The reduction of aircraft noise over the past decades has generated a growing awareness that the characteristics of a signal can be equally or more important to annoyance than the sound pressure level. Sound can be perceived as more annoying, depending on the frequency content or tonal components. The sound quality metrics loudness, roughness, sharpness, and tonality are important tools to characterize sound. Flyover measurements of landing and takeoff aircraft are investigated in terms of sound quality metrics. The experimental dataset includes 141 measurements of 14 landing aircraft types and 160 measurements of 12 takeoff aircraft types. The sound quality metrics are compared for different aircraft types, and their variability within the same aircraft is investigated. Possible correlations of the sound quality metrics with the airframe, engines, and aircraft operational conditions are investigated. This analysis provides empirical expressions that show a good agreement with experimental data for loudness, sharpness, and roughness for takeoff aircraft. For landing aircraft, empirical expressions could only be obtained for loudness and tonality.

The pursuit of greener aviation led to the investigation of new aircraft concepts. Disruptive aircraft configurations show great potential in reducing the ground noise impact, but extensive research is required before they can be manufactured. Tube-and-wing aircraft with over-the-wing engines, benefiting from noise shielding, are a more feasible option for midterm noise reduction targets. This work explores a low-noise version of the B747-400 considering over-the-wing engines, and a multidisciplinary procedure is used to calculate the aircraft and engine performance, the flight procedure, and the noise impact. Engine positions are found providing maximumengine noise shielding, reflected in a decrease in the sound exposure level (SEL) contours. These contours are calculated considering the wing leading edge as both a sharp and a curved edge. This work investigates whether a sharp leading-edge approximation is acceptable for first estimates of the optimal engines position and corresponding noise reduction values. From the results it is found that the maximumdecrease in theSEL obtained for the modified aircraft is 2 dB considering a sharp leading edge and 1.5 dB for a curved leading edge, for departure. For approach, the SEL contours do not show significant differences for the sharp and curved leading edge, with maximum noise reduction values of 3.5 dB for both cases.

Engine noise shielding is an important measure towards low-noise aircraft configurations. Such designs are supported by prediction tools that indicate high values for shielding of engine noise. Most prediction models approximate the complex nature of engine noise to simple noise sources such as monopoles or dipoles. This work compares predictions of noise shielding with experiments using different noise sources and shielding body geometries. The experiments considered in this work concern a monopole source shielded by a flat plate and a NACA 64-008 A wing, and a propeller shielded by the same wing. Comparisons between models and measurements are made by analysis of noise levels at individual microphones and using conventional beamforming. Results show that for the monopole cases the model predictions are in agreement with the experimental data, with an average deviation of 2-3 dB. The curvature of the leading edge of the wing influences the noise shielding results. The measured values of noise shielding of propeller noise are lower than those measured for the omni-directional source. Different types of source directivity are used to approximate the propeller in the predictions: monopole, dipole and a multi-source. The dipole approximation shows the best agreement with the experiments for the case of the propeller.

Ducted propellers are an interesting design choice for unmanned aerial vehicle (UAV) concepts due to a potential increase of the propeller efficiency. In such designs, it is commonly assumed that introducing the duct also results in an overall noise reduction. The objective of this work is to experimentally analyze and quantify noise of a ducted propeller suitable to be installed on a medium size UAV (wingspan 5–10 m). A microphone array is used for recording the noise levels at each microphone position and used collectively to localize noise sources with beamforming. Different types of noise sources are considered (an omni-directional source and a propeller). In addition, the effect of the presence of an incoming airflow is assessed. With no incoming airflow, it is found that the duct significantly modifies the noise radiation both in the frequency and the spatial domain. With an incoming airflow, the effect of the duct on the frequency content of the signal is almost eliminated. The fact that for this case the harmonics become lower results in a reduction of the received noise levels. Also the directivity changes. These insights are of importance in efforts towards modeling the effects of ducts for complex noise sources such as propellers.

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.

The shielding of engine noise by the aircraft wings and fuselage can lead to a significant reduction on perceived noise on ground. Most research on noise shielding is focused on BlendedWing Body (BWB) configurations because of the large dimension of the fuselage. However, noise shielding is also considered relevant in conventional tube and wing configurations when the engines are mounted above the wings. Therefore, it is important to have a noise shielding method adaptable to different aircraft geometries without compromising the accuracy of the predictions or resulting in very slow computations. In this work two methods are used to calculate noise shielding and compared in terms of accuracy and computational time, the Diffraction Integral Method (DIM) and a method built on the Modified Theory of the Physical Optics (MTPO). Both methods are based on the Kirchhoff integral and are considered accurate for sharp-edged objects. It was verified that the two methods are comparable in terms of their predictions, but the MTPO-based method is more efficient computationally. Noise shielding predictions for different frequencies are presented for a flyover of the Fokker 70 considering a realistic geometry and flight conditions. These predictions indicate significant values of noise shielding, which demonstrates its importance in current aircraft.