Transient acoustic fields studied by pulsed TV holography

Abstract: Many of the acoustic events that reach our ears are produced by impacted and vibrating bodies. Pulsed holographic interferometry is a suitable technique to record transient events, like sound waves in air from an impacted body or the wave propagation in solid objects. Unlike point measuring techniques, this is a whole-field, non-contacting optical method that gives the status of the entire object at a specific instant of time. Quantitative measurements of acoustic pressure fields are conducted using the double-exposure technique with photographic film as recording medium. Sound fields in air generated by impacted plates (paper A), and acoustic waves in an elliptical cavity are studied (paper B). Results are obtained in form of spatial maps that represent two-dimensional projections of the acoustic fields. For a full three-dimensional description, several recordings from different angels followed by a tomographic reconstruction are needed. The classical holographic technique with optical hologram reconstruction is, however, not practical for this purpose. Therefore, a new, all-electronic version of the measuring technique is developed, referred to as pulsed TV holography. Both the time-consuming wet processing and the hologram reconstruction are avoided. Instead, quantitative results are computed directly from two subsequent CCD-recorded holograms. The system is demonstrated in experiments with propagating flexural waves in impacted objects (paper C) and in recordings of an acoustic field from several viewing directions followed by tomographic reconstruction (paper D). A tomographic reconstruction of a circular symmetric sound field outside an impacted plate is carried out (paper E). A numerical model for sound radiation from an impacted plate is studied in cooperation with the acoustic group at Ecole National Supérieure des Télécommunicatons (ENST) in France. The transient plate vibration and sound radiation are modelled in the time domain using finite difference methods. The simulated results compare quite well to the optically measured sound wave pattern (paper F). In conclusion, pulsed TV holography is proved to be a feasible method for the measurements of transient acoustic fields. It is successfully used in combination with tomographic techniques for full three-dimensional evaluation of the pressure distribution. This is valuable, for example, in experimental studies in acoustics and fluid mechanics and for verifying numerical models.