An acoustic technique is described which permits precise measurement of phase velocity in liquids or gases with high attenuation ( α >10 cm-1) utilizing a thin, variable path length acoustic cavity formed by a quartz transducer and a piezoelectric bimorph. The path length can be changed manually or in feedback where the bimorph motion is driven by the output of a cw spectrometer. Measurement of phase velocity is then simply measurement of cavity size, which is done capacitively to high precision. The path length modulation method can allow absolute measurement of attenuation as well as resolution of ∼10-5 in phase velocity with a path length as short as 50μ. It is anticipated that this approach can be used successfully to explore regimes of collective modes and pair breaking in superfluid 3He, as well as textural effects in restricted geometries.
ASJC Scopus subject areas
- Electronic, Optical and Magnetic Materials
- Condensed Matter Physics
- Electrical and Electronic Engineering