Attenuation of sound by rigid spheres: measurement of the viscous and thermal components of attenuation and comparison with theory

An acoustic cavity technique has been developed to separately measure the viscous and thermal components of the attenuation of sound by rigid spheres in various gases. A rigorous quantitative test of theoretical predictions for the individual components of attenuation has been carried out for spheres of known size in argon, nitrogen, and dichlorodifluoromethane (Freon−12), gases selected to cover a wide range in the relative influence of thermal damping to viscous damping. Theory and experiment are in good agreement in regard to both qualitative and quantitative features. Qualitatively, the most striking feature observed is the sinusoidal variation of attenuation with displacement of the spheres along the cavity axis, as predicted by theory. The absolute values of the viscous and thermal damping constants agree quite well with theory. Even better agreement with theory is obtained for the difference between viscous and thermal attenuation, a measurement that does not require opening the cavity to insert or...