A brief history of the discovery of the low frequency sound absorption mechanism in seawater and its pH dependence.

In the 1960s tests with a new surface ship sonar indicated that low‐frequency (less than 10 000 Hz) propagation loss was greater than predicted by the existing MgSO₄ absorption model. Propagation measurements in the SOFAR channel showed that the anomalous absorption was up to ten times greater than predicted and could be fitted by a 1‐kHz relaxation. This was not without controversy at the time, but interest in this fundamental parameter spurred a major measurement effort by the U.S. Navy Underwater Sound Laboratory and other laboratories in the US, Canada, New Zealand, Europe, and Australia, resulting in at‐sea measurements from Hudson Bay to the Tasman Sea. Yeager and Fisher conducted laboratory t‐jump measurements that identified a boron based low‐frequency relaxation mechanism in seawater. Simmons and Fisher followed with resonant sphere measurements to quantify the resulting acoustic absorption. A compilation of the at‐sea results showed, somewhat surprisingly, a correlation with pH: the lower the pH, the lower the absorption. Mellen quantified the pH dependence by extensive laboratory resonator measurements. These results were confirmed by measurements conducted in China. The possible application of acoustically monitoring ocean pH was first suggested by Browning and Mellen in 1990.