14:00
Session 4: Experimental Aeroacoustic Measurements – Field 2
Chair: Camilo Ignacio Andino Cappagli
14:00
20 mins
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Effect of Propeller Blade Spacing on Quadcopter’s Aeroacoustic Noise and Sound Quality
Carlos Ramos-Romero, Dima Usov, Antonio Filippone, Antonio Torija Martinez
Abstract: Previous studies have demonstrated that propeller and blade geometry are primary contributors of aeroacoustic noise in propeller‑driven vehicles. This raises the question: how does the noise signature of an Unmanned Aircraft System (UAS) equipped with two aerodynamically similar, but geometrically different propeller set change? To investigate this, an outdoor measurement campaign was conducted to characterise the acoustic response of the vehicle operating with two aerodynamically equivalent, two-bladed, low-Reynolds-number propeller sets with different blade spacing angles.
The acoustic energy distribution across the frequency spectrum induced by propeller geometry is examined for three distinct flight operations: take-off, hovering, and landing. These operations were characterised using a set of acoustic descriptors ( LAmax, LAeq and LAE). By keeping operational conditions (e.g., altitude and speed) and design parameters (e.g., number of blades and motors, and vehicle mass) constant, significant differences in the emitted noise were observed as a function of propeller blade-spacing angle.
A shift of acoustic energy toward the low-frequency range and a redistribution of high harmonics were observed when the vehicle operated with the altered blade-spacing-angle propeller compared with the baseline propeller configuration. This spectral modification affected the evaluated sound quality metrics (e.g., loudness, sharpness and tonality). Variations exceeding the just noticeable difference thresholds in the sound quality metrics indicate that modifying propeller design can plausibly alter perceived noise while maintaining aerodynamic performance.
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14:20
20 mins
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Influence of acoustic ground reflections on the sound perception of a hovering quadcopter drone
Roberto Merino-Martinez, Renatto Yupa Villanueva, Josephine Pockelé, Amy Morin, Mirjam Snellen
Abstract: The rapid growth in the use of unmanned aerial vehicles (UAVs), commonly referred to as drones, over the past decades has raised increasing concerns regarding their noise emissions, which constitute a major barrier to their widespread societal acceptance. As a result, substantial research efforts have been devoted to characterizing and analyzing the acoustic emissions of these devices. From an experimental perspective, a large proportion of drone noise measurements are typically conducted under anechoic laboratory conditions, corresponding to free-field sound propagation (i.e. without significant reflections). Such measurements often serve as the basis for noise modelling and auralization tools.
In realistic outdoor operational environments, however, sound reflections from the ground and surrounding surfaces (e.g. buildings) inevitably occur, altering the acoustic signal received by an observer. Although these reflection effects can be modelled analytically to some extent within auralization frameworks, their influence on the perceptual attributes of drone noise remains insufficiently understood.
To address this gap, acoustic measurements were conducted in the anechoic chamber of the Faculty of Applied Sciences at Delft University of Technology using a DJI Mavic 3 Enterprise quadcopter. Two experimental configurations were considered: a fully anechoic setup (without floor) and a hemi-anechoic setup (with a solid wooden floor). Recordings were performed using three vertical 16-microphone arrays positioned at three different azimuthal emission angles relative to the drone. Measurements were obtained at two hover heights, 1 m and 2 m, thereby covering a broad range of emission angles (see Fig. 1).
The recorded signals will be analyzed using psychoacoustic sound quality metrics, including loudness, sharpness, tonality, roughness, and fluctuation strength, computed with the open-access Sound Quality Analysis Toolbox (SQAT) and a small-scale listening experiment. Differences between the fully anechoic and hemi-anechoic configurations will be evaluated in relation to the corresponding just-noticeable differences (JNDs) per SQM, providing insight into the perceptual relevance of ground reflections on drone noise. Lastly, comparisons with well-known ground reflection models (e.g. Delany & Bazley) will be made.
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14:40
20 mins
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Wireless accelerometers: can they be useful for understanding drone vibration at flight?
Bin Liu, N. Costa, D. Tcherniak
Abstract: This paper introduces a recently developed wireless vibration measurement system by Hottinger Bruel & Kjaer that is thought to be useful for experimental structure analysis of flying drones. Analysing the noise emission from drone often requires a good understanding of how the drone structure vibrates. Obtaining the vibration data of a flying drone with existing measurement techniques could be troublesome due to the weight of the data acquisition system. The wireless accelerometers make it possible. The lightweight miniature wireless accelerometers (22 g each) can collect high frequency vibration data (up to 1.6 kHz frequency range) in three measurement axes. The acquired data is synchronized between the sensors, which enables the phase information between the different measurement locations. The measured vibration data of a flying drone will enable further analysis, which eventually contributes to make drones quieter. Due to the lack of access to sufficiently big drones, the paper exemplifies sensors’ use on a lightweight sport bike, where the accelerometers are mounted on the rotating elements, such as wheels and pedals.
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