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

Abstract

High-fidelity measurement of sonic boom waveforms requires microphones and data acquisition hardware with flat frequency responses extending below 1 Hz. Hardware limitations can pose challenges meeting these requirements. This letter describes an engineering method involving digital pole-shift filtering that can be used in post-processing to extend effective hardware bandwidth. This approach is evaluated for sonic boom recordings from NASA's Quiet Supersonic Flights 2018 measurement campaign. Recordings of several booms at multiple measurement sites using different hardware/microphone combinations were used to design filters. Results demonstrate that the measurement-designed filters significantly reduce the mean square error between original and benchmark waveforms. Digital filter designs based on hardware manufacturer specifications also reduce error, but not as much. Residual errors after filtering, method limitations, and transferability to a launch vehicle reentry boom measurement are discussed.