SOUND SPEEDS IN SUSPENSIONS IN THERMODYNAMIC-EQUILIBRIUM

This work considers sound propagation in suspensions of particles of constant mass in fluids, in both relaxed and frozen thermodynamic equilibrium. Treating suspensions as relaxing media, thermodynamic arguments are used to obtain their sound speeds in equilibrium conditions. The results for relaxed equilibrium, which is applicable in the limit of low frequencies, agree with existing theories for aerosols, but disagree with Wood's equation. It is shown that the latter is thermodynamically correct only in the exceptional case when the specific heat ratios of the fluid and of the particles are equal to unity. In all other cases discrepancies occur. These may be significant when one of the two phases in the suspension is a gas, as is the case in aerosols and in bubbly liquids. The paper also includes a brief discussion of the sound speed in frozen equilibrium.