Roberto Merino-Martínez

Functional Beamforming Applied to Imaging of Flyover Noise on Landing Aircraft

Functional beamforming is a state-of-the-art nonlinear algorithm based on the conventional frequency domain beamformer. In general, it is found to provide improved array spatial resolution and dynamic range. The computational time required for the functional beamforming is approximately the same as that for the conventional frequency domain beamformer and, in general, notably shorter than those of the deconvolution methods. In this paper, several simulations are presented comparing the performance of this algorithm with other imaging methods. Moreover, this beamforming technique is applied to 115 flyover measurements performed with a 32 microphone array on landing aircraft. The simulated and experimental results show good agreement. It is found that, for both synthetic and experimental data, functional beamforming offers better quality acoustic images, with a dynamic range (i.e., the difference in decibels between the main lobe and the highest sidelobe) approximately 30 times larger and an array spatial r...

Analysis of landing gear noise during approach

Airframe noise is becoming increasingly important during approach, even reaching higher noise levels than the engines in some cases. More people are affected due to low flight altitudes and fixed traffic routing associated with typical approaches. Formost air- craft types, the landing gear system is a dominant airframe noise source. However, this element can only be modeled in an approximate manner in wind tunnel experiments. In this research, flyovers of landing aircraft were recorded using a 32 microphone array. Fun ctional beamforming was applied to analyze the noise emissions from the landing gear system. lt was confirmed that for some aircraft types, such as the Airbus A320 and the Fokker 70, the nose landing gear is a dominant noise source du ring approach. The correlation between the noise levels generated by the landing gear and the aircraft velocity was found to be significant, explai ning about 70% of the varia bility found in the noise levels, which is in good agreement with all known theory. Moreover, the experimental resu lts for the Airbus A320 measurements were compared with those obtained using the DLR system noise prediction tool PANAM. Whereas the total aircraft noise levels were in good agreement. the measurements indicate a higher contribution from the nose landing gear noise compared to the predictions.

High-resolution CLEAN-SC: Theory and experimental validation:

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.

Assessment of Noise Variability of Landing Aircraft Using Phased Microphone Array

Enforcing noise control environmental laws around airports is a difficult task due to the large variability in the noise levels observed for flyovers of the same aircraft type under similar conditi...

On the use of global optimization methods for acoustic source mapping

Conventional beamforming with a microphone array is a well-established method for localizing and quantifying sound sources. It provides estimates for the source strengths on a predefined grid by determining the agreement between the pressures measured and those modeled for a source located at the grid point under consideration. As such, conventional beamforming can be seen as an exhaustive search for those locations that provide a maximum match between measured and modeled pressures. In this contribution, the authors propose to, instead of the exhaustive search, use an efficient global optimization method to search for the source locations that maximize the agreement between model and measurement. Advantages are two-fold. First, the efficient optimization allows for inclusion of more unknowns, such as the source position in three-dimensional or environmental parameters such as the speed of sound. Second, the model for the received pressure field can be readily adapted to reflect, for example, the presence of more sound sources or environmental parameters that affect the received signals. For the work considered, the global optimization method, Differential Evolution, is selected. Results with simulated and experimental data show that sources can be accurately identified, including the distance from the source to the array.

Analysis of nose landing gear noise comparing numerical computations, prediction models and flyover and wind-tunnel measurements

The noise emissions of a full-scale nose landing gear, measured in a wind tunnel and obtained from computational simulations, are compared with those of three regional aircraft types recorded in flyover measurements. The results from these three approaches are also compared with the predictions of two airframe noise models (Fink and Guo). The geometries of the nose landing gears in all cases were similar. Microphone arrays and acoustic imaging algorithms were employed to estimate the sound emissions of the nose landing gears. A good agreement was found between the overall trends of the frequency spectra in all cases. Moreover, the expected 6th power law with the flow velocity was confirmed. On the other hand, strong tonal peaks (at around 2200 Hz) were only found for the flyover tests and computational simulations and are not present in typical noise prediction models. As the frequencies of the tones did not depend on the flow velocity, they are likely to be caused by cavities found in structural components of the nose landing gear. Removing these tones would cause overall noise reductions up to 2 dB in the frequency range examined. The noise emissions in the side direction did not present tonal peaks. The acoustic source maps showed that the dominant noise sources were located in the middle of the wheel axle, followed by the main strut and the bay doors. It is, therefore, recommended to further investigate this phenomenon, to include cavity-noise estimations in the current noise prediction models, and to eliminate such cavities where possible with the use of cavity caps, for example.

Comparison between analog and digital microphone phased arrays for aeroacoustic measurements

Microphone arrays are useful measurement devices for estimating the location and strength of sound sources. Numerous comparative studies have been conducted regarding the performance of acoustic imaging methods in the past, but literature lacks of a systematic investigation on the role of the hardware on the measurements. This research focuses on the performance differences between two 63-microphone arrays: one with digital MEMS (Micro Electro Mechanical Systems) microphones and the other with analog condenser microphones. Both systems are used on an aeroacoustic experiment performed in an anechoic open-jet wind tunnel featuring two airfoils (NACA 0012 and NACA 0018) equipped with trailing-edge serrations. Whereas both arrays provided similar frequency spectra when analyzing trailing-edge noise emissions (which are in agreement with previous research), the analog array seems to offer source maps of higher quality with a higher dynamic range (lower sidelobe level). Moreover, the results of the digital array featuring trailing-edge serrations show a noise increase at the higher frequencies (4 kHz) with respect to the straight-edge case, which is not expected from the findings of previous experimental research. The results of the analog array do not present such behavior. This manuscript is the result of a collaboration project between the University of Twente (UTwente) and Delft University of Technology (TU Delft).