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. There are other types of detector which record bat calls so that they can be analysed afterwards, but these are more commonly referred to by their particular function.
Bats emit calls from about 12 kHz to 160 kHz, but the upper frequencies in this range are rapidly absorbed in air. Many bat detectors are limited to around 15 kHz to 125 kHz at best. Bat detectors are available commercially and also can be self-built.
Read more about Bat Detector: Using Bat Detectors, Sampling Frequency For Digital/TE Detectors, Non-acoustic Detection