The parallel phase-shifting interferometry (PPSI) technique has been employed as a powerful tool for measuring several dynamic phenomena. Particularly, this technique was used in aeroacoustics to measure either the flow or the generated sound wave. One of the major advantages of using PPSI in aeroacoustics is its ability to probe the flow without modifying it. The letter by Ishikawa et al. presents an improvement of this approach that allows a simultaneous imaging of the flow and the sound, at a high speed frame rate. A gas, with a different density from ambient air, is injected. The flow is then visualized with the PPSI, based on the changing of the refractive index caused by the injected gas. Moreover, the sound wave is retrieved after applying a time-directional processing on the high-speed recorded video. The authors validate their new method on an experimental case, where they visualize the flow around a whistle and measure the generated sound. A comparison with the signal recorded by microphone shows a high fidelity retrieval of the sound. The work of Ishikawa et al. paves the way to new investigations on complex phenomena, not only in aeroacoustics, but also in other fields such as fluid dynamics and thermodynamics.
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