Massive Microphone Array Enables Detailed Aeroacoustic Analysis of Aircraft Models in Open Wind Tunnels

Core Concepts

A novel 7200-channel MEMS microphone array enables detailed aeroacoustic analysis of aircraft models in open wind tunnels, including improved source localization, quantification, and directivity assessment.

Abstract

This paper presents the development and application of a large 7200-channel MEMS microphone array for aeroacoustic wind tunnel studies. The array features a 6 m x 3 m aperture with modular 800-microphone panels, allowing for flexible configuration and optimization of the array geometry.
The key highlights are:

The MEMS microphone sensors provide high-density spatial sampling with low noise and sufficient precision for aeroacoustic research. The array's modular design enables time-synchronized measurement of arbitrary aperture sizes.

Beamforming analysis on the array data shows excellent agreement with conventional microphone arrays and far-field measurements, validating the performance of the MEMS array.

The large array size enables the extraction of sub-arrays at different observation angles, allowing for the assessment of source directivity. This reveals the impact of shear layer decorrelation on beamforming results at high frequencies and large downstream angles.

A frequency-dependent sub-array approach is introduced, which counters the shear layer effects and enables accurate prediction of the far-field sound levels based on the near-field beamforming results.

The authors conclude that the massive MEMS microphone array provides significant advantages over conventional arrays for aeroacoustic research in open wind tunnels, enabling improved source identification, quantification, and directivity analysis.

Stats

The array has a total of 7200 MEMS microphones with a rectangular aperture of 6 m × 3 m.
The MEMS sensors have a sensitivity accuracy of ±1 dB and phase precision of ±1° at 1 kHz.
The MEMS sensors maintain a ±2 dB accuracy in frequency response over the range of 60 Hz to 6 kHz.
The noise floor of the MEMS sensors is 25 dB(A), comparable to a standard 1/2" condenser microphone.

Quotes

"The main advantage of a large array is the flexibility to perform beamforming with sub-arrays from different incident angles to take the model rotation into account for different Angles of Attack (AoA) or evaluate source directivities."
"With an increase in sensors of a factor of 10 to 100, the inverse problem is getting over-determined."
"The frequency-dependent aperture is optimal for beamforming: At low frequencies sources typically do not have strong directivities, so a large aperture is not problematic for their assessment. Further, decorrelation effects are weak, so that beamforming profits from the large aperture because of the increased spatial resolution."

Key Insights Distilled From

Enhancing Aeroacoustic Wind Tunnel Studies through Massive Channel Upscaling with MEMS Microphones

by Daniel Ernst... at arxiv.org 05-07-2024

https://arxiv.org/pdf/2405.03322.pdf

Deeper Inquiries

How can the array's capabilities be further expanded beyond acoustic measurements, such as integrating additional sensors and actuators for advanced aeroacoustic research

To expand the array's capabilities beyond acoustic measurements, integrating additional sensors and actuators can significantly enhance advanced aeroacoustic research. By incorporating temperature sensors, the speed of sound in the fluid medium can be estimated, providing valuable data for aeroacoustic analysis. Acoustic actuators can be utilized for self-calibration purposes, ensuring the accuracy and reliability of the measurements. Automatic detection of defective sensors can be implemented to maintain the array's functionality and data integrity. Furthermore, advanced active source detection methods can be integrated to identify and isolate specific aeroacoustic sources, allowing for a more detailed analysis of the acoustic environment around the aircraft model. These enhancements would enable a comprehensive understanding of aeroacoustic phenomena and facilitate the development of innovative noise reduction strategies.

What are the limitations of the Amiet open wind tunnel correction, and how can it be improved to better account for shear layer effects at large downstream angles

The Amiet open wind tunnel correction, while effective in addressing sound refraction through the shear layer in open wind tunnels, has limitations when dealing with large downstream angles. One of the main drawbacks is its assumption of refraction along a plane, which may not accurately capture the complex scattering and decorrelation effects induced by the shear layer at significant angles. To improve the correction method, a more sophisticated model that considers the three-dimensional nature of sound propagation through the shear layer is essential. This enhanced model should account for the varying thickness of the shear layer, the angle of incidence, the frequency of the sound waves, and the distance between sensor pairs. By incorporating these factors into the correction algorithm, a more accurate and comprehensive correction for shear layer effects at large downstream angles can be achieved, leading to more precise aeroacoustic measurements and analysis in open wind tunnel experiments.

What insights could be gained by applying advanced source identification and separation techniques on the high-density array data to study the individual contributions of different aeroacoustic sources on the aircraft model

Applying advanced source identification and separation techniques on the high-density array data can provide valuable insights into the individual contributions of different aeroacoustic sources on the aircraft model. By utilizing clustering algorithms and expert decision support systems, it is possible to categorize and differentiate between various sources of noise generated by the model. This approach allows for the identification of specific noise sources, such as engine noise, airframe noise, or turbulent flow noise, enabling a detailed analysis of their characteristics and impact on the overall acoustic environment. Moreover, by separating and isolating these sources, researchers can quantify their relative contributions to the total noise emission, facilitating targeted noise reduction strategies and design optimizations. Advanced techniques like tomographic directivity analysis and frequency domain beamforming can further enhance the understanding of source localization and directivity, providing comprehensive insights into the aeroacoustic behavior of the aircraft model.