Improving low-frequency response of sonic boom measurements through digital filtering

Area of Science:

• Acoustics
• Signal Processing
• Aerospace Engineering

Background:

• Accurate sonic boom waveform measurement necessitates specialized hardware with flat frequency responses below 1 Hz.
• Existing hardware limitations present challenges in achieving the required low-frequency fidelity for sonic boom analysis.

Purpose of the Study:

• To introduce and evaluate a digital pole-shift filtering method for extending the effective bandwidth of measurement hardware.
• To assess the efficacy of this post-processing technique using real-world sonic boom data.

Main Methods:

• A digital pole-shift filtering technique was developed for post-processing sonic boom recordings.
• Filters were designed using data from NASA's Quiet Supersonic Flights 2018 campaign, employing various hardware and microphone setups.
• Custom-designed filters were compared against those based on manufacturer specifications.

Main Results:

• Measurement-designed digital filters significantly reduced the mean square error between original and benchmark sonic boom waveforms.
• Filters based on hardware manufacturer specifications also improved accuracy but were less effective than custom-designed filters.
• Analysis included residual errors, method limitations, and potential application to launch vehicle reentry booms.

Conclusions:

• Digital pole-shift filtering is an effective engineering method to enhance low-frequency measurement capabilities for sonic booms.
• Custom filter design based on specific measurement data yields superior results compared to generic manufacturer specifications.
• The technique shows promise for improving sonic boom data fidelity and has potential applications in other low-frequency acoustic measurements.