Beampattern control of a microphone array to minimize secondary source contamination

Area of Science:

• Acoustics
• Aerodynamics
• Signal Processing

Background:

• Aeroacoustics is crucial for aircraft noise reduction.
• Measuring noise from individual components like propellers is challenging.
• Traditional beamforming methods may not isolate specific noise sources effectively.

Purpose of the Study:

• To adapt a null-steering technique for precise measurement of individual propeller noise.
• To quantify the acoustic energy generated by a single propeller on a twin-propeller aircraft model.
• To validate the effectiveness of the adapted null-steering method in aeroacoustic measurements.

Main Methods:

• Applied a null-steering technique to a linear delay-and-sum beamformer.
• Utilized array shading with weights derived from polynomial roots for null placement.
• Conditioned the cross-spectral matrix to mitigate directional aeroacoustic system effects.
• Employed a plane-wave formulation based on microwave antenna theory.

Main Results:

• Successfully steered nulls onto the second propeller, effectively isolating its acoustic contribution.
• Observed a 6 dB difference in measurements before and after null steering.
• Demonstrated the technique's efficacy in eliminating noise from one propeller.
• Provided a quantitative measure of acoustic energy from the remaining propeller.

Conclusions:

• The adapted null-steering technique is effective for isolating and measuring noise from individual propellers.
• Pre-conditioning the cross-spectral matrix is essential for accurate results with real-world aeroacoustic data.
• This method offers a valuable tool for detailed aeroacoustic analysis and noise source identification.