Echolocating bats emit sounds in patterns—
characteristic of each species—that contain both
frequency-modulated (FM) and constant-frequency (CF)
signals. The broadband FM signals and the narrowband
CF signals travel out to a target, reflect from it, and
return to the hunting bat. In this process of transmission
and reflection, the sounds are changed, and the changes
in the echoes enable the bat to perceive features of the
target.
The FM signals report information about target
characteristics that modify the timing and the fine
frequency structure, or spectrum, of echoes—for
example, the target's size, shape, texture, surface
structure, and direction in space. Because of their narrow
bandwidth, CF signals portray only the target's presence
and, in the case of some bat species, its motion relative to
the bat's. Responding to changes in the CF echo's
frequency, bats of some species correct in flight for the
direction and velocity of their moving prey.