where f is the acoustic resonance frequency for the resonance mode ‘‘m’’, n is the eigenvalue for the resonance mode ‘‘m’’ and a is the inner radius. The magnitude measured is the frequency; the eigenvalues are calculated from Bessel’s spherical equations [7] ; the internal radius of the spherical resonator must be calculated, the resonance frequencies for argon, which equation of state and speed of sound are well known [10] , were measured for such calculations