Sound quality metrics applied to aircraft components under operational conditions using a microphone array

Abstract

Aircraft noise is an increasingly important issue that causes annoyance and complaints for the communities living in the vicinity of airports. The conventional sound metrics (such as the A-weighted sound pressure level) typically used for assessing the impact of aircraft noise often fail to conveniently represent the actual annoyance experienced. More sophisticated sound quality metrics (such as loudness, tonality and sharpness) can be used to determine the psychoacoustic annoyance perceived by the human ear. In this paper, an Airbus A320 landing flyover under operational conditions recorded with a microphone array is analyzed. The application of functional beamforming to the acoustic data allows for the separation of the emissions of different noise sources on board of the aircraft. For this case, the nose landing gear (NLG) and the turbofan engines were selected, due to their expected dominance during approach. It was found that, despite being more quiet than the turbofan engines, the NLG system emits a strong tonal sound that makes it overall more annoying than the noise of the engines. Airframe noise prediction models (Fink's and Guo's methods) do not consider this tone and considerably underpredict the noise levels and the annoyance of the NLG. Thus, it is highly recommended to employ these sound quality metrics to study aircraft noise and to evaluate the potential improvement in annoyance of future low-noise aircraft concepts, rather than just a difference in the sound pressure level in decibels.

Implementation of tonal cavity noise estimations in landing gear noise prediction models

“…Out of all measurements, only the recordings corresponding to seven flyovers of the Airbus A320 aircraft family were considered for this manuscript. This aircraft type was selected because its NLG system is known to be one of the dominant noise sources during landing [21,44,45], due to its strong tonal signature (see Hz. An optical camera (Datavision UI-1220LE [88] with a Kowa LM4NCL lens recording with a sampling frequency of 30 Hz) was placed at the center of the array facing straight up from the ground, which provided video footage synchronized with the microphone data.…”

“…1b. A remarkable example is the Airbus A320 NLG which emits tones at around 1700 Hz that protrude more than 10 dB from the surrounding broadband noise [21,45].…”

“…Indeed, current noise prediction models, such as the methods by Fink [20] and Guo [51,52], only provide the estimated broadband emissions of the landing gear, but do not consider parasitic noise sources, such as cavities. Previous research [43,45] showed that prediction models can considerably underpredict the emissions of the NLG because of this fact, compared to aircraft flyover measurements under operational conditions.…”

“…The sound pressure levels (L p ) of these noise sources can dominate the overall airframe noise signature during landing [13] and even be perceived in cruise configuration [50]. In addition, sounds with a high tonality are perceived by the human ear as more annoying than a broadband sound with the same L p [45,[55][56][57]. Moreover, the strong amplitudes of tonal cavity noise can cause acoustic fatigue, stability problems, drag increase and even damage in aircraft or ground vehicles [58].…”

“…A microphone array was employed, which, combined with acoustic imaging algorithms [61][62][63][64][65][66], allows for the visualization of sound and the separation of the noise emissions generated by different elements on board [61,67]. This case study focuses on the Airbus A320 aircraft, which is a relevant example, given its popularity (at the moment of writing this manuscript, it is the world's best-selling airplane) and the aforementioned presence of a strong tonal component in the noise signature of its NLG system [21,45].…”