Echolocation is the primary tool dolphins depend on for their survival underwater. Without this ability, the localization of food and navigation would be much harder for the animals. This active sonar is characterised by narrow transmission and reception directivity patterns, over a variety of ranges. In Cetaceans, only toothed whales and dolphins (odontocetes) are able to echolocate. The main concept for echolocation is to emit sound and listen to the returning echoes. Where sound enters the head first and how the signals are transmitted to the mandibular fats and inner ears is not fully understood. There are several theories on how sound reaches the fatty tissues in the lower jaw: through a thin region in the lower jaw called the pan bone, through the throat region and an opening in the rear mandibles and through the teeth, acting as an end-fire array and enhancing the directional hearing.The hypothesis on sound reception via the teeth was investigated further in this thesis. The sound pressure was modelled at the teeth of a bottlenose dolphin (Tursiops truncatus) using acoustic characteristics of different components of the entire jaw. The angle of the jaw was changed from 0° to ±10° in 1 degree steps and from ±10° to ±90° in 5 degree steps. The pressure was measured at 3 front and back teeth and the difference between pressure values compared. The pressure was also measured at each of the 22 teeth on both sides of the jaw for two different angles, 0° and +5°. Multiple scattering and the influence from the relative positions and sizes of teeth as well as the effect of missing teeth was investigated. Results show that the propagation of sound varies with size and position of the teeth. The simulations display the relation between neighbouring teeth and the attenuation or amplification of the signals at other teeth. Teeth cannot be considered as individual point-like receivers. A pressure tendency can be seen at the back teeth for the first ±15°, which indicates that directional hearing occurs for the first degrees the dolphin moves its head. It also shows that hearing takes place primarily at the back of the jaw.
|Date of Award
|16 Nov 2015
|Sally Clift (Supervisor), Philippe Blondel (Supervisor) & William Megill (Supervisor)