Formation of the broadband biosonar beam of dolphins: Melon-focusing hypothesis not correct

Abstract

In 1974, Norris and Harvey measured the sound velocity profile of the melon of a just deceased bottlenose dolphin and found a low-velocity core in the melon with the velocity increasing towards the surface. This was the genesis of the “melon focusing” hypothesis in the formation of the biosonar beam. Aroyan solved the wave equation for a signal produced at the nasal plug using a finite difference approach and found that the skull was responsible for the biosonar beam. Aroyan’s results were never taken seriously and the notion that the melon was mainly responsible for the biosonar beam continues to this day. We used a finite element approach to solve the wave equation for a broadband signal generated at the phonic lips. CT scan data were used to provide a map of the acoustic impedance of the dolphin’s head and our results compared well with different measurements made around the dolphin’s head and in the far field. Our findings also showed that the skull and air sacs are the most influential structures in the formation of the biosonar beam and the melon was responsible for directing the axis of the beam. Our results should put to rest the “melon-focusing” hypothesis.