Correction Notice In the Acknowledgements section the project VIRLWINT should also be listed. The following statement should be included: This publication is part of the project VIRLWINT (Virtual acoustical twin of distributed propulsion systems) funded by the German Aerospace Center (DLR). The affiliation for author S. Schade should be changed to: German Aerospace Center, Bismarckstraße 101, 10625 Berlin, Germany The affiliation for author P. Ratei should be changed to: German Aerospace Center, Hein-Saß-Weg 22, 21229 Hamburg, Germany The affiliation for author S. Bartels should be changed to: German Aerospace Center, Linder Höhe, 51147 Köln, Germany The affiliation for author R. Jaron und A. Moreau should be changed to: German Aerospace Center, Bismarckstraße 101, 10625 Berlin, Germany.

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The noise emissions of an optimized low-noise drone propeller design were measured experimentally using a polar array consisting of fifteen microphones within a circular arc covering 105 degrees. The experiments were conducted in the anechoic chamber at the Faculty of Aerospace Engineering at the Technion - Israel Institute of Technology. A comparison between the optimized low-noise drone propeller design and the baseline case (a commercial off-the-shelf APC 14 x5.5 propeller) was performed using conventional sound metrics (e.g. equivalent A-weighted sound pressure level or perceived noise level), as well as state-of-the-art psychoacoustic sound quality metrics (loudness, sharpness, tonality, roughness, and fluctuation strength). These sound quality metrics (SQMs) were also combined into a global psychoacoustic annoyance metric to assess the predicted noise annoyance a human observer would experience. Despite increasing noise levels at the lower frequencies, the optimized configuration presents consistently lower noise emissions in the directivity angles considered for all the sound metrics employed. These results encourage further research into the perceptioninfluenced design of drone propellers by focusing on psychoacoustic metrics that capture human hearing more accurately than conventional sound metrics typically used in certification.@en

Correction Notice • Place s5 in front of the symbols ‘greater than or equal to (≥)’ and ‘less than (<)’ in Equation 2, as shown below. (Formula Presented). • Replace the correlation coefficient ‘0.886’ showed in Table 6 to ‘-0.886’. This correction also apply to the paragraph below Section 3 (Correlation between Annoyance and Drone Characteristics) and in the last paragraph of the Conclusions. This negative value indicates an inverse relationship between the installation ratio (d/D) and the annoyance computed using the More PA model. • Replace ‘0.337 (Zwicker PA model)’ by ‘0.362 (Willemsen PA model)’ in the paragraph below Section 3 (Correlation between Annoyance and Drone Characteristics). This value (0.362) is also reported in Table 6.

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This preliminary study evaluates the expected psychoacoustics annoyance of an array of distributed propellers operating at different synchrophasing angles between the propeller blades. The study, conducted in a hybrid test section wind tunnel, focused on a three-propeller array in a tractor configuration. The noise emissions were measured using a phased microphone array. Both conventional noise metrics and state-of-the-art psychoacoustic metrics were calculated, enabling a thorough analysis of the noise characteristics produced by these configurations and their expected noise perception. The study found that the relative blade-phase angle significantly influences the levels of the conventional noise metrics and psychoacoustic metrics. These results highlight the potential of controlling relative blade-phase angles to reduce noise annoyance. They also emphasize the importance of conducting psychoacoustic analysis in design configurations like the one examined here.@en

This study investigated the acoustic and psychoacoustic properties of five quadcopters drones during realistic flyover scenarios, utilizing a 64-microphone array for outdoor recordings. Acoustic analyses encompassed signal-to-noise ratio (SNR) values, time-frequency sound pressure levels, and noise spectra at overhead positions. An analysis based on A-weighted SNR revealed discernible drone noise despite background noise. Significant noise levels were observed up to 12 kHz. Harmonics of blade passage frequencies were evident, influencing noise spectra up to 1 kHz. Unlike traditional aircraft, drones' proximity to the ground limits the atmospheric absorption effects of high-frequency noise. A psychoacoustic analysis focused on sound quality metrics (SQMs) and annoyance assessment. SQMs exhibited consistent patterns across attributes, such as sharpness, tonality, roughness, and impulsiveness, with notable drone-specific perceptions. Different annoyance models indicated varying degrees of annoyance perception, with the Autel EVO II drone (lowest installation ratio, defined as the ratio between the drone diagonal size and the propeller diameter) perceived as the most annoying and the DJI Phantom 4 (heaviest) as the least one. Propeller positioning, represented by the parameter of installation ratio, correlated significantly with annoyance levels, suggesting an influence on both noise signature and psychoacoustic response. These findings highlight the importance of understanding the acoustic and psychoacoustic impact of drones, particularly in urban environments.@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 attachment of porous media to a blunt trailing edge (TE) can significantly suppress vortex shedding processes and the related tonal noise, yet the near-wall and internal flow fields of porous media are difficult to analyze experimentally and rely on numerical simulations to elucidate the internal flow features. A structured porous trailing edge (SPTE) has been recently designed that follows a methodology of a structured porous coated cylinder. The SPTE acoustic response was compared against randomized porous media with 10 and 30 pores/in. in an anechoic wind tunnel over a range of flow velocities. Acoustic beamforming revealed that the dominant acoustic sources were at the end of the solid plate, even when a porous TE was attached. A region of integration was used to extract acoustic spectra without additional noise sources, revealing that the SPTE possesses superior noise reduction capability. Dipolar directivity patterns were observed at the vortex shedding frequency for each TE, and the coherence between microphones revealed the complex acoustic propagation of the high-frequency content. A wavelet analysis revealed how the SPTE breaks periodic vortex shedding cycles into smaller cycles over a wider frequency range, leading to an overall noise reduction relative to the other TEs.

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An experimental aeroacoustic study on the influence of the collective blade pitch angle in the noise emissions by an isolated propeller under different turbulent inflow conditions is presented. Acoustic and aerodynamic measurements are conducted in an anechoic, open-jet wind tunnel facility. Different inflow turbulence characteristics are achieved by employing square-mesh, square-bar turbulence grids positioned ahead of an additional 2:1 contraction at the wind tunnel's exit. It is found that the ingestion of grid-generated turbulence does not significantly impact the thrust produced by the propeller for any of the tested collective blade pitch angles. On the other hand, turbulence ingestion greatly increases noise production in both broadband and tonal components. The grouping of broadband noise around the Blade Passing Frequency (BPF) and its harmonics ("haystacking'') does not prove to be a phenomenon of particular relevance in grid-generated turbulence ingestion. A directivity analysis shows that an increase in inflow turbulence intensity is responsible for increased noise emissions downstream of the propeller.@en

Optimal procedure design through high-precision navigation is one of the most effective ways towards sustainable aviation. By re-designing approach procedures, it is possible to reduce the noise exposure on the ground and minimize the amount of population negatively affected by it in the near-airport areas. In this study, an optimal Required Navigation Performance Authorisation Required (RNP AR) approach procedure designed using statistical historical wind data for minimum noise impact is presented. An analytic comparison of the existing and designed RNP AR procedure around Landvetter Airport in Gothenburg (Sweden) is performed in terms of sound exposure level contours, amount of affected population, auralization, and psychoacoustic sound quality metrics. It is demonstrated that, by re-constructing the lateral profile and allowing the aircraft to turn near the runway, a significant reduction of over 1/3 in the number of people initially affected by noise exceeding 70 dB(A) in terms of sound exposure level can be achieved. However, this comes at the expense of shifting the noise contour towards urban areas that were not previously affected. Auralization and perception-based analyses are used to evaluate the perceived annoyance by the people in the near-airport area, and it is shown that a relatively small reduction in overall psychoacoustic annoyance can be achieved by the new procedure.@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 aircraft 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 according 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

Vertical-axis wind turbines (VAWTs) are considered promising solutions for urban wind energy generation due to their design, low maintenance costs, and reduced noise and visual impact compared to horizontal-axis wind turbines (HAWTs). However, deploying these turbines close to densely populated urban areas often triggers considerable local opposition to wind energy projects. Among the primary concerns raised by communities is the issue of noise emissions. Noise annoyance should be considered in the design and decision-making process to foster the social acceptance of VAWTs in urban environments. At the same time, maximising the operational efficiency of VAWTs in terms of power generation and actuation effort is equally important. This paper balances noise and aero-servo-elastic performance by formulating and solving a multi-objective optimisation problem from a controller calibration perspective. Psychoacoustic annoyance is taken as a novel indicator for the noise objective by providing a more reliable estimate of the human perception of wind turbine noise than conventional sound metrics. The computation of the psychoacoustic annoyance metric is made feasible by integrating it with an accurate and computationally efficient low-fidelity noise prediction model. For optimisation, an advanced partial-load control scheme – often used in industrial turbines – is considered, with the Kω2 controller as a baseline for comparison. Optimal solutions balancing the defined objectives are identified using a multi-criteria decision-making method (MCDM) and are subsequently assessed using a frequency-domain controller analysis framework and mid-fidelity time-domain aero-servo-elastic simulations. The MCDM results indicate the potential application of this controller in small-scale urban VAWTs to attain power gains of up to 39 % on one side and to trade off a reduction in actuation effort of up to 25 % at the cost of only a 2 % power decrease and a 6 % increase in psychoacoustic annoyance on the other side compared to the baseline. These findings confirm the flexible structure of the optimally calibrated wind speed estimator and tip-speed ratio (WSE–TSR) tracking controller, effectively balancing aero-servo-elastic performance with noise emissions and marking the first instance of integrating residential concerns into the decision-making process.@en

New aircraft concepts with distributed propulsion systems are currently developed for urban and regional air mobility. Typically, these propulsion systems consist of a large amount of propulsors, such as ducted fans or propellers. As the emitted sound fields of individual propulsors may interact with each other, the noise emission is characterized by interference effects. These interactions change the psychoacoustic characteristics of distributed propulsion systems compared to conventional designs with two or four engines. Therefore, in this paper acoustic interactions due to rotational speed fluctuations are investigated for a distributed propulsion system equipped with 26 ducted, low-speed fans. Synthesized flyover sounds are generated using an analytical auralization process for different ranges of rotational speed fluctuations and the impact on noise immission as well as psychoacoustic sound quality metrics is assessed. Our study indicates that temporal fluctuations decrease with increased rotational speed fluctuations. Time signals are smoothed when speed fluctuations are applied, resulting in lower values for psychoacoustic loudness, tonality and fluctuation strength.@en

Current validation of wind turbine noisemodels primarily focuses on sound levels averaged over time, typicallyexpressed in metrics such as the equivalent A-weighted sound pressure level(LA,eq). Whereas valid for regulatory purposes, these methods are notsufficient for psychoacoustic research, as time-averaged levels alone do notfully explain the measured noise annoyance. Therefore, this research aims toestablish whether psychoacoustic sound quality metrics (SQMs) provideadditional value when analysing wind turbine noise models. This work employsthe Horizontal Axis Wind turbine simulation Code 2 (HAWC2) to generate noisesource spectrograms, which are propagated through the atmosphere with aGaussian beam-tracing approach. The final sound signals are retrieved throughan inverse Short-Time Fourier Transform (iSTFT). This methodology is applied toa case study featuring a stall-controlled, horizontal-axis, Nordtank NTK 500/41wind turbine. The results are evaluated against measurements by consideringLA,eq, SQMs, and a comparative listening experiment. The LA,eq metric shows aconsistent underprediction of the simulations with respect to measurements,which is partly explained by a ground reflection modelling error. In thehigh-frequency range, stall noise is known to be significantly underpredictedby the aero-acoustic simulation model. This usually translates in increasingdiscrepancies between measurements and models as the wind speed increases. Thecomparative listening experiment confirms that participants experience thesimulations and the measurements as significantly different. The differenceratings show a good agreement with the differences in the psychoacousticannoyance and loudness metrics. It is more difficult to relate the results fromthe listening experiment to LA,eq. These findings confirm that an evaluationwith psychoacoustic metrics next to conventional methods provides additionalvalue in validating wind turbine noise models for human perception research. @en

Authorities are starting to pay attention not only to the noise levels of Unmanned Aerial Vehicles (UAVs) but also to their quality for acceptance. This manuscript presents a study of four types of propeller-driven UAVs (single-propeller quadcopter, coaxial-propeller quadcopter, quadplane eVTOL (electric vertical take off and landing) and tailsitter eVTOL) to assess their acoustic and psychoacoustic signatures. Experimental outdoor recordings are conducted under realistic flyover conditions. An acoustic analysis showed that quadcopters present higher noise levels compared to the eVTOLs, where the coaxial-propeller configuration revealed to be the noisiest and the quadplane the quietest. A psychoacoustic analysis demonstrated that the coaxial-propeller quadcopter was roughly three times more annoying than its single-propeller counterpart, whereas the quadplane and tailsitter eVTOLs showed similarly lower annoyance values. Additionally, the coaxial-propeller quadcopter exhibited the highest levels of loudness and impulsiveness, while the tailsitter had the lowest. Conversely, the tailsitter exhibited contrasting behavior in terms of sharpness. Regarding tonality, the quadplane was the most tonal, and the tailsitter eVTOL the least. In terms of modulation frequency characteristics, the single-propeller UAV emitted the harshest and most pulsating sound, while the tailsitter had lower values.@en

Literature suggests that psychoacoustic metrics, which are established by models firmly based on the human auditory system, are able to better approximate perceptual attributes of sounds in a quantitative manner than conventional metrics based on the sound pressure level. Therefore, psychoacoustic metrics are commonly used as sound quality indicators in practical applications, such as product design or environmental noise assessment, in which human perception plays an important role. However, their calculation is not straightforward and the non-availability of verified, well-documented, open-source implementations in a single environment poses a challenge to reproducible research. In this contribution, we introduce the sound quality analysis toolbox (SQAT), an open-source compilation of MATLAB codes containing the implementation of the following key psychoacoustic metrics: loudness (ISO 532-1:2017), sharpness (DIN 45692:2009), roughness (Daniel & Weber’s model), fluctuation strength (Osses et al. model), and tonality (Aures’ model). A comprehensive description of their implementation is provided. Furthermore, a verification study is presented in order to demonstrate the reliability of the implementations, supporting the use of SQAT as a reliable tool for future works in the field of sound quality assessment based on psychoacoustic metrics. @en

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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Exterior vehicle sounds have been introduced in electric vehicles and as external human–machine interfaces for automated vehicles. While previous research has studied the effect of exterior vehicle sounds on detectability and acceptance, the present study takes on a different approach by examining the efficacy of such sounds in deterring people from crossing the road. An online study was conducted in which 226 participants were presented with different types of synthetic sounds, including sounds of a combustion engine, pure tones, combined tones, and beeps. Participants were presented with a scenario where a vehicle moved in a straight trajectory at a constant velocity of 30 km/h, without any accompanying visual information. Participants, acting as pedestrians, were asked to hold down a key when they felt safe to cross. After each trial, they assessed whether the vehicle sound was easy to notice, whether it gave enough information to realize that a vehicle was approaching, and whether the sound was annoying. The results showed that sounds of higher modeled perceived loudness, such as continuous tones with high frequency, were the most effective in deterring participants from crossing the road. The tested intermittent beeps resulted in lower crossing deterrence than continuous tones, presumably because no valuable information could be derived during the inter-pulse intervals. Tire noise proved to be effective in deterring participants from crossing while being the least annoying among the sounds tested. These results may prove insightful for the improvement of synthetic exterior vehicle sounds.

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Sound evaluation based on listening experiments is very costly and time-consuming. Alternatively, the sounds under evaluation can be compared using psychoacoustic metrics, under the hypothesis that sounds that differ by more than one just-noticeable difference (JND) are perceived as significantly different from each other. Wellestablished psychoacoustic metrics are obtained from models of loudness, sharpness, roughness, fluctuation strength, and tonality. In the present study, we use these psychoacoustic models to analyse a selection of aircraft flyovers and train pass-by sounds, as well as fluctuating sounds obtained from resonances of a rotating flexible tube. In addition to the description of the adopted model implementation of each psychoacoustic metric, our discussion focuses on technical and practical aspects including (1) the dependency of the results on the appropriate sound and model calibration, and (2) how to interpret the results based on reported JNDs from the literature. We reflect on how much we can rely on a specific model implementation considering the status of the metric—standardised (or not), accessibility to algorithms and to their validation results (or not)—and external factors that may influence the obtained estimates. @en

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