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.

The recently introduced high-resolution (HR)-CLEAN-SC algorithm for acoustic imaging provides ‘super-resolution’, i.e. the ability to discern sound sources located closer than the Rayleigh resolution limit. This is achieved by allowing the source markers to be relocated from the actual source locations within a certain constraint to avoid the combined influence of the other sound sources. The freedom to relocate the source markers to increase the performance of the algorithm depends on the maximum sidelobe level of the acoustic array used. This paper presents an ‘enhanced’ version of the HR-CLEAN-SC algorithm which benefits from low maximum sidelobe level array design. The source marker constraint μ is adapted to the maximum sidelobe level at each frequency. Application to up to four synthetic sound sources shows that the sources can be resolved at half the frequency associated with the Rayleigh resolution limit, when an acoustic array optimized for low maximum sidelobe level is used in combination with Enhanced HR-CLEAN-SC. This improves source discrimination compared to when the HR-CLEAN-SC algorithm is used with a benchmark acoustic array design. The results are confirmed by experimental validation in which up to four loudspeakers and the same array configurations as in the synthesized data case are used.

Phased microphone arrays have become a well-established tool for performing aeroacoustic measurements in wind tunnels (both open-jet and closed-section), flying aircraft, and engine test beds. This paper provides a review of the most well-known and state-of-the-art acoustic imaging methods and recommendations on when to use them. Several exemplary results showing the performance of most methods in aeroacoustic applications are included. This manuscript provides a general introduction to aeroacoustic measurements for non-experienced microphone-array users as well as a broad overview for general aeroacoustic experts.

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.

Most acoustic imaging methods assume the presence of point sound sources and, hence, may fail to correctly estimate the sound emissions of distributed sound sources, such as trailing-edge noise. In this contribution, three integration techniques are suggested to overcome this issue based on models considering a single point source, a line source, and several line sources, respectively. Two simulated benchmark cases featuring distributed sound sources are employed to compare the performance of these integration techniques with respect to other well-known acoustic imaging methods. The considered integration methods provide the best performance in retrieving the source levels and require short computation times. In addition, the negative effects of the presence of unwanted noise sources, such as corner sources in wind-tunnel measurements, can be eliminated. A sensitivity analysis shows that the integration technique based on a line source is robust with respect to the choice of the integration area (shape, position, and mesh fineness). This technique is applied to a trailing-edge-noise experiment in an open-jet wind tunnel featuring a NACA 0018 airfoil. The location and far-field noise emissions of the trailing-edge line source were calculated.

In this article, a high-resolution extension of CLEAN-SC is proposed: high-resolution-CLEAN-SC. Where CLEAN-SC uses peak sources in ‘dirty maps’ to define so-called source components, high-resolution-CLEAN-SC takes advantage of the fact that source components can likewise be derived from points at some distance from the peak, as long as these ‘source markers’ are on the main lobe of the point spread function. This is very useful when sources are closely spaced together, such that their point spread functions interfere. Then, alternative markers can be sought in which the relative influence by point spread functions of other source locations is minimised. For those markers, the source components agree better with the actual sources, which allows for better estimation of their locations and strengths. This article outlines the theory needed to understand this approach and discusses applications to 2D and 3D microphone array simulations with closely spaced sources. An experimental validation was performed with two closely spaced loudspeakers in an anechoic chamber.