LiquidHe4: A Tunable High-Pass Phonon Filter

Various experiments on the propagation of high-frequency phonons in liquid ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ at $T=0.1\mathrm{}$ K lead to the conclusion that there is a critical wave vector $q_c$ such that phonons with wave vector $q<\mathrm{}{q}_c$ have very short mean paths while those with $q>\mathrm{}{q}_c$ have very long ones. As $q_c$ is adjustable over a wide range we recognize the possibility for phonon spectroscopy, using the helium as a tuneable high-pass phonon filter.