Visualisation et optimisation des effets des vac damps sur le champ vectoriel d'intensité acoustique d'une structure soumise à des impacts répétés

Hearing impairment can be caused by long term exposure to industrial impact noises such as that produced when riveting large structures. The purpose of this thesis was to evaluate how vacuum applied viscoelastic pads (Vac Damp) could be used for impact noise reduction.

The research involved two steps. The first one was the development of a low cost acoustic field visualisation tool, to be used on a single source. It was decided that a Nearfield Acoustical Holography (NAH) system should be used.

The second part of this research involved experiments on Vac Damps. The purpose was to evaluate the pads for the impact noise control, in the direction of a worker's relative position to the impact point.

A number of observations are exposed herein, that corroborate the occurrence of slippage at the pad-plate interface. A direct relationship is confirmed between the pads' thickness and the acceleration noise attenuation. The inverse relationship between the duration of the acceleration noise and the size of the pads is explained. It is also shown that the neighbouring vibration mode of the system dominates the low-frequency acoustic field, for adhesive-bonded pads configurations.

For the first time, the propagation of the acceleration noise from a Vac Damp applied structure is studied. The near field duration of this noise along the impact axis is deemed comparable to the contact duration. The low-frequency components are rather diffuse, whereas the higher-frequency components are concentrated along the normal axis to the plate. The supersonic frequencies are propagated partly by flexural waves along the plate. The Vac Damps dampen this flexural wave and thus reduce the acceleration noise amplitude, away from the impact axis.

Mathematical and experimental determination of the acceleration pressure signals show that the acceleration noise is symmetrical about the impact axis.

This research concludes to one most important result. It is the presence, due to the use of non viscoelastic layered Vac Damps, of low-pressure areas in the acceleration noise field. These low-pressure zones could become a great advantage for impact noise reduction around a riveter's head. (Abstract shortened by UMI.)