Observations and explanation of low frequency clicks in blue whale calls

A brief study has been conducted on the low frequency clicks and tones observed in calls recorded in recent years from Blue Whale aggregations at locations off the Rottnest trench. As has been suggested previously, it is concluded that the 20 Hz clicks most likely are the self-excitation of the bubble resonance of the gas within the lungs by the whale. This is probable, as the click waveform has the appearance of the decay of a resonance, and as there are no other conceivable mechanisms which might give rise to a resonance within a whale body at 20 Hz. This is explained with recourse to bubble physics, which are extended to include a treatment of both spherical and elongated bubbles, and to considerations of the effects of various depths at which a whale might call. The amplitude of the bubble oscillations required to generate the observed signal levels is shown to be surprisingly large, and appears to be close to the theoretical maximum for a resonant bubble driven by any means.