Michrochiropteran EcholocationMeg Moeller Neil Evans |
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Mechanism
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FM | CF | |
Purpose | Extreme precision of target | Detects motion and wing beats |
Frequencies transmitted | Broadband sweep | Small frequency band |
Improved by | Can be boosted further with harmonic frequencies | The bat will lower the frequency to compensate for Doppler shifts to detect motion |
Advantage | Ignores background noise | Distance motion detector using Doppler shifts |
Quick call allows obstacle avoidance and up to date prey information | ||
Disadvantage | Range of detection (Fenton 1995) | Long range of detection |
Reason? | Echo is in the right frequency range only for a moment (Grinnell 1995) | Only uses a narrow frequency band so the echo can be interpreted |
Environment used | Dense vegetation | Open enviroments |
Most species emit only one type of pulse. The hunting strategies of the species is directly related to the characteristics of its biosonar.
Differences between the bats’ ears in intensity and arrival time of sound give the azimuth of the target, whereas the interference pattern of sound waves reflected within the structure of the outer ear gives the elevation. (Suga, 1990)
Bats transmit two types of acoustic signals: either constant frequency (CF) or frequency modulated (FM). Some transmit only CF, some only FM, some both. For ‘both’ bats: There are three distinct phases to the echolocation attack sequence of a bat approaching airborne prey: detection, approach, and terminal. During the detection phase, the bat transmits long CF pulses, at a low pulse repetition frequency. The approach pulses are shorter CF and are transmitted at higher pulse repetition frequency. Terminal pulses are descending FM of short duration and high pulse repetition frequency. Acoustic wave forms vary widely between species. Even within a species individual bats very their acoustic transmissions considerably, depending on environment. (Mark Denny, 2004)
During detection phase, bats can discriminate between different target types of the same size, and identify the species.
During approach phase, the target has been detected and classified as prey and is being tracked. This requires knowledge of the targets spatial location and velocity
During the terminal phase, the bat can resolve in range features of the prey that are separated by just a few millimeters. It can follow these features in time, pulse by pulse, to see hw the insect oscillates as it beats its wings. (Mark Denny, 2004)
Echolocation of a FM bat and the waves produced. Image from sholarpedia.