Large-Aperture 12k-Channel MEMS Array for Open-Rotor Acoustic Measurements - Part I: Hardware, Setup, and Beamforming

Abstract

This paper presents open-rotor acoustic measurements obtained with a large-aperture Micro-Electro-Mechanical Systems (MEMS) microphone array comprising 12000 sensors, far exceeding conventional array sizes and enabling beamforming and directivity analyses that are not feasible with sparse arrays. By moving the 5x6 array in the streamwise direction, rotor noise is observed from multiple viewing angles while preserving a high local sensor density. The measurements show that shear-layer decorrelation strongly reduces the usable coherent aperture and directly biases beamforming levels, to the extent that the apparent directivity can even be reversed if left uncorrected. To address this effect, channel-quality screening, dense sub-array selection, conventional beamforming, and a first-order correction based on the sub-array-averaged linear coherence are combined. The corrected results reveal asymmetric low-frequency directivity with stronger downstream radiation in the blade-passing-frequency-dominated range, while the higher-frequency directivity remains upstream but less pronounced.