Editing Animal echolocation (section)

==== FM signal advantages ====

[[File:Yannick Dauby - Bats echolocation (CC by).ogg|thumb|Echolocation call produced by ''[[Pipistrellus pipistrellus]]'', an FM bat. The ultrasonic call has been "[[heterodyne]]d" – multiplied by a constant frequency to produce frequency subtraction, and thus an audible sound – by a bat detector. A key feature of the recording is the increase in the repetition rate of the call as the bat nears its target – this is called the "terminal buzz".]]

The major advantage conferred by an FM signal is extremely precise range discrimination, or [[sound localization |localization]], of the target. J. A. Simmons demonstrated this effect with a series of experiments that showed how bats using FM signals could distinguish between two separate targets even when the targets were less than half a millimeter apart. This ability is due to the broadband sweep of the signal, which allows for better resolution of the time delay between the call and the returning echo, thereby improving the cross correlation of the two. If harmonic frequencies are added to the FM signal, then this localization becomes even more precise.<ref name="Jones_2006"/><ref name="Grinnell 1995"/><ref name="Simmons_1980"/>

One possible disadvantage of the FM signal is a decreased operational range of the call. Because the energy of the call is spread out among many frequencies, the distance at which the FM-bat can detect targets is limited.<ref name="Fenton_1995"/> This is in part because any echo returning at a particular frequency can only be evaluated for a brief fraction of a millisecond, as the fast downward sweep of the call does not remain at any one frequency for long.<ref name="Grinnell 1995"/>