Employing digital pole-shifting filters to improve low-frequency response of sonic boom measurements

• Published in: The Journal of the Acoustical Society of America Vol. 146; no. 4; p. 2754
• Main Authors: Rasband, Reese D., Gee, Kent L., Gabrielson, Thomas B., Loubeau, Alexandra
• Format: Journal Article
• Language: English
• Published: 01-10-2019
• ISSN: 0001-4966; 1520-8524
• DOI: 10.1121/1.5136533

High-fidelity measurement of sonic boom waveforms requires microphones and data acquisition hardware with flat frequency responses that extend well below 1 Hz. However, digital pole-shifting filters that account for the microphone and data acquisition system roll-offs of conventional hardware can be used in post-processing to extend the effective bandwidth. This approach is demonstrated for sonic boom recordings from the NASA Quiet Supersonic Flights 2018 (QSF18) measurement campaign. Recordings of several booms at multiple measurement sites using different data acquisition hardware and microphone combinations were used to understand the robustness of the processing technique. In particular, manufacturers report nominal hardware roll-off frequencies with which filters can be designed. However, the QSF18 data can be used to derive filters computationally using a mean-square error method. Results, including residual errors and other limitations, are discussed. The transferability of the technique is further described through application to a launch vehicle reentry sonic boom, which had markedly different characteristics than the QSF18 recordings. [Work supported by NASA Langley Research Center through the National Institute of Aerospace.]