Propeller Noise in Confined Anechoic and Open Environments
Jeremiah Whelchel, William N. Alexander, Nanyaporn Intaratep
Abstract
An experimental study of four commercially available eVTOL propellers was conducted in an anechoic chamber and drone park at Virginia Tech using 11 microphones and a JR3 Multi-Axis Force/Torque Sensor. Noise and thrust produced by each propeller operating at take-off conditions was investigated. Varying solidity screens were used in the anechoic chamber as an attempt to mitigate the effects of recirculation. Spectrograms of the noise suggest that the screens may have been effective for small propellers, but results were inconclusive. Operating the same propellers in an open environment produced consistent tonal levels at the BPF and its harmonics. Propeller airframe interaction was also studied. The most significant increase in SPL was observed when the surface normals from the support arm were projected toward the microphones. Single propeller data were compared with noise from a DJI Matrice 600 Pro hexacopter in hover. Tonal results agreed well if single propeller sources were assumed to combine coherently and in-phase, a poor assumption for this vehicle. In contrast, high frequency broadband noise compared well by assuming the addition of six incoherent single propellers.