Getting to Grips with Aircraft Noise

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10 - MEASURING NOISE 80 10. MEASURING NOISE This section quickly describes the basics of noise measurement. 10.1. INTRODUCTION The equipment needed to quantify noise levels is basically composed of : • A microphone for the signal measurement • A recorder • A spectrum analyzer for frequency distribution determination. Though the dB(A) is directly measurable, other quantities (such as the EPNdB) have to be calculated. This is one of the tasks achieved by the analyzer. 10.2. MICROPHONES The mikes typically used for the estimation of aeroplanes noise emissions are pressure and capacitor microphones. Indeed the physical quantity that can be directly accessed is the air pressure (as a function of the time) through the capacity variation of a condenser. For aircraft acoustics, the mikes will be mounted “grazing” (see picture). This configuration allows to keep an quasi-constant incidence of the acoustic wave on the mike during the event duration, which is a pre-requisite to determine the corresponding EPNL (for instance during certification tests). 0° Grazing incidence (90°) Mike Flight Operations Support & Line Assistance Getting to grips with aircraft noise
10.3. ANALYSERS Flight Operations Support & Line Assistance Getting to grips with aircraft noise INDEX Today, analyzers have adopted a digital technology allowing a real-time signal interpretation. They are needed in order to quantify the above-mentioned descriptors, as they are not directly accessible. For certification purposes, the retained bandwidth in terms of frequency is the third of octave. In this very case, the analysis is performed for each third of octave thanks to 24 filters centered on the normalized frequencies from 50 Hz to 10000 Hz. 81
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3.1. ICAO ANNEX 16 VOLUME 1 CHAPTER
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Getting to Grips with Aircraft Noise

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