Sound wave channelling in near-critical sulfur hexafluoride (SF6)

Strong density and speed of sound gradients exist in fluids near their liquid-vapor critical point under gravity. The speed of sound has an increasingly sharp minimum and acoustic waves are channelled within a layer of fluid. Geometrical acoustic calculations are presented for different isothermal fluid columns of sulfur hexafluoride (SF6) under gravity using a semiempirical crossover equation of state. More than 40% of the emitted acoustic energy is channelled within a 20 mm high duct at 1 mK above the critical temperature. It is shown how, by changes in temperature, frequency, and gravitational strength, the governing length scales (wavelength, radius of ray curvature, and correlation length of the critical density fluctuations) can be varied. Near-critical fluids allow table-top sound channel experiments.