The Larynx–Melon–Vestibular Lips (LMVL) model of the dolphin sonar. II. The melon beam former

The LMVL model of the dolphin sonar is based on the following system components: (1) the Larynx, controlled by the palatopharyngeal sphincter and operating as a ‘‘sawtooth’’ relaxation oscillator, is the source of the echolocation pulses (the so-called ‘‘clicks’’), and its acoustic output is coupled to the melon through the impedance matching nasal ducts; (2) the Melon, because of its low sound velocity, acts as a ‘‘slow waveguide’’ to form the observed forward-looking beam patterns; and (3) the residual air in the nasopharyngeal cavity acts as an air-pressure reservoir to energize relaxation oscillations of the Vestibular Lips, resulting in the lip-modulated tonal phonations (the so-called ‘‘whistles’’) that radiate through the corresponding vestibular sac. The larynx sawtooth generator has been treated previously [J. W. Fitzgerald, Part I Oceans ’94, Vol. I, pp. 271–276 (Sept. 1994)]. The measured sound velocity in the melon decreases with temperature from v≂1.39×105 cm/s @10 °C to v≂1.28×105 cm/s @40 °C. Melonlike acoustic slow waveguides, with intermediate sound velocities, yielded average pulse beamwidths of θ1≂11.5° (−3 dB) and θ2≂20.0° (−10 dB). This compares to accepted average values from measurements on live dolphins of θ1≂10.0° (−3 dB) and θ2≂21.8° (−10 dB) [W. Au, The Sonar of Dolphins (Springer-Verlag, New York, 1993), p. 105]. These preliminary measurements of melonlike acoustic slow waveguides, and the dissertation of Part I of this series, support the LMVL model of the dolphin sonar.