Abstract
Historically, the schlieren-based optical deflectometer has been successfully used to characterize high-speed compressible flows. This article describes the use of the instrument to measure significantly smaller density gradients associated with the one-dimensional acoustic wave field in a quiescent medium inside a plane wave tube. Results of the static calibration are presented. Cross-spectral analysis between the light intensity fluctuations in a schlieren image and a reference microphone signal is then used to determine the quantitative density gradient field in the normal incidence impedance tube. The results are rigorously compared with those obtained using the standard two-microphone method. The lowest sound pressure level measured with reasonable accuracy is a 100 dB (re 20 μPa), 5 kHz tone. Expressions for the sensitivities of both the optical system and the photodetector device are derived, as well as an expression for the minimum detectable signal. These results are used to discuss potential improvements and fundamental limitations.
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