Acoustic Impedance of LiquidHe3

We present a derivation of the acoustic impedance of liquid ${\mathrm{He}}^3$ at very low temperatures, where the behavior of the liquid is of the collisionless type. Assuming the collective mode is dominant, we show that the energy transfer at the boundary can be calculated directly by equating the work done by the vibrating wall against the pressure of the liquid to the energy flowing from the wall in the collective mode. The comparison of our result with the exact result obtained by Bekarevich and Khalatnikov demonstrates the accuracy of our method. The application to experiment is discussed.