## 3 Results
### Measurements on Test Objects  
In Figure 8 we show the measurement carried out on test object 1, using only the cross-correlation index by the audio data. This object was made using a 20 \(\times\) 20 cm sheet of PMMA (Polymethyl-methacrylate also known as acrylic glass or plexiglass) with a thickness of 4.8 mm, glued with cyanoacrylate glue to a base consisting of a white tile of the same size. To obtain the gaps, paper thicknesses were used; in Figure 8a the glue was colored to improve the contrast of the image. Using this method, three pseudo-rectangular cavities were created which are highlighted in figure. The central point (Ref) was chosen as the test point, certainly not adherent, and used as a reference point for the entire measurement of the object. The data were then processed (after the Kriging gridding steps) using the contour map technique by the software Surfer, and the recognition of the three pseudo-rectangular areas is evident in Figure 8b. Only the rectangle on the left is not well defined, probably because the paper thicknesses introduced in this area were not sufficient to create a cavity as in the central one. It is also visible that the cyanoacrylate glue in the part bordering the central rectangle with the left area has not adhered effectively to the base, i.e., it is too thin (in width) to delimit the area, as can also be seen from the level curve visible in the Figure 8b. Figure 9 shows the measurement carried out on Test Object 2, using both the impact-test data (Figure 9b) and the audio cross-correlation index (Figure 9c), to demonstrate the fusion of data. This object was made using a 20 \(\times\) 20 cm sheet of transparent PMMA with a thickness of 4.8 mm, glued to a base consisting of a tile of the same size covered with quick-setting cement mortar. To obtain a certainly empty chamber, a circular (7.5 cm in diameter) 4.8 mm thick was used in the central part. The evident non-uniformity in the thickness of the cement mortar revealed few points of actual contact with the PMMA slab. Furthermore, the bonding of the plate to the mortar was only partial, and in fact this circumstance was measured and displayed in Figure 9b. The center point of the test object was chosen as the test point for the audio cross-correlation, certainly non-attached, and used as a reference for the entire measurement of the object. The data were then processed using the contour map technique (after kriging gridding) in Surfer. In this case, unlike the previous one in Figure 8, the recognition of the zones is less evident, also because the measured mesh is much more complex. Figure 9c shows high-level contour lines in the vicinity of the point chosen as the test point (as it was obvious to expect), while the areas evidently more adherent are those represented by level curves of lower value and  
Figure 8: The test object 1 made in the laboratory. It is a 4.8 mm thick PMMA slab, glued using cyanoacrylate as shown in (**a**) on a 20 \(\times\) 20 cm tile used as a base. The measurement carried out was reported in (**b**) using the contour lines technique. colored blue to highlight them. In Figure 9b, the same test-object was measured using the impact-test method: the information by this measurement is in a good agreement with the effective structure of the test-object depicted in Figure 9a and with the audio measure of Figure 9c. Fusion of the data for this test object made it possible to highlight with certainty the points of effective intimate contact with the PMMA plate: in fact the only different color assumed by the cement mortar could be misleading in relation to the areas actually adhered. In Figure 10 we show an audio cross-correlation measurement made on a sample of real mosaic (floor of Roman origin, coming from the archaeological area of Tivoli, Roma) measuring 20 \(\times\) 20 cm. This mosaic comes from a restoration carried out in the 1970s, and was glued using cementitious material to a tile used as a base.