De-Dopplerisation and acoustic imaging of aircraft flyover measurements

The technique described in this paper eliminates the Doppler effect from aircraft flyover noise measurements and generates narrowband spectra at required angles. This new capability allows more accurate interpretation of flight data, and is essential for a detailed comparison with predictions and static measurements. Until now, only 1/3 octave levels, before de-Dopplerization, have been available, and these yield little information on tonal content. The paper 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 contra-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. Narrowband spectra at selected angles, density plots of complete flyovers, and field shapes at constant frequencies are all presented. Acoustic imaging, achieved by focusing the de-Dopplerized signals from an array of microphones, is also described, and results from a Lockheed TriStar graphically illustrate its capability.