Acoustic UAV-Tracking in 6 Degrees of Freedom for Directivity Measurements

Abstract

The increasing deployment of Unmanned Aerial Vehicles (UAVs) in civil and commercial applications has intensified the need for accurate characterization of their acoustic emissions,particularly with respect to societal acceptance and regulatory requirements. This paper presents a comprehensive experimental and methodological framework for determining the three-dimensional sound directivity (sound sphere) of UAVs during realistic flight maneuvers. The approach combines a large-scale sound sphere array with a high-resolution acoustic tracking array and an optical motion tracking system, enabling synchronized acquisition of acoustic and optical data. A key contribution is the development of a model-based acoustic tracking method using frequency-domain beamforming and a minimization algorithm to estimate the full six-degree-of-freedom trajectory of the UAV. Validation against optical motion tracking demonstrates high accuracy, with mean deviations of 7.6 cm in position and 4.7° in attitude. Based on the reconstructed trajectories, a processing pipeline is introduced to derive sound spheres, including Doppler correction, distance normalization, and directional mapping of microphone signals. Experimental results for two quadcopter UAVs under varying conditions—such as flight speed, payload, and platform configuration—reveal distinct, frequencydependent directivity patterns. A predominantly dipole-like radiation characteristic is observed, with notable variations in high-frequency behavior and sensitivity to operational parameters. Increased payload leads to higher sound pressure levels and asymmetries in the radiation pattern, while flight speed primarily affects the orientation of directivity lobes. The presented methodology enables detailed and reproducible analysis of UAV noise emissions under realistic conditions and provides a foundation for improved noise assessment, comparative studies, and the development of standardized certification procedures.