ACOUSTIC DIAGNOSTICS VERSUS THERMOGRAPHY: COMPARISON AND INTEGRATION AS A BASIC APPROACH FOR CULTURAL HERITAGE PRESERVATION

Innovative technologies are increasingly being applied in the cultural heritage domain, leading to specific requirements in the effectiveness and harmonization of instrumental methods. Funded on metrological basis, the assessment of the reliability of diagnostic tools includes the comparison between different methods revealing the same quantity or kind of degradation. The investigation deals with an integrated approach to detect the conservation state of frescoes. A comparison of two diagnostic methods, identifying detachments in mural paintings, was carried out in the upper hall of the Oratorio of San Rocco in Padua, a restoration yard become a scene of a participated experimentation. The activity was coordinated between two working units and the restorers for assessing the range of validity of both methods, their strengths and limits, their ability to turn the outcomes into a meaningful indication for the restorers. The frescoes were analyzed through the Acoustic Energy Absorption Diagnostic Device - ACEADD method and the IR thermography applying three different processing algorithms. The ACEADD evaluates the acoustic absorption in detached portion of frescoes. Hidden cavities within walls, excited by a sound wave, behaves as selective acoustic absorbers vibrating at specific frequencies related to their thickness, while a rigid wall reflects all the incident energy. Employing a non contact setup, the equipment automatically scans an area, radiating towards the surface an acoustic wave with audible frequency content and recording the reflected wave. The Cepstrum algorithm extracts the impulse response of the surface and provides an acoustic absorption image of that area, localizing the defects where the absorption is high. Using different frequency contents of the exciting wave the sensitivity of the method to defects size is enhanced. IR thermography measures the surface temperature in real time. When a heat flux is imposed to the surface, a sub-surface air cavity represents a thermal resistance to heat diffusion inside the wall, reducing the thermal flux, becoming visible through a local higher temperature. Three different heating procedures with suitable heat sources and two processing algorithms were applied making the method more effective, the tested area increased and the duration of the measurement reduced, while achieving the needed sensitivity to defects. The processing techniques are dynamic, based on the analysis of the temporal evolution of the surface thermal response. The Principal Component Analysis (PCA) was applied both to thermograms of the heating and cooling phase. Finally Pulsed Phase Thermography (PPT) was similarly applied to the data, separating the amplitude and phase components of the dynamic thermal response Fourier Transform. Acoustic and Thermal images equally reveals the presence of defects in specific areas. The acoustic method suitably detects detachments of consistent extension and thickness, with the lower frequency range, while detailed images emerge with the upper frequency range and low step size. Thermograms reveal important indications when the PCA third component is examined or the PPT in phase. The testing results of the two methods were also compared with the restorers investigation, widening the panorama of information the restorers may use to match the insights from their own experience.