Some Intense, Low-Frequency, Underwater Sounds of Wide Geographic Distribution, Apparently of Biological Origin

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Abstract
A certain intense, underwater, transient sound has been observed over extended areas of the Atlantic and Pacific Oceans. The power spectrum for a single pulse of this sound is strongly peaked, energy being confined to a narrow band centered in the vicinity of 20 cps; the sounds, accordingly, have been named twenty-cycle pulses. A single pulse typically lasts about one second; the pulses normally occur in trains, or groups, characterized by remarkably regular repetition rates. A common rate is one pulse approximately every 10 sec, although others are observed. A typical pulse train is of several minutes' duration; separated by silent periods two to three minutes in length, pulse trains often repeat for many hours. On the continental shelf south of New England, the positions of scores of sources of this sound have been determined by remote acoustic ranging; the sources are found to move. Speeds are typically a few knots. Radiated sound levels are so high that the sources are detectable tens of miles away. Source densities of only one in several hundred square miles are most common. Movements appear random, suggesting that the source is biological. The low areal density, the high source level, and speeds of a few knots that are sustained for hours suggest a large animal. Characteristics possibly correlated with the sounding cycles of whales are apparent. The hypothesis that the sounds originate from the heartbeats of whales appears credible on several counts.

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