Sound Absorption in Gases in the Critical Region

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Abstract
A theory is based on a dynamic heat capacity associated with long-wavelength density fluctuations in the critical region. Evaluation of the dynamic heat capacity requires the time-dependent response of the long-range radial distribution function to a uniform harmonic variation of temperature with time. This response is calculated on the assumption that the motion of matter in the neighborhood of a fixed molecule is determined by macroscopic laws of momentum and energy transport, except for the addition of a familiar critical contribution to the pressure. Relaxation of the important Fourier space components of the radial distribution function is not hindered appreciably by fluid inertia or viscous resistance. Rather the relaxation follows a generalized heat diffusion law. The calculations are applied to data on Xe and HCl, and satisfactory agreement between theory and experiment is found.

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