Acoustics of echolocating dolphins and small whales

Abstract

One of the most effective methods for an animal to probe an underwater environment for the purpose of navigation, obstacle and predator avoidance, and prey detection is by the use of underwater sounds or acoustic signals. Dolphins and small whales emit sounds and analyze returning echoes to detect and recognize objects underwater, a process referred to as echolocation. We will first discuss the acoustic reception system of dolphins and consider topics such as auditory sensitivity, spectral analysis capabilities and directional hearing. We will then focus on the acoustic transmission system of dolphins, discussing topics such as properties of echolocation signals and propagation of the echolocation signals from the animals’ head. Dolphins echolocate by emitting high intensity broadband acoustic pulses in a directional beam and listening to echoes reflected from objects in their environment. Echolocation studies on three species, the Atlantic bottlenose dolphin (Tursiops truncatus), white whale (Delphinaterus leucas), and false killer whale (Pseudorca crassidens) have been conducted extensively in Kaneohe Bay, Oahu, Hawaii. Measurements of dolphin echolocation signals in the open waters of Kaneohe Bay indicate that the signals are of short duration (less than 50–70 us), high intensity (up to 230dB re 1 μPa peak‐to‐peak), broadband (30–40 kHz 3‐dB bandwidth) and of high frequency (peak frequencies between 100 and 130 kHz). Evidence indicates that the frequency of the signals may be controlled by intensity, with high intensity signals having high peak frequencies. Echolocation signals are emitted in a beam that is directed forward in the horizontal plane for Tursiops and Delphinaplerus, upwards at an angle of 5 to 10° in the vertical plane. The vertical beam of Pseudorca is directed between 0° and –5° downward. All three species use a pulse mode of transmission in which the repetition rate of the signal is adjusted so that the desired echoes are received before another pulse is transmitted.