Abstract
One component of the success of the Westervelt analysis of the parametric array was that sound propagation in most gases and liquids exhibit very little dispersion. Therefore, the pump waves and their nonlinearly generated product propagate at very nearly the same speed. When an acoustic medium can support more than one sound mode with different propagation speeds, then a parametric array can be created by the nonlinear interaction of the two slower sound waves to produce the faster wave, if the slow wave propagation directions are not collinear. Bulk superfluid helium, at temperatures below 2.1 K, will support a slower (thermal) sound mode and a faster (mechanical) sound mode. An experiment performed in 1977 will be described that used a waveguide to create thermal (temperature‐entropy) sound waves while controlling their angle of intersection. A mechanical (pressure‐density) wave was produced and its amplitude determined the thermodynamic coupling constant between density changes and the Galilean invariant square difference in the normal and superfluid particle velocities that characterize the thermal sound wave amplitude. An earlier experiment will also be described that demonstrated the nonlinear conversion of two shear waves to a longitudinal wave in aluminum. [Work supported by the Office of Naval Research.]
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