The effect of gas loading on the RUS spectra of spheres

Resonant Ultrasound Spectroscopy (RUS) of a spherical sample in a pressurizing gas atmosphere was investigated experimentally and theoretically. Measurements were made on a fused silica sphere in He, Ar, and N2 gases up to pressures of 120 bar. The pressure-dependent shift in the resonant frequency, delta f, and the Q-factor were measured for the S00, S11, and T02 modes. A theoretical model based on acoustic radiation impedance was used to calculate delta f and the radiation-resistance component, Qr, of the Q-factor. Agreement between theory and experiment was good for Qr, but there were discrepancies for delta f. It was found that the theoretical delta f due to gas-loading effects associated with acoustic radiation was very small and consistent with the observed dependence on pressure and gas species for the T02 mode but not for the S00 and S11 modes. We conclude that the T02 mode is the most reliable of these modes to use in measuring third-order elastic constants by RUS.