Fourier frescos: Applying Fourier acoustics to locate delamination defects in a fresco painting surrogate

Fresco painting is an artistic technique in which an artist spreads layers of plaster onto a rigid masonry substrate and then applies dry pigments to the wet plaster to create the artwork. As frescos age, a common mode of failure is the delamination of the plaster layers from themselves, or from the underlying masonry substrate. Historically, to mitigate failure, these delaminated regions are mapped by experienced conservators who strike the plaster and assess the radiated sound. Our team has demonstrated the use of scanning laser Doppler vibrometry (LDV) to identify regions of delamination in frescoes in the US Capitol Building; however, it is difficult to determine with certainty if the correct delamination shape is resolved without prior knowledge of the shape. In this talk, we apply Fourier-based signal processing techniques to LDV measurements of a period-accurate fresco surrogate with a delamination of known shape. The 3-D wave field is reconstructed via Rayleigh’s integral, and the normal acoustic intensity is computed. Further, wavenumber-space filtering is shown to improve accuracy in resolving the simulated delamination shape, opening the door for computer programs or simple smartphone applications that put the conservator’s expertise into the hands of the novice.