Underwater 20-Hz Signals Recorded near Midway Island

From time to time, strong underwater acoustic signals with frequencies near 20 Hz have been recorded from hydrophones in nearly all oceans. Further details, such as source level, waveform, and movement, are not so well known. These signals may be divided into two main classes: short (∼1 sec) and long (up to 35 sec) emissions of sound at or near 20 Hz. We describe some low-frequency long signals that were recorded on hydrophones near Midway Island, thus providing a further example of these interesting phenomena. The slow (∼1 knot) meandering of the source, its relatively high signal level (∼70 dB re 1 dyn/cm2 at 1 yd) and low frequency (23–25 Hz) suggest a large biological source.