ACOUSTICS OF ECHOLOCATING DOLPHINS AND SMALL WHALES

One of the most effective methods for an animal to probe an underwater environment for the purpose of navigation, obstacle and predator avoi dance, and prey detection is by the use of underwater sounds or acoust ic signals. Dolphins and small whales emit sounds and analyze returnin g echoes to detect and recognize objects underwater, a process referre d to as echolocation. We will first discuss the acoustic reception sys tem of dolphins and consider topics such as auditory sensitivity, spec tral analysis capabilities and directional hearing. We will then focus on the acoustic transmission system of dolphins, discussing topics su ch as properties of echolocation signals and propagation of the echolo cation signals from the animals' head. Dolphins echolocate by emitting high intensity broadband acoustic pulses in a directional beam and li stening to echoes reflected from objects in their environment. Echoloc ation studies on three species, the Atlantic bottlenose dolphin (Tursi ops truncatus), white whale (Delphinaterus leucas), and false killer w hale (Pseudorca crassidens) have been conducted extensively in Kaneohe Bay, Oahu, Hawaii. Measurements of dolphin echolocation signals in th e open waters of Kaneohe Bay indicate that the signals are of short du ration (less than 50-70 mu s), high intensity (up to 230 dB re 1 mu Pa peak-to-peak), broadband (30-40 kHz 3-dB bandwidth) and of high frequ ency (peak frequencies between 100 and 130 kHz). Evidence indicates th at the frequency of the signals may be controlled by intensity, with h igh intensity signals having high peak frequencies. Echolocation signa ls are emitted in a beam that is directed forward in the horizontal pl ane for Tursiops and Delphinapterus, upwards at an angle of 5 to 10 de grees in the vertical plane. The vertical beam of Pseudorca is directe d between 0 degrees and -5 degrees downward. All three species use a p ulse mode of transmission in which the repetition rate of the signal i s adjusted so that the desired echoes are received before another puls e is transmitted.