You know what I'm talking about; that accelerating high-pitched beep bleeding into a squeal as the deadly projectile locks-on and closes on your position. It's the "Oh crap I'm dead" sound for fighter pilots and whatever "Oh crap I'm dead" translates into for bugs as well, apparently.
This squeal is known as a "terminal buzz" and, as you can see, is preceded by short chirps at an ever-increasing pace, as fast as 160-190 chirps per second as the bat homes in on its prey.
To determine the upper limits of this terminal buzz, Coen Elemans and John Ratcliffe at the University of Southern Denmark first recorded Daubenton's bats as they hunted for meal worms suspended throughout a chamber wired to 12 microphones. They soon discovered that the bats vocal prowess was beyond the limit of normal muscles.
The next step was to connect a bat's vocal cords to a motor and monitor its maximum frequency -- which turned out to be as high as 200Hz. Until 2008, when songbirds were discovered to have that ability, only rattle snakes were capable of creating those sorts of sounds. For reference, these oscillations are 20 times faster than the quickest human muscle twitch.
Eventually, the team traced the source of this high-speed chirping to a previously-unnoticed, highly-specialised muscle group that powers the rapid call rates. This discovery also proved that it was a mechanical limitation (in terms of the larynx's ability to vibrate), not a shortcoming in the bat's echolocation system (overlap between call production and reception) as some scientists had previously believed, to be the real reason bats chirp at the frequency they do.