Echolocating bats cannot simply call faster all the time. Each call must return before the next goes out, or the echoes become ambiguous, so call rate is tied to how far away the target is. Search-phase calls are long, widely spaced and relatively low in bandwidth, typically around five to twenty per second.
Once a target is detected, the bat enters an approach phase, shortening call duration and raising the rate. In the terminal phase, the buzz, big brown bats and many other aerial hawkers reach call rates near 190 to 200 per second, with call durations under a millisecond. The buzz is audible on a bat detector as a distinctive raspberry sound and is the standard field marker that a bat has attempted a capture.
The capture itself is usually not made with the mouth. The bat scoops the insect with a wingtip or with the tail membrane and transfers it to the mouth in flight, which is why hunting bats often appear to somersault. Whether every buzz represents a successful capture is not knowable from acoustics alone.