Application of UAVs for low-frequency outdoor sound measurements

A comprehensive multi-point sensor network is required to measure low frequency noise in atmosphere with respect to emissions from renewable energy sources and seismic activity monitoring. Uncrewed Aerial Vehicles (UAVs) offer a solution as sensor carrier for measuring outdoor noise. The feasibility of employing UAVs in low frequency acoustic measurement is examined here. Noise signature of different drones is measured in an anechoic chamber and compared with the atmospheric background noise. Frequencies below the first rotor tone had minimal disturbance from the inherent noise of UAVs. Directivity study with an automized sensor array revealed that sensors mounted in the rotor plane are least affected by the rotor noise. An infrasound reference source working on the principle of a quarter wave resonator is then used for in situ calibration of a microphone mounted on a UAV. Results of sound pressure sampled at multiple points with the UAV were in level with the ground-based measurements. Furthermore, a wind shield array is designed for filtering the noise caused by atmospheric turbulence, which is critical in the low frequencies. Several configurations of wind shields are tested to increase the frequency range of the measurement and to minimize internal resonance effects of the wind shield.