Acoustic imaging in bat sonar: Echolocation signals and the evolution of echolocation
- Cite this article as:
- Simmons, J.A. & Stein, R.A. J. Comp. Physiol. (1980) 135: 61. doi:10.1007/BF00660182
Echolocating bats behave as though they perceive the crosscorrelation functions between their sonar transmissions and echoes as images of targets, at least with respect to perception of target range, horizontal direction, and shape. These data imply that bats use a multi-dimensional acoustic imaging system for echolocation with broadband, usually frequencymodulated signals. The perceptual structure of the echolocation signals used by different species of bats was investigated using the crosscorrelation functions between emitted signals and returning echoes as indices of perceptual acuity.
Thebandwidth andaverage period of echolocation signals are identified as the principal acoustic features of broadband sonar waveforms that determine the quality of target perceptions. The multiple-harmonic structure of echolocation sounds, which is characteristic of the broadband signals of the majority of species of bats, yields a lower average period (separation of peaks in the crosscorrelation function) than would be expected from the average frequency of the signal as a whole, sharpening target localization.
The frequency-modulation of the harmonics in the sonar sounds of bats reduces the heights of side-peaks in the crosscorrelation functions of the signals, promoting sharp, unambiguous determination of target position, and leads to the well-known coupling of perception of range and velocity for moving targets. The shapes of the frequency sweeps and bandwidths of frequency modulation contribute to reducing this range-velocity coupling. Harmonic organization nearly eliminates range-velocity coupling.
The use of multiple-harmonics and fairly broad frequency modulation in sonar signals yields especially sharp resolution of target position to reject clutter interference. Such signals are commonly used by bats in cluttered environments. Very broad frequency sweeps with fewer harmonics may accomplish the same effect, but the low signal periodicity contributed by harmonic structure is an important factor in “banishing” side-peaks in the crosscorrelation function from perception.
linear frequency sweep
linear period sweep