An experiment to determine some characteristics of long‐range underwater sound propagation was undertaken by the U.S. Naval Ordnance Laboratory in April 1964. In this experiment, Project Neptune, sound signals were dropped at various ranges from a listening station in Bermuda. To supplement this and other stations, one was established, by the New Zealand Naval Research Laboratory, off southern New Zealand to record the sound signals dropped on the final phase between Cape Town, South Africa, and Perth, Australia. The recorded energy was analyzed in13‐oct bands to determine the transmission‐path characteristics for low frequencies. The signal envelopes were found to differ in shape from the usual solar case, and the attenuations were much larger than previously obtained for either RSR (refracted‐surface reflected) or sofar propagation. These differences may be explained in terms of the different velocity structure of the Southern Ocean from that sound in the Atlantic or Pacific Oceans. In particular, the thermocline is not as pronounced as in temperate or tropical regions and is absent south of the Antarctic convergence. Thus, the energy was transmitted by a mixture of RSR and sofar modes, little sofar energy arriving from those shots whose tracks crossed the Antarctic convergence.