Acoustic counter-UAS sensors are passive and use high-sensitivity microphone arrays coupled with audio analysis applications to detect, track and identify sounds produced by UAV motors and propellers. The spinning of different types of UAV propellers produce unique acoustic patterns, which makes it possible to create a library of these acoustic signatures to identify different types of UAV and determine the general direction of the sound source.

The approximate location of a UAV can be determined using triangulation methods with multiple spatially separated microphone stations. By analyzing the Doppler-induced frequency shift of the source signal over a period of time, some C-UAS may be able to report the approximate speed and direction of the sound source. High levels of sound pollution, like those in urban environments, can degrade the performance of C-UAS that rely on acoustic sensors. Acoustic sensors are ideally employed to “quiet” remote locations such as rural prisons or for border protection applications.

Reference- DHS Counter-UAS Technology Guide