Another variant of gas thermometry, acoustic thermometry, is based on the temperature dependence of the speed of sound ca. In an ideal gas, the speed of sound is given by
ca2 = (cp/cV)ĚkT/m
m being the mass of a gas particle and cp/cV the ratio of the specific heat capacities at constant pressure or volume, respectively. For single-atom gases, in particular the rare gases, cp/cV = 5/3.
Two methods have been used to measure the speed of sound. In older works, a fixed-frequency, variable-path, cylindrical acoustic interferometer was used. Nowadays, variable-frequency, fixed-path spherical resonators are preferred. Their figure of merit is about an order of magnitude higher than that of cylindrical resonators. Furthermore, boundary layer effects and the problems due to excitation of different modes are essentially smaller.
The present value of the Boltzmann constant is essentially based on acoustic thermometry.