Signal Processing Considerations on the Optical Measurement of Acoustic Particle Velocities in Free-Field Conditions

To overcome the limitations of existing acoustic standards based on microphones, the optical method based on photon correlation has been considered as a potentially new primary standard. In this article, the experimental measurement and signal processing procedures to measure acoustic pressures optically, thus realizing the acoustic pascal in a direct manner, from captured photon sequences are described. The assessment of specific major contributing parameters associated with the signal processing of the photon correlation method is investigated and discussed in detail for free-field measurements. To extract the information relating to the acoustic particle velocities resulting from captured photon sequences, specific signal processing procedures were investigated. First, the captured electrical pulse sequences are converted to a series of binarized photon event sequences. Next, the sequences are divided into time sets related to the time length of individual acoustic cycles at each frequency step; a gating window equal to a short part of the acoustic cycle itself is applied on the aforementioned time sets. The gating process is performed by shifting the time window through the acoustic cycle, with the autocorrelation function calculated for each phase offset step, which yields the acoustic particle velocity and, hence, pressure directly. Single and double gating are applied and their effects are investigated. In addition, the effect of specific signal processing parameters is studied and selection criteria are proposed. Comparison between optical and reciprocity calibrations in terms of sound pressure and device sensitivity in the frequency range of 0.5–16-kHz range revealed discrepancies within 0.01–1.3 dB.