observed with many substances, Υmax increases with frequency approaching saturation value as ν approaches infinity. Experimentally, an output of 43 watts/cm2 has been achieved at a frequency of 1000 kc. The determination of the power output by a calorimetric substitution method is described.
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Epstein et al. (1947) studied this question.