This paper presents the proof of principle of high-speed holographic measurements applied to solve an inverse problem in the domain of vibroacoustics. In order to get a robust and efficient set-up, a compact holographic interferometer which includes a Fresnel configuration equipped with a negative zoom for large surfaces and a diffractive optical element to improve the photometric efficiency of the set-up was developed. The vibration measurements from the digital holographic set-up are applied to the “RIFF” method which provides identification of the force distribution at the surface of the vibrating object, by solving a regularized inverse problem. Experimental results demonstrate the advantage provided by full-field measurements from multi-point holographic vibrometer with fine spatial and temporal resolutions.
This paper describes the basics of high-speed holographic metrology, its limitations, and its application to the investigation of traveling acoustic waves propagating in mechanical structures. Limits are related to a few parameters that must be carefully adjusted for the recording. A full numerical simulation of the recording–reconstruction holographic process is presented and used to investigate the decorrelation phase noise induced by spatial resolution, active surface of pixels, and short exposure time. Applications to vibroacoustics of structures consider the case of waves propagating after a shock by impact hammer and wave interaction in one-dimensional and two-dimensional acoustic black hole extremities.
Structural vibrations can be measured with optical digital holography. Such a method provides measurements with a very high spatial resolution and is a nonintrusive technique. This method is based on the interference between a reference laser beam and the field diffracted by the studied object. Using a high speed camera, it can also be implemented in the time domain to investigate non-stationary problems.
In this paper, we present a recent investigation which shows that the high-speed digital holography is comparable with classical laser vibrometry. Furthermore, an experimental investigation of the vibratory field inside the Acoustic Black Hole is discussed. The principle of digital holography is explained and it is used here to provide a full field measurement of the velocity field at the extremity of the structure.