Interaction of Ultrasonic Third Sound with Substrate Surface Defects

A new attenuation mechanism has been identified for third sound propagating in superfluid ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ films. Attenuated transmissions and phase-inverted reflections have been observed to occur for both macroscopic and microscopic surface defects in the substrate used to support superfluid ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ films. The data were obtained by using sound frequencies in the 20 to 200 kHz range.