This paper presents a low-order method for assessing tonal noise from full-electric propeller-driven aircraft during outdoor operations. A high-fidelity numerical simulation and several outdoor measurements were performed to validate the approach and identify the dominant noise source. Outdoor measurements involve constant-altitude level flight and three take-off flights. The low-order method focuses exclusively on blade and hub geometry, while the numerical simulation considers propeller blades and the spinner. Comparison of outdoor measurements, numerical simulations, and low-order model predictions reveals the propeller as the primary noise source for the specified aircraft configuration, with negligible interactions with the airframe. The study further demonstrates that during take-off flights noise levels at higher harmonics are more sensitive to variations in propeller disk angles of attack. These findings underscore the importance of addressing propeller noise in full-electric propeller-driven aircraft. Additionally, the paper emphasizes the practical application of the low-order approach for evaluating the aircraft's noise footprint during take-off flights, providing crucial insights for early design stages.@en

In recent years, the search for more sustainable propulsion systems for aviation has led to an increased interest in electric propulsion aircraft. These aircraft are often claimed to be more silent due to the lack of an internal combustion engine which eliminates the combustion and exhaust noise. However, the dominant noise source for single-propeller aircraft, independent of the propulsion method, is often the propeller itself. Additionally, lower noise levels simply assessed with conventional sound metrics like the sound pressure levels might not necessarily correspond to lower noise annoyance. Therefore, this research investigates the noise levels and expected psychoacoustic annoyance of the electric air- craft Pipistrel Velis in comparison to its fuel-burning counterpart, the Pipistrel Virus. The Velis can serve as a direct replacement for the Virus as they have the same number of seats and the same take-off weight, raising the question of what the effect of this replacement is on community noise annoyance. The research is based on experimental measurements during flyovers of both aircraft. Overall, it was found that the Velis is found to have lower noise emissions and lower psychoacoustic annoyance ac- cording to its sound quality metrics. This is reflected in the loudness, tonality, and impulsiveness, likely due to the presence of fewer tones below 500 Hz.@en

This manuscript presents a psychoacoustic analysis of the noise emissions from the Airbus A320 aircraft family, with a special focus on the tonal noise emitted by its nose landing gear (NLG) system. The study is based on microphone array measurements of aircraft flyovers under operational conditions performed next to Amsterdam Airport Schiphol. It was found that the NLG system is a dominant tonal noise source for all aircraft subtypes measured (A319, A320, A320neo, A321, and A321neo) around 1700 Hz. The magnitude of the tonal noise observed was strongly correlated with the aircraft velocity, whereas the tonal frequency remained relatively constant. A preliminary psychoacoustic analysis between the different aircraft subtypes showed that, on average, the neo versions presented higher metric values for effective perceived noise level (EPNL), psychoacoustic annoyance, and loudness but lower tonality than their older ceo counterparts. However, given the low number of neo samples available in this study, no strong conclusion can be drawn from this analysis. Overall, the A321 aircraft measured presented lower average values for all noise metrics evaluated than the A320 ones, despite being larger and heavier. These claims will be evaluated in upcoming dedicated psychoacoustic listening experiments.

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The design, development, and acoustic characterization of the Psychoacoustic Listening Laboratory (PALILA) recently established at Delft University of Technology are presented in this manuscript. This laboratory comprises a soundproof room with a modular design and specialized audio equipment. Its primary objective is to conduct experimental investigations into the human perception of aeroacoustic noise sources, such as aircraft, drones, or wind turbines. Furthermore, PALILA is certainly suited for studying other sound sources (e.g. household appliances, ground vehicles, etc.). The manuscript outlines the fundamental characteristics of the facility (i.e. dimensions and materials). A thorough acoustic characterization is provided, including assessments of the background noise levels, reverberation time, free-field sound propagation, and transmission losses of the walls (with respect to the exterior). Overall, PALILA is deemed to be a suitable quiet environment to conduct high-quality psychoacoustic listening experiments.

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Complex acoustic systems typically present three-dimensional distributions of noise sources. Conventional acoustic imaging methods with planar microphone arrays are unsuitable for three-dimensional acoustic imaging, given the computational demands and the incapability to explicitly account for the presence of multiple sources. This paper proposes the use of global optimization methods to solve these shortcomings. An experiment with three incoherent speakers proved that this method can accurately determine the three-dimensional location and the respective sound level of each individual source. In addition, super-resolution is achieved beyond half the Rayleigh resolution limit. VC 2022 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

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Complex test models in aeroacoustic experiments often present an arrangement of noise sources within a three–dimensional space. Planar microphone array normally have difficulties in separating sound sources in the direction normal to the array plane due to their poorer spatial resolution in this direction. This paper evaluates the benefits of combining asynchronous microphone array measurements for three–dimensional acoustic source. An experimental setup consisting of three out–of–plane speakers was considered. A planar microphone array was employed for the acoustic measurements in a baseline position and then displaced around the speakers to provide different points of view. The acoustic source maps obtained from each array position were combined using the geometric mean of their source autopowers. The performance of this approach in combination with the following acoustic imaging methods was investigated: conventional frequency domain beamforming (as baseline), functional beamforming, orthogonal beamforming, robust adaptive beamforming, CLEAN–SC, Richardson–Lucy deconvolution, and global optimization methods. For each case, the performance is evaluated in terms of accuracy in source position localization and spectral quantification in sound pressure level. In general, it was determined that combining additional views considerably improved the accuracy in terms of position localization (especially in the depth direction).@en

The contribution of the engine and the airframe to the total noise generated by an aircraft varies with the operating conditions. Semi-empirical models are able to account for such variations but require detailed engine and airframe data as input that is not readily available for most aircraft types and operations. This hinders the validation of these models through comparison between predictions and experimental data. This work investigates the sensitivity of semi-empirical models of engine and airframe noise to slight variations of the input data, representative of uncertainties in geometrical parameters and variability of the aircraft operating conditions during flyovers. In addition, the predictions are compared to measurements of A320, A330, and B777 landings and departures. This, together with the sensitivity analysis, indicates frequency regions where a mismatch between measurements and predictions exists. The deviation between predictions and measurements for landings can be partially explained by the underestimation of the sound pressure level of the higher harmonics of the fan. For takeoff, the models predict lower levels than measured. This is hypothesized to be associated with jet-installation noise, which is not accounted for in the semi-empirical models. The predicted spectra of the Airbus A320 and A330 were adjusted to account for jet installation noise, using levels available in the literature. This resulted in a better agreement between modeled and measured spectra at low frequencies.@en

Full-scale propeller measurements are useful to study the total noise contribution of a propeller-driven aircraft, including installation effects. Full-scale measurements under operational conditions also provide an accurate validation opportunity for propeller noise prediction models. These studies are, therefore, necessary to quantify and reduce the noise annoyance of propeller-driven aircraft. For propeller aircraft, rotating sources need to be considered. In this research, propeller noise is studied for a full-scale propeller using an acoustic microphone array. The acoustic imaging techniques used are Conventional Frequency-Domain or Time-Domain Beamforming for the stationary noise sources and the ROtating Source Identifier for the rotating noise sources. By applying these two acoustic imaging methods simultaneously, in addition to filtering in the spatial and frequency domain, also filtering in the source velocity domain can be exploited. These methods were applied to an engine run-up of a Pipistrel Velis Electro, the first fully-electric certified aircraft. This electric aircraft, placed on the ground, allows for an initial study on the present noise sources and their relative contributions. Ultimately, this information can be used to separate the measured spectrum into spectra of different noise components which in turn can be used for full-scale validation and improvement of propeller noise prediction models.@en

In sustainable aviation important aspects are to reduce the greenhouse gas emissions during flight, which have a worldwide impact, and to reduce the noise during take-off and landing. However, in addition airports have an increasing interest in reducing emissions during ground operations to improve the local air quality and potential noise issues. In 2020, Amsterdam Airport Schiphol started a pilot for sustainable taxiing with a pilot-controlled hybrid-electric aircraft-towing vehicle called TaxiBot. During the operational testing, a noise level assessment was performed to evaluate the noise levels to which ground workers are exposed.
For the noise measurements a phased microphone array was used. This allowed for noise source identification through beamforming. In addition, the microphones are used for a noise directionality assessment. Both methods are used to compare the noise sources and levels of a taxibotted versus a conventional taxiing operation. The results show that a taxibotted pass-by produces 7.1 dBA less at a 90deg emission angle at the source position and thus significantly impacts the noise levels on the airport. The directionality assessment shows that the taxibotted operation is especially silent while approaching the observer.@en