De-Dopplerization and acoustic imaging of aircraft flyover noise measurements

Abstract

The technique described in this paper eliminates the Doppler effect from aircraft flyover noise measurements and generates narrow band spectra at required angles. Such a capability allows more accurate interpretation of flight data, and is necessary for a detailed comparison with predictions and static measurements, since 1 3 octave or narrow band levels, before de-Dopplerization, yield limited information on tonal content. The paper first explains how a single microphone output is de-Dopplerized, and includes details of aircraft tracking and computer simulation of flyover measurements. The technique is especially relevant to the analysis of noise from counter-rotating propeller driven aircraft, and results are shown for an Avro Shackleton. It is also applied to a Boeing 757, with high bypass ratio turbofan engines. Narrow band spectra at selected angles, density plots of complete flyovers, and field shapes at constant frequencies are all presented. Acoustic imaging, achieved by focussing the de-Dopplerized signals from an array of microphones, is also described, and results from a Lockheed TriStar graphically illustrate its capability.