Echolocation by bats and other animals in air

Abstract

Bats use a biological sonar for remote sensing and spatial perception, emitting frequencies from 5 to over 200 kHz in a variety of frequency-modulated, constant-frequency, and harmonically-structured signal patterns. Using echolocation, bats (Eptesicus fuscus) can detect 5-mm spheres at distances up to 3 m, and 20-mm spheres at up to 5 m. Range (echo-delay) related gain control of echoes regulates echo strength to amplitudes for target-feature processing and helps to reject interference from a variety of causes. The accuracy of bearing (azimuth) determination is ±1.5° and of elevation determination is ±3°. The distance to targets is perceived from echo arrival time with an accuracy of 0.2 mm (1–2μs). Signal processing of echoes yields the capacity to perceive multiple targets, and adaptive changes in signals further control interference and match the bat's acoustic-imaging system to the requirements of different tasks. These capabilities are achieved by Eptesicus with 20–100 kHz, multiple-harmonic FM signals. Many other species of bats use long constant-frequency signals in a Doppler-sensing sonar which detects both overall velocity and target fluttering movements. Frequency accuracy of 0.02% is achieved using high-Q filters. A few species of birds echolocate with clicks in the 1–10 kHz range, and the fruit-bat Rousettus, echolocates with 10–40 kHz clicks. Across all echolocating species, the capacity to detect and avoid small obstacles is primarily determined by emitted wavelengths.