Complex sentence: A bat detector is a device used to detect the presence of bats by converting their echolocation ultrasound signals, as they are emitted by the bats, to audible frequencies, usually about 300 Hz to 5 kHz.
Split into simpler sentences: A bat detector is a device used to detect the presence of bats by converting their echolocation ultrasound signals to audible frequencies as they are emitted by the bats. Bat detectors work by converting the bat call sound to a lower, audible frequency, usually about 300 Hz to 5 kHz.