Flight Acoustics Measurement Techniques And Applications

This paper will review flight acoustics technology at NASA Langley Research Center developed over the past decade. In particular, the paper will focus on flight experiments performed for three diverse objectives: (1) research applications, such as noise prediction code validation, (2) noise impact modeling, and (3) noise abatement flight procedures. To best achieve these diverse objectives, different deployments of microphone systems on the ground are required, and different data analysis techniques are needed. In all cases, accurate positioning of the aircraft synchronized in time with the data recording is necessary. However, there are some restrictions on flight operations unique to each case for the methods to properly work. 2. MEASUREMENT AND ANALYSIS Research Applications For flight acoustics research applications, such as noise prediction code validations studies or wind tunnel/flight data comparisons, it is often desired to have a high quality, well documented, and very accurate data set. A technique called ensemble-averaging can be implemented to greatly decrease the error band of the amplitude of flight noise spectra. It can be argued that the acoustic source field produced by an aircraft moving at constant altitude, speed, attitude, and engine power setting through a uniform atmosphere represents a stationary random process. The acoustic signal received from a moving aircraft at a fixed observer position, i.e., microphone location, however, is clearly nonstationary. In addition to the well-known Doppler effect, the characteristics of the spectrum of the received signal change because of the directionality of the source, spherical spreading, atmospheric absorption, and ground reflection and attenuation. Since the techniques of time series analysis are valid on..