Written by Whitlow Au
Bisonar research at the Marine Mammal Research Program has been the main focus of the program until 2017. Below, I provide a brief description of the four distinctive areas of echolocation research the lab has worked on over the years.
I. Determining the parameters for the noise limited version of the sonar equation and the reverberation limited version of the sonar equation. There are 8 – 11 parameters and the values of these parameters need to be measured in controlled experiments with dolphins. Most of the experiments have been conducted with bottlenose dolphins, although research with a beluga whale and a false killer whale as well as a short study with a Risso’s dolphin were also performed. The figure below is a schematic of a target detection experiment in the presence of artificially induced Gaussian noise.
We also performed experiments to determine how sounds enter into the auditory system of dolphins. The figure below shows a dolphin beaching on a mat so that a sound source could be placed at different positions on its head and the brain wave caused by the reception of the sound could be detected. The sound source was moved to various positions of the dolphin’s head and we found that the whole lower jaw acted as a receiver with some areas more sensitive than others.
II. Measuring the echolocation signals of dolphins in the wild using a “inverted star” array as shown in the picture below. Field trips were taken to Iceland to measure the white-beaked dolphin, in the Pacific northwest to measure killer whales, to the Bahama sand bank to measure the Atlantic spotted dolphin, in Kikura Bay, New Zealand to measure dusky dolphins and Waimea Bay to measure spinner and spotted dolphins. The figure below shows the inverted star array with a video camera attached to it, a killer whale approaching the measurement platform and an underwater image to the killer whale swimming past the array.
III. The foraging behavior of spinner dolphins at night close to shore in the Hawaiian islands and deep-diving odontocetes in various locations in the world. Deep moored ecological acoustic recorders (EARs) have obtained foraging signals of pilot, sperm and beaked whales as well as Risso’s dolphins in several locations around the world. EARs have been deployed off Kauai, Niihau and the Kona coast of the Big Island, Guam and Saipan, Tinian, Okinawa, the Josephine sea mount off Portugal and in the Mediterranean Sea.
IV. Investigating how sound propagates within the head of bottlenose dolphins to determine how the biosonar beam is formed. The left side of the figure below shows specially-built suction cup hydrophones that were used to measure the sound field at the surface of a dolphin’s head. The right side of the figure shows how the acoustic impedance of a dolphin’s head was mathematically divided into many parts so that the finite element analysis technique could determine how the biosonar signals propagate in the head to form the biosonar beam external to the head.
