Measurement of the ratio of the speed of sound to the speed of light
James B. Mehl, Michael R. Moldover
Measurements of the resonance frequencies of the acoustic modes and of the microwave modes of a single cavity can determine u/c, the ratio of the speed of sound of a gas to the speed of light. Such measurements with a monatomic gas would determine the thermodynamic temperature T with unprecedented accuracy. By judicious choices of cavity geometry and resonance modes, u /c can be measured to part-per-million accuracy using cavities whose geometry is known only to parts per thousand. These techniques can also be applied to measurements of the universal gas constant R. A measurement of 8 would also require an accurate determination of the average atomic mass of the monatomic gas.
and Moldover, M.
Measurement of the ratio of the speed of sound to the speed of light, Physical Review A, [online], https://doi.org/10.1103/PhysRevA.34.3341, https://tsapps.nist.gov/publication/get_pdf.cfm?pub_id=925220
(Accessed September 27, 2023)