It’s an Acoustic Recording Unit, a tool that records the high-frequency echolocation calls bats use while flying and hunting. These calls typically range from 14,000 to over 100,000 Hz—well beyond the range of human hearing (20,000 Hz). These detectors are specifically designed to detect only those ultrasonic sounds, enabling researchers to study bat activity and behavior.

This unit was deployed by the Biodiversity Research Institute, a non-profit organization based in Portland, Maine. We work globally on wildlife research and conservation.

Bat detectors are tuned to hear only ultrasonic bat calls. They filter out human voices and most other everyday sounds to conserve battery and storage, which are the two biggest limiting factors. These units typically run from one hour before sunset to one hour after sunrise—when bats are most active.

High-frequency sounds (like bat calls) don’t travel far. When your neighbors are having a noisy party playing the Bee Gees' greatest hits from the other end of the street, it is the bass (low-frequency) sound that keeps you up, not the higher-pitched singing. A large bat with a strong echolocation call might be picked up from as far as 30 meters away, but most bat calls are reliably detected within about 20 meters.

Each species has a distinct echolocation signature—differences in frequency, duration, and call pattern—that suits its environment and foraging style. Scientists use software like Kaleidoscope Pro and SonoBat to compare recorded calls with known reference calls. Some species have nearly identical calls (like the Indiana bat and little brown bat), so experts manually review recordings when needed. If a call could belong to either, it’s labeled “LUSO”—a shorthand indicating it could be one or the other. If only one of the two species is known to occur in the area, researchers can still draw useful conclusions from those calls.

These ARU detectors do not count individual bats. If one species is recorded 100 times in a night, it could be 100 different bats—or just one bat flying loops and calling a lot. However, we can determine if a species is present and if an area is essential to it based on the number of calls we record. The timing of the calls can also indicate the distance to the roost. Early evening calls suggest that the bats are roosting nearby; if they are only detected several hours after dusk, they are likely commuting to the release site. All this information is valuable for conservation and can guide the timing of construction or other activities to minimize impacts on bats.