Acoustic emission in bulk normal and superfluid 3He

We present measurements of the damping experienced by custom-made quartz tuning forks submerged in He-3 covering frequencies from 20 to 600 kHz. Measurements were conducted in the bulk of normal liquid He-3 at temperatures from 1.5 K down to 12 mK and in superfluid He-3-B well below the critical temperature. The presented results complement earlier work on tuning fork damping in He-3, removing possible ambiguities associated with acoustic emission within partially enclosed volumes and extend the probed range of frequencies, leading to a clearly established frequency dependence of the acoustic losses. Our results validate existing models of damping and point toward the same mechanism of wave emission of first sound in normal He-3 and liquid He-4 and zero sound in superfluid He-3. We observe a steep frequency dependence of the damping similar to f (5.5), which starts to dominate around 100 kHz and restricts the use of tuning forks as efficient sensors in quantum fluids. The acoustic emission model can predict the limiting frequencies for various devices, including micro-electromechanical and nano-electromechanical structures developed for quantum turbulence and single vortex dynamics research.