Application of near-field acoustic holography in presence of high temperature gradient: Experimental uncertainty analysis

Near-field Acoustic Holography (NAH) has been shown to be a powerful tool for the study of sound radiation from vibrating structures. It provides the reconstruction of acoustic pressure, intensity and particle velocity in the space starting from pressure measurements made on a plane array near the vibrating structure. The reconstruction of the sound field is based on measurement of cross-spectra in the scan plane and on the application of 2D-spatial Fourier transformations. As a basic hypothesis it is clearly required the wavelength of sound propagation to be constant over the considered space. However, this hypothesis may not be satisfied if measurements are performed in presence of high temperature gradients, as the wavelength will be different along the reconstruction path. The effect of this modifying input can be a significant uncertainty in the final results. In order to assess uncertainty of NAH results in presence of significant temperature gradients, in this work an experimental analysis has been performed on a muffler for automotive applications heated at 170°C. The NAH results have been checked in terms of velocity distribution computed on the structure surface. To this aim, measurements taken by a scanning laser Doppler vibrometer (SLDV) have been used for comparison. The present work has been carried out in the framework of the "ACES" project GRD1-1999-11202 founded by European Community.