Experimental analysis of acoustical properties of irregular cavities using laser refracto-vibrometry

In this paper, the Scanning Laser Doppler Vibrometer (SLDV) is used to measure acoustic pressure in small regions of cavities for the study of acoustical localization. It is shown that this optical method leads to interesting information on localized acoustical modes inside irregular cavities, which are very difficult to observe using conventional microphone measurements. Indeed, localization regions are often of comparable size or even smaller than a typical microphone which can make this type of sensor intrusive. The SLDV is used to measure sound pressure after deriving the refracto-vibrometry method from its standard use. Data are obtained in a large area with a high spatial resolution and the method is validated on some cavities. On irregular cavities, localized modes have been measured and visualized precisely, showing a good agreement to numerical simulations. Owing to this tool, the increase of damping induced by irregularities is experimentally demonstrated through the study of the quality factor. The method is also used to estimate the sound absorption coefficient of quarter-wave resonators. The expected application of this work is the use of irregular geometries to dissipate acoustical energy, especially at frequencies at which porous materials are not efficient.