Experimental observation of the sound field around a moving source using parallel phase-shifting interferometry

Railway noise is still one of the issues in the wayside environment despite of various countermeasures. For effective countermeasures, it is important to reveal characteristics of sound sources and sound propagation. Using parallel phase-shifting interferometry (PPSI) which measure the air density by interfering the reference light with object light, we tried to observe the sound field around a moving source. This system utilize laser and high-speed camera makes it possible to observe unstedy phenomena and visualize sound waves accurately. As a moving source, a speaker emitting 40kHz sinusoidal sound was mounted on a model, and the model was launched at 100 km/h. As A result, we succeed in observing the sound waves generated from the moving source clearly and visualizing the frequency modulation by Doppler effect. Furthermore, the result was averaged in sub-pixel to understand easily. These results clearly show the difference in frequency depending on the relative position to the sound source as it is in theory.