Variability of low-frequency sound absorption in the ocean: pH dependence

Abstract

Experimental measurements of the excess sound absorption in sea water below 1 kHz show a marked regional dependence. For example, the excess in the North Pacific Ocean is found to be only half that in the North Atlantic Ocean. Recent laboratory measurements have demonstrated that the absorption mechanism is a boric acid–borate relaxation. Since this reaction is also a buffer in sea water, the equilibrium and, consequently, the absorption coefficient should depend strongly on pH. A pH correction factor for Thorp’s formula is proposed. Over the range of pH values encountered in the oceans (7.5–8.2), the predicted absorption is, approximately, an exponentially increasing function of pH with a range of values of about 5 to 1. Comparison of experimental absorption coefficients with predictions based on reported local pH values shows reasonably good agreement.