This study examines the role of turbulence distortion in predicting inflow turbulence (IT) noise generation from large wind turbines via Amiet's theory. Two subsequent distortion mechanisms are investigated: (i) the streamtube expansion in the rotor induction zone and (ii) the interaction with the surface of thick-blade profiles. Large-eddy simulations reveal that the turbulence spectra, which reflect distortion effects, remain largely unaffected by rotor induction within the frequency range relevant for noise generation. As for the other mechanism, the distortion of the turbulence approaching a blade leading edge is modeled with a simplified closed-form solution of Goldstein's rapid distortion theory. This model, based on vorticity deflection, is extended here beyond the high-frequency approximation and integrated into an analytical Amiet-based IT noise tool. Applications to representative test cases show that while distortion effects are minimal for current turbine sizes, they become relevant for future configurations featuring larger rotor sizes and thicker airfoils. The developed model reveals that IT noise levels do not necessarily scale with rotor size but are shaped by spectral changes induced by the blade geometry, operational parameters, and inflow conditions. This model offers a physically consistent, computationally efficient framework for the aeroacoustic assessment of next-generation wind turbine design. ... Read more

Unmanned aerial vehicles are increasingly considered for urban operations, yet their noise emissions remain a key limitation due to their directional and perceptually complex character. This paper presents a three-dimensional acoustic and psychoacoustic characterization of a quadcopter drone measured under controlled free-field conditions in an anechoic chamber. A 112-channel phased microphone array was used to record the drone sound radiation for 19 azimuth angles and 11 vertical positions (polar angles). The overall sound pressure level (OSPL) and its A-weighted counterpart (OASPL) were employed to quantify the acoustic directivity and conventional frequency-domain beamforming was used to generate two-dimensional source maps at selected one-third-octave bands. Moreover, sound quality metrics, together with a psychoacoustic annoyance model, were evaluated to examine the directional dependence of perceived sound quality. The results show that energy-based metrics exhibit a dipole-like directivity pattern, characterized by a quasi-axisymmetric bi-lobed dependence with respect to the polar angles. The lowest and most nearly-isotropic radiation occurs close to the rotor plane (i.e., polar angles close to zero), and higher levels occurring as the polar angle is steered away from the drone's horizontal plane. The beamforming source maps reveal compact sources associated with the rotors at higher frequencies. Regarding perception, loudness and psychoacoustic annoyance broadly follow the OSPL and OASPL trends. In contrast, sharpness shows a more diffuse angular dependence. Tonality, roughness, and fluctuation strength show localized maxima, indicating that the loudest directions are not necessarily the most perceptually salient or annoying. Furthermore, the perceptual minima do not coincide with the minimum OSPL and OASPL directions. These findings confirm that purely energy-based metrics are insufficient to fully characterize the perceived nature of drone noise and contribute to a more complete characterization of drone noise for source identification, auralization, and mitigation-oriented design. ... Read more

The influence of ambient noise in the perception of wind turbine noise is evaluated in this exploratory study. For this purpose, experimental field measurements of an NTK wind turbine at different wind speeds and background noise levels are considered. Synthetic wind turbine noise auralizations are then computed to replicate the weather and operational conditions during the experiments. Different background noise recordings were then synthetically added to the simulated auralizations to investigate the effect in sound quality metrics, such as loudness, roughness, or the psychoacoustic annoyance model by Zwicker. A least-squares analysis was applied to the resulting sound signals. It was found that adding background noise to the auralisations notably reduced the differences in metrics between simulations and experiments. However, the behaviour with respect to the A-weighted signal-to-noise ratio then becomes background-noise dependent and, hence, more challenging to predict. Therefore, for perceptual studies, it is recommended to use experimental recordings with low background noise as a ground truth. ... Read more

Most aircraft noise research is solely based on audio recordings. Nevertheless, the use of virtual reality (VR) environments provides a more immersive experience and, hence, a higher level of realism when conducting psychoacoustic listening experiments in laboratory conditions. Moreover, this approach enables the analysis of non-acoustical factors (e.g. visual cues). This study evaluates the influence of different (audio-)visual parameters in the perceived noise annoyance reported in VR experiments. For this purpose, an open-source application developed in Unity was employed to simulate 16 different VR scenarios based on real-life locations. These scenarios were characterized by different binary visual aspects (e.g. rural vs. urban, sunny vs. cloudy, or artificial vs. natural). In each scene, the same binaural aircraft flyover recording was employed to focus on the effect of the different environmental conditions. However, the background noise differed per soundscape, providing different signal-to-noise ratio (SNR) values. The influence of the aircraft visibility (not rendered in some cloudy scenarios) was also evaluated. The results show that, in general, cloudy, rural, and natural environments were perceived as slightly more annoying. Moreover, a significant and moderate correlation was observed between the annoyance ratings and the SNR, showing that background noise can partly mask the presence of aircraft. ... Read more

The issue of acoustic reflections caused by an axisymmetric nozzle is addressed both numerically and experimentally. The contraction (with an exit diameter of 420 mm and a contraction ratio of 2) is representative of typical open-jet wind tunnel exits. Such a category of wind tunnels are extensively used in aeroacoustic studies due to the possibility of acoustic treatment of the test chamber (plenum) walls and the placement of acoustic sensors outside of the flow. The distance between the wind tunnel’s nozzle exit and the model being tested is usually limited to around one hydraulic diameter in order for the model to be fully contained within the exit jet’s core. Partly for this reason, the wind tunnel nozzle normally protrudes inside the anechoic plenum to distance the tested model from the (acoustically-treated) walls. This effectively creates a cavity, which is in communication with the test section. Acoustic simulations through a commercial finite-element code (COMSOL Multiphysics) show that the presence of the nozzle leads to interference patterns within the test section and a substantial modification of a source’s measured directivity pattern. Experimental measurements in a fully anechoic chamber confirm these results. Melamine foam inserts on both the exit flange and part of the inner walls of the contraction are shown to mitigate the issue partly. ... Read more

The increasing adoption of electric vehicles (EVs), which operate more quietly than internal combustion engine vehicles, raises concerns about their detectability, particularly for visually impaired road users. Regulations mandate exterior sound signals for EVs, ensuring minimum sound pressure levels at low speeds. However, these signals are often used in already noisy urban environments, creating a challenge: enhancing detectability without adding excessive noise pollution. This study explores the use of synthetic exterior sounds that balance high noticeability with low annoyance. An audiovisual experiment was conducted with 20 participants in 15 virtual reality scenarios featuring an EV passing in front of them. Different sound signals, including pure, intermittent, and complex tones at varying frequencies, were tested alongside two baseline cases (a diesel engine and tyre noise alone, i.e., no synthetic sound added). Participants rated sounds for annoyance, noticeability, and informativeness using 11-point ICBEN scales. Trigger measurements provided additional insights into their willingness to cross in front of the EV. The results highlight optimal sound characteristics for EVs, offering guidance on improving pedestrian safety while minimising noise pollution. By refining exterior sound design, this research contributes to the development of effective and user-friendly EV sound standards, ensuring safer and more inclusive urban environments. ... Read more

This study investigates the relationship between sound quality metrics (SQMs) and noise annoyance caused by airborne wind energy systems (AWESs). In a controlled listening experiment, 75 participants rated their annoyance on the International Commission on Biological Effects of Noise (ICBEN) scale in response to recordings from in-field measurements of two fixed-wing and one soft-wing ground-generation AWES. All recordings were normalized to an equivalent A-weighted sound pressure level of 45 dBA. The results revealed that sharpness was the only SQM predicting participants' annoyance. Fixed-wing kites, characterized by sharper and more tonal and narrowband sound profiles, were rated as more annoying than the soft-wing kite, characterized by higher loudness values. In addition, the effect of some SQMs on annoyance depended on participant characteristics, with loudness having a weaker impact on annoyance for participants familiar with AWESs and tonality having a weaker effect on annoyance for older participants. These findings emphasize the importance of considering psychoacoustic factors in the design and operation of AWESs to reduce noise annoyance. ... Read more

Psychoacoustic listening experiments were conducted to investigate the human response to propeller noise under turbulent inflow conditions. The sound stimuli considered were recorded in aeroacoustic wind tunnel experiments featuring an isolated six-bladed propeller. The propeller operated under different isotropic inflow turbulence conditions and collective blade pitch angles. Higher inflow turbulence intensity levels caused a growth in the broadband noise emissions of the propeller, increasing the overall loudness levels. On the other hand, sharpness and tonality consistently decreased due to the increase in low-frequency noise and the tone-masking effect by the aforementioned broadband noise, respectively. The listening experiments aimed to elucidate the effect of these contradicting trends on sound perception. It was observed that lower noise annoyance ratings were reported for higher inflow turbulence intensity levels and lower collective blade pitch angles. Overall, only the tonality metric provided a statistically significant correlation with the annoyance ratings, indicating the importance of this perceptual aspect in propeller noise, in combination with loudness and sharpness. Other energy-based sound metrics, like the effective perceived noise level, failed to correctly describe the results of the listening experiment. In general, this analysis is valuable for the perception-influenced design of devices equipped with propellers. ... Read more

This study explores the potential of simulation methodologies in the early stages of the acoustic design of advanced air mobility cabins. With perceptual assessment as a priority, the approach includes conducting listening experiments based on the auralization of simulation results. For this, a cabin was simulated under the stochastic load of a turbulent boundary layer and auralized as representative cabin noise. The listening experiments investigated the impact of cabin parameter variations—specifically Young's modulus, skin thickness, and fluid bulk modulus—on the participants' perception and preferences. The findings show that utilizing the presented methodology within an early design scope produced audible differences for these parameter variations. With significant changes to the signals' preference probabilities, the proposed method is able to provide a better understanding and statistical depth to the cabin acoustic design process. Loudness and A-weighted sound pressure levels reliably predicted preferences, whereas other psychoacoustic metrics were of little significance, mainly due to the stochastic, stationary, and low-frequency characteristics of the noise samples. Furthermore, the position of the passenger within the cabin model significantly affected the preferences. Adding authentic cabin sounds to the auralizations did not significantly alter the parameter variations' preference distributions. ... Read more

The human perception of two wind turbines of different sizes, a small NTK turbine and a larger NREL model, was evaluated through their synthetically auralised sound. A wide range of wind speed conditions and observer locations was considered. The simulated sounds were analyzed using equivalent sound pressure levels and psychoacoustic sound quality metrics. Moreover, listening experiments were conducted to evaluate the human response to the same sounds. The least-squares models fitted to the results provided scaling laws for the different sound metrics as a function of wind speed (divided into low- and high-speed regimes) and distance to the observer. At lower wind speeds, the NREL turbine’s noise and annoyance levels increase faster with increasing wind speed than the NTK turbine. The results of the NREL turbine at high wind speeds seem to indicate that turbulent boundary layer trailing-edge noise contributes more to annoyance than leading edge turbulent inflow noise. In the listening experiments, the larger wind turbine was perceived roughly 30% more annoying than the smaller one for the same conditions. The equivalent A-weighted sound pressure level and the psychoacoustic annoyance model by Zwicker were reported to closely represent the annoyance ratings reported in the listening experiment. ... Read more

Novel propulsion concepts are being developed for urban air mobility (UAM). This study analyses the noise emissions of a virtual UAM vehicle equipped with 26 distributed, ducted, low-speed fans. Synthetic flyover sounds are generated using an auralization framework, which involves noise predictions, sound propagation, and the binaural audio rendering at the observer position. Since noise emissions of distributed propulsion are characterized by interference effects, this paper discusses the influence of rotational speed fluctuations of the distributed fans on the sound perception of the auralized sounds. These rotational speed fluctuations are modelled as different ranges of random and constant deviations from the nominal speed, in contrast to the baseline case with the 26 fans operating synchronously. The results of a listening experiment performed to evaluate the perceptual differences between the different configurations seem to indicate that relatively small fluctuations in the rotational speed of the fans (e.g. $\pm1\%$ with respect to the nominal rpm) already notably improve the perceived noise annoyance. The observed differences in annoyance ratings are reasonably well explained by sound metrics that consider the tonal nature of sound, such as the effective perceived noise level (EPNL), tonality, and the psychoacoustic annoyance model by Di et al. ... Read more

The far-field acoustic emissions of a six-bladed propeller were investigated in aeroacoustic experiments in an openjet wind tunnel. The propeller was operating in different isotropic inflow turbulence conditions generated by turbulence grids placed upstream of the exit plane of the wind tunnel nozzle. In addition, the collective pitch angle of the propeller blades was also varied throughout the measurements. A preliminary directivity analysis of different acoustic and psychoacoustic metrics was performed to investigate the influence of the inflow turbulence intensity and collective pitch angle on the noise emissions and sound perception. In general, increasing the inflow turbulence levels did not modify the conventional metrics recorded, e.g. equivalent sound pressure level. Nevertheless, it considerably increased the broadband noise emissions of the propeller, the loudness, and the overall psychoacoustic annoyance metrics. However, notable reductions in tonality (due to partial tone masking because of the higher levels of broadband noise) and sharpness were reported for increasing turbulence intensities. Overall, this analysis is valuable for the perception-influenced design of devices equipped with propellers, such as drones or urban air mobility vehicles, to account for installation effects. ... Read more

Distributed propulsion systems are developed to power a new generation of aircraft. However, it is not known yet which noise emissions these propulsion systems produce, which psychoacoustic characteristics such systems exhibit, and how the generated noise is perceived. This paper investigates how fans with fewer stator than rotor blades affect the noise perception of a distributed propulsion system intended for an urban air mobility vehicle, which is equipped with 26 low-speed ducted fans. Three fan designs with different tonal to broadband noise ratio and opposite dominant noise radiation directions are examined. An analytical process is applied to determine the noise emission, propagate the sound through the atmosphere, auralize the flyover signals, and calculate psychoacoustic metrics. A validation and comparison with A320 turbofan engines at takeoff is provided. The results indicate that the distributed propulsion system generates noise signatures with complex directional characteristics and high sharpness. By applying tonal noise reduction mechanisms at source, a significant effective perceived noise level reduction is achieved for the considered fan stages with fewer stator than rotor blades. In addition, tonality, loudness and roughness are reduced well above one noticeable difference compared to a baseline fan and similar or even lower values are achieved than with turbofans. ... Read more

The application of an acoustically absorbent material (melamine foam) is investigated for the treatment of turbulence grids in an anechoic open-jet wind tunnel facility featuring an axisymmetric contraction. A comparative study of both the generated turbulence and the grids’ self-noise is performed. It is found that the application of melamine foam on the downstream side of the grids marginally affects the produced turbulence, while providing an efficient suppression of tonal peaks in the grids’ self-noise spectrum. Broadband noise levels instead show opposing trends depending on the frequency range considered. On the one hand, a general decrease due to the acoustic treatment is observed for Strouhal numbers lower than unity. On the other, an increase, in the form of broad peaks, is seen to occur over certain higher frequency ranges. ... Read more

The growing adoption of electric vehicles, known for their quieter operation compared to internal combustion engine vehicles, raises concerns about their detectability, particularly for vulnerable road users. To address this, regulations mandate the inclusion of exterior sound signals for electric vehicles, specifying minimum sound pressure levels at low speeds. These synthetic exterior sounds are often used in noisy urban environments, creating the challenge of enhancing detectability without introducing excessive noise annoyance. This study investigates the design of synthetic exterior sound signals that balance high noticeability with low annoyance. An audiovisual experiment with 14 participants was conducted using 15 virtual reality scenarios featuring a passing car. The scenarios included various sound signals, such as pure, intermittent, and complex tones at different frequencies. Two baseline cases, a diesel engine and only tyre noise, were also tested. Participants rated sounds for annoyance, noticeability, and informativeness using 11-point ICBEN scales. The findings highlight how psychoacoustic sound quality metrics predict annoyance ratings better than conventional sound metrics, providing insight into optimising sound design for electric vehicles. By improving pedestrian safety while minimising noise pollution, this research supports the development of effective and user-friendly exterior sound standards for electric vehicles. ... Read more

Aircraft noise annoyance is inherently subjective, and its accurate quantification represents a challenging task. There is a lack of consensus in the scientific community regarding which metrics are best for effectively representing this type of annoyance. The present study aims at relating various sound metrics to noise annoyance ratings measured in listening experiments featuring 60 aircraft flyover recordings (30 landings and 30 takeoffs). This is done by considering different sound quality metrics (SQMs), psychoacoustic annoyance models, and more conventional noise certification metrics, such as the effective perceived noise level (EPNL) or the sound exposure level. A correlation analysis was subsequently performed on a large pool of sound metrics considering both linear and non-linear functions. The results show that, in general, metrics derived from psychoacoustic annoyance models (especially those proposed by Zwicker and Di et al.) present considerably better correlations compared to conventional metrics and most individual SQMs. The metrics of loudness, EPNL, and maximum perceived noise level (PNL) also exhibit strong correlations and capacity to predict a substantial portion of the variance observed in the reported annoyance ratings. Moreover, considering non-linear functions (e.g. logarithmic or hyperbolic tangent power) further improves the prediction performance. ... Read more

The far-field acoustic emissions of an isolated scale-model propeller ingesting turbulent inflow are experimentally investigated in wind-tunnel measurements. Phase-averaging and phase-shifting signal processing techniques are used to separate the deterministic (tonal) and random (broadband) components of the recorded sound signals. It is reported that a combination of the two techniques prevents tones unrelated to the shaft rotational frequency from contaminating the estimate of the broadband part of the signal. Scaling and directivity analyses of the obtained broadband component highlight the presence of two noise generation regimes depending on the considered frequency range. In particular, a quasi-omnidirectional directivity pattern is observed for frequencies for which the Helmholtz number is much lower than unity when turbulence is ingested. On the other hand, a dipole-like pattern with minima close to the propeller’s rotational plane gradually appears for higher frequencies. A time- and frequency-domain analysis through the Continuous Wavelet Transform (CWT) method shows how the increase in broadband noise is due to a large number of short-duration pulses linked to the ingestion of turbulence. ... Read more

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. ... Read more

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. ... Read more

Experimental aeroacoustic measurements conducted in wind tunnels are crucial for informing the design process of various devices, including aircraft components or wind turbine blades. Whereas closed-section wind tunnels typically offer better aerodynamic conditions compared to their open-jet counterparts, they often introduce challenges related to noisier test environments and the optimal placement of acoustic sensors, such as microphone arrays, within the test section. As advancements in noise reduction measures lead to quieter test models and our knowledge regarding the location of their main noise sources improves, accurately quantifying the sound sources within these models becomes increasingly important. The present manuscript offers valuable guidelines aimed at enhancing the precision of sound source quantification. It provides practical recommendations regarding microphone placement and the utilization of post-processing techniques, such as manipulations of the cross-spectral matrix. The experimental setup consists of an array of 16 microphones in two different configurations (flush-mounted and recessed in cavities). To assess the quantification accuracy, a speaker playing broadband noise at different sound pressure levels outside of the flow serves as a known reference sound source. A wide range of signal-to-noise ratios (SNRs) are achieved by employing different flow velocities and speaker settings. The results indicate that relatively accurate sound source quantification can be achieved with SNRs down to −10 dB. Lastly, a scaling law for the expected quantification error is proposed in terms of the number of microphones within the array and the SNR. In this manner, the experimental setup can be adapted accordingly to obtain the required level of accuracy. ... Read more