Correction Procedures for Aircraft Noise Data. Volume II. Background Noise Considerations

摘要

The impact of background noise on the value of PNL, PNLT, and the resulting EPNL noise metric in aircraft certification to FAR Part 36 is examined in this report, the second in a series of reports on aircraft noise measurement correction procedures. Procedures to remove background noise effects from data measured in the form of one-third octave band sound pressure levels for jet and large propeller aircraft, or data in the form of A-weighted noise levels for light propeller driven aircraft, are defined. After evaluating various techniques for different ratios of signal-to-background noise, one simple correction method for turbojet/turbofan aircraft noise is proposed. The recommended method consists of applying an energy correction, up to a maximum of -10 dB, for that portion of the background noise spectra dominated by energy-adding or predetection background noise. For the remaining portion of the background noise spectra, the non-additive postdetection background noise floor tends to mask out bands very close to or below this noise floor. A simple spectrum extrapolation procedure is recommended in ths case. Another background noise correction method for light propeller aircraft noise is also proposed. This procedure simply involves application of an energy correction to the 'as measured' A-weighted aircraft signal using the A-weighted background noise level. Procedures are also suggested for measuring the background noise level in order to account for the randomness of the fluctuating background noise level. (Author)