Measurement of sound pressure inside tube using optical interferometry

Measurement of sound pressure inside a tube is important for duct acoustics and microphone calibration. Inserting microphone directly into sound field will disturb the field and produce inaccurate measurement result. Recently, non-intrusive sound pressure measurement using optical techniques have been proposed. A laser Doppler vibrometer is used to measure line integrals of sound pressure yield projections and a reconstruction technique is then applied to recover the original sound field from projections. In this paper, measurement of sound pressure distribution using optical method is proposed to realize direct pressure measurement for microphone calibration. A simulation of sound field reconstruction from projections using filtered back-projection technique was developed and the performance was evaluated. The reconstruction performance was evaluated for the projection of plane wave and point source wave of frequency from 1000 Hz to 16000 Hz. The implementation of the proposed method for reconstruction of sound field inside an acrylic tube diameter 61.75 mm, length 22 mm, and thickness 3.5 mm for 1000 Hz sound source from projection using laser Doppler vibrometer was performed. The result shows that the proposed method was able to reconstruct the sound field inside tube and measure pressure distribution.