Effect of pressure on sound absorption in synthetic seawater and in aqueous solutions of MgSO₄

Measurements have been made, at 25 °C, of the effect of pressure on sound absorption in Lyman and Fleming (LF) synthetic seawater from 30–300 kHz up to 307 atm. Similar measurements were made in 0.02M MgSO₄ solution in a 0.02M MgSO₄–0.6M NaCl solution. The decrease with pressure of absorption per wavelength (αλ) in LF seawater differs substantially from those reported by other investigators. The relaxation frequency, within experimental error, was found to be independent of pressure. For LF seawater, increasing pressure to 307 atm reduced absorption by 26.3%, very nearly the same as was observed previously (28.2%) for 0.5M MgSO₄ solutions. The addition of 0.6M NaCl to the 0.02M MgSO₄ solution decreased the absorption by a factor of 4 and the effect of pressure was found to be independent of the NaCl concentration. The Eigen and Tamm multistate dissociation parameters which best describe the pressure dependence of sound absorption and electrical conductance in a 0.02M MgSO₄ solution predict a 21.4% decrease in absorption at 307 atm compared with the observed 19.6% decrease. However, these parameters yield an equilibrium constant which differs from the conductance derived value by a factor of 3.