Global Model for Sound Absorption in Sea Water

Abstract

The attenuation term in the sonar equation for propagation loss can be taken to include all losses that are proportional to range. Absorption in the medium is usually the dominant mechanism; however, interface scattering, volume scattering and diffraction can also become important components under certain conditions. Sound absorption in sea water is an order of magnitude greater than in fresh water at sonar frequencies. Resonator experiments in the 1950's identified the mechanism as an ionic relaxation of magnesium sulfate in the 100 kHz range. Sea experiments in the 1960's showed another anomaly in the 1 kHz range. T-jump measurements in the 1970's showed that boric acid is involved. Details of the mechanism were investigated using the resonator method in the 1980's. Other relaxations were also discovered but the only one of these that plays a significant role in sea water is the magnesium-carbonate relaxation. A three-relaxation model of sea water absorption was developed based on both laboratory and sea experiments. The main feature of the new model is the pH dependence of two components: boric acid and magnesium carbonate. In the nominal sea-water pH range 7.7-8.3, the low-frequency absorption changes by nearly a factor of 4. Model tests, using available sea data and archival pH values, show good agreement. Error analysis indicates that predictions can be expected to be accurate to within + or - 15%, providing that local pH is known to within + or - 0.05 units. Variability of pH with depth is usually much larger than this.