A Theory of the Spermaceti Organ in Sperm Whale Sound Production
At a depth of 2500 m from where William Whitney recorded sperm whales sounding normal (unpublished), the density of air is about 1/4 that of water but the velocity of sound is approximately unchanged from the surface. (Air is not quite a perfect gas: at 250 atmospheres volume is 20% too large.) Acoustic impedance (velocity x density) thus is still less than that of water, and an air film remains a good sound reflector. Reflection of a sound pulse from the frontal air sac of a sperm whale showed excellent reflection with a phase change of 180° (inversion as in Fig. 1) suggesting the spermaceti organ might serve as a frequency controlling resonator or delay line analogous to an open ended rather than closed pipe. A reverberation model (Norris and Harvey, 1972) and a bugle model (Mackay, 1972) could be relevant. The energy release mechanism can involve air but frequency is depth independent if controlled by a liquid or solid resonator (or air if geometry could be fixed, viscosity changing little).
KeywordsAcoustic Impedance Sperm Whale Sound Pulse Short Pulse Length Closed Pipe
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