Assessment of Noise Variability of Landing Aircraft Using Phased Microphone Array

Abstract

Enforcing noise control environmental laws around airports is a difficult task due to the large variability in the noise levels observed for flyovers of the same aircraft type under similar conditions. These variations are not properly considered by the current models, such as the Noise-Power-Distance tables. In this research, noise measurements of aircraft flyovers are used to assess this variability and investigate its causes. 115 flyovers of landing aircraft were recorded using a 32 microphone phased array. It was assumed that the main cause of this variability is the change of the emitted aircraft noise, as previous studies showed that the effect of the variable atmosphere (for the distances considered) is negligible. The noise level variability for the Boeing 737 and Fokker 70 cases was approximately 7 dB. The fan rotational speed of each flyover was determined by analyzing the spectrograms. Functional beamforming was applied to the acoustic data. For the Boeing 737 case it was found that the engines are the dominant noise source for all flyovers and that over 75% of the variation of the noise levels can be explained by variations in the engine settings. The Fokker 70 flyovers were dominated by airframe noise sources. Differences in the aircraft velocity explain almost 70% of the noise level variability in this case.

Comparing flyover noise measurements to full-scale nose landing gear wind tunnel experiments for regional aircraft

“…In general, both cases present significant correlations if the typical p−value threshold of 0.05 is considered. Similar results were found by Snellen et al in [35].…”

“…Functional beamforming [33,34] was selected in this case, as it provides better dynamic range and array spatial resolution than conventional frequency domain beamforming, and these features are very important for flyover measurements, due to the relatively large distance between source and observer [18,22,35].…”

Analysis of landing gear noise during approach

“…It is interesting to notice that the frequency of the aforementioned tonal peaks for all the seven Airbus A320 in this research does not seem to have a significant correlation with the aircraft velocity (see Fig. 3) because the correlation coefficient, ρ ≈ 0.16, is considerably low and the p−value ≈ 0.73 is much larger than the 0.05 threshold typically used for determining whether a correlation is significant or not [22]. This fact indicates that these tones are likely to be caused by a flow-induced cavity resonance in the landing gear system, as suggested by Michel and Qiao [5] and Dobrzynski [3].…”

“…Interestingly, analogous frequency peaks and source location were found by Michel and Qiao [5] for a single-aisle aircraft. The main landing gear system (both bogies) was not a dominant sound source at this particular frequencies, but it usually has a similar contribution than the nose landing gear [3,22], as will be shown later in Sec. IV.B.…”

“…These vehicles have been selected for this initial assessment due to their prominent airframe noise [22] and their market share and, hence impact on community noise. Figure 4 contains an example of a source plot for each aircraft, which depicts the Overall SPL (OASPL) when the nose landing gear is above the array.…”