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James A. Simmons
Postal:
Department of Neuroscience Brown University Providence, Rhode Island 02912 USA
Email:
james_simmons@brown.edu
Theories About Target Ranging in Bat Sonar
The frequency-modulated biosonar sounds transmitted by bats are well- suited to the accurate determination of target range and target.
Introduction
Echolocating animals achieve high levels of performance for guidance, orientation, and target finding in surroundings that go from simple, open spaces to densely cluttered scenes. Developing man-made radar and sonar systems that can perform at comparable levels has been an important motivation for studying echolocation from the time of its discovery (Griffin, 1958) to the present (Au and Simmons, 2007; Simmons et al., 2017). The most obvious comparison with man-made sys- tems is with target ranging, determining the distance to an object from the outward and returning travel time of echoes. Figure 1 illustrates the problem of determining target range from echo delay. The earliest theories of echolocation recognized the distinction between a pulse and a chirp to explain target ranging. Inspired by these theories, behavioral studies established that bats do indeed perceive target range from echo delay (Simmons et al., 1995). Furthermore, they exploit the properties of their frequency-modulated (FM) signals to work back from long chirps to brief pulses (Simmons et al., 1996).
Discovering Echolocation
The first theory, of course, is that animal sonar exists at all. The full story leading to the discovery of echolocation and subsequent research has been described in detail elsewhere (Griffin, 1958; Grinnell et al., 2017). Beginning with Spallanzani and Ju- rine in the late 18th century and extending into the first years of the 20th century, experiments had established that these animals guided flight in the dark using their ears, not vision. Lacking knowledge about ultrasonic sounds, early researchers did not consider the bat as the source of signals. This idea was proposed by Sir Hiram Maxim in 1912 (see Griffin, 1958). He asserted that bats radiated low-frequency sounds and picked up echoes from objects as a way to avoid collisions.
In 1920, Hamilton Hartridge reported a series of observations on bats flying from room to room where the lights could be turned on or off and obstacles placed in their path (see Griffin, 1958; Grinnell et al., 2017). He concluded that bats must be able to perceive objects when they are a considerable distance away, most likely by emitted high-frequency sounds inaudible to humans, and listening for echoes re- turning to their ears. The crucial part of Hartridge’s argument was acoustical, that obstacles typically encountered by bats do not cast “shadows” of audible sounds (i.e., low-frequency sounds heard by humans) nor do they return strong reflections of these sounds because the wavelengths are too long. However, if the sounds are of high frequency so that their wavelengths are short, then an obstacle will intercept an appreciable part of each incident sound, creating a sound shadow behind it and returning the intercepted part of the sound to the front as a reflection.
Armed with a newly developed electronic device for picking up ultrasonic sounds and rendering them into audible signals, Donald R. Griffin and Robert Galambos
©2017 Acoustical Society of America. All rights reserved. volume 13, issue 4 | Winter 2017 | Acoustics Today | 43