Determination of Geometric Farfield for Ducted and Unducted Rotors

Abstract

In this research, the requirements for microphones to be in the geometric farfield for ducted and unducted rotors have been established. The approach is to acquire acoustic data at various geometric locations relative to the source. This is performed for different rotor configurations and compared with the Inverse Square Law (ISL). This paper produced a measurement requirement that satisfies the ISL. It is presented in terms of rotor diameter and/or duct length. If this requirement is met then data for rotors with different dimensions acquired at any anechoic facility may be compared. Furthermore, the present work defines the shortest distance where farfield microphone should be placed for accurate extrapolation of the data to the rotor noise of a full-scale system by the ISL.