A history of the study of low frequency sound absorption in the sea—An active 50 years and it isn't over yet

In 1965, Thorp published a compilation of low frequency data from SOFAR propagation that indicated absorption below 10,000 Hz was greater than predicted. It was suggested this may be an artifact, but the data were shown to fit a 1 kHz relaxation mechanism. NUWC (formerly NUSC) began an extensive at-sea measurement program, ranging from Hudson Bay (jointly with Canadian DREP) to Lake Tanganyika, while several other laboratories obtained data in other oceans. Yeager and Fisher, using the T-jump method at the Case-Western Laboratory, determined there was a 1 kHz relaxation in seawater associated with boron. The data collected at-sea showed, however, a variation between ocean areas, notably values from the North Pacific were 1/2 those in the North Atlantic. This puzzled supporters and encouraged critics. At Scripps, Simmons and Fisher, using a resonating sphere, showed that the boron relaxation did indeed cause the absorption of low frequency sound. Browning and Mellen discovered the key to the at-sea variation was pH, and at NUWC Mellen confirmed this dependence in the laboratory using Simmons' sphere. This direct connection between low frequency absorption and ocean acidification has produced several recent papers on possible implications, and increasing global warming suggests an interesting future.