Physical factors influencing refraction of the echolocative sound beam in delphinid cetaceans

The delphinid melon has been postulated to function as a sound lens in which lipids refract and collimate the ultrasonic beam used for echolocation. To identify the major physical factors influencing sound-beam refraction, lipid samples from the 4 regions of the Tursiops truncatus melon were examined for variation of sound velocity with temperature. These results were then combined with available sound velocity data for Tursiops sp. melon tissue, Globicephala melaena melon lipids, G. melaena melon tissue, and seawater to model the total melon/seawater system encountered by the projected sound beam in both delphinids. Our analysis suggests that much of the refractive collimation occurs at the tissue/seawater interface rather than in the main body of the delphinid melon. Abrupt changes in the temperature and sound velocity at or near this convex interface (the forehead) cause this refraction.