The speed of sound in gases
The speed of sound in various gases was measured using two different acoustic resonators. The first, a fixed-pathlength variable-frequency cylindrical resonator, was operated between 50 and 100 kHz, while the second, a spherical resonator of radius 60 mm, was operated between 2 and 15 kHz. The temperatures and pressures of the gases were accurately controlled and measured. Measurements were made on argon, xenon, helium, and 2,2-dimethylpropane at various temperatures between 250 and 340 K, and at pressures below 110 kPa. The results obtained in 2,2-dimethylpropane were used to derive values of the perfect-gas heat capacity and the second acoustic virial coefficient at temperatures between 250 and 340 K. The second acoustic virial coefficients determined using the spherical resonator have a precision of about ±0.1 per cent and have been used to calculate second virial coefficients. Measurements of the acoustic losses in the spherical resonator indicate that the vibrational relaxation time of 2,2- dimethylpropane at 298.15 K and 100 kPa is 4 ns. Detailed measurements of the speed of sound in argon indicate that a precision approaching 1 x10-6 is possible in acoustic thermometry using a spherical acoustic resonator. The second acoustic virial coefficients obtained in argon are in close agreement with values calculated from the interatomic pair-potential-energy function.