FOR extended solids, the ultrasonic frequencies to be used for any reasonable propagation of the waves through them must be low, say, 500 kilocycles and less, for the vibrations of higher frequencies will be excessively damped. For determination of velocities at higher frequencies, a method based on the Debye–Sears ultrasonic diffraction of light has already been indicated1, and in the following we will describe the method found for the lower frequencies.
Parshad, R., Curr. Sci., 13, 14 (1944).
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Reports on Progress in Physics (1947)