# Cross-Laminated Timber Floor: Analysis of the Acoustic Properties and Radiation Efficiency
## 3 Results and Discussion
### Experimental Results: Vibration-Derived Parameters  
A grid of accelerometer measurement positions is used to evaluate the vibration velocity of the CLT slab. Firstly, the frequency trends are plotted to investigate the velocity levels. From the velocity levels in Figure 13, it is possible to observe how those produced by the rubber ball, for frequencies below 125 Hz, are higher than those produced by the tapping machine and the dodecahedron. Conversely, the velocity level's trend of the standard tapping machine and of the rubber ball are partially overlapped, only in the range between 160-250 Hz. For frequencies below 80 Hz, however, the levels recorded are lower than those of the rubber ball, but higher than those of the dodecahedron. For frequencies above 250 Hz, the velocity levels of the standard tapping machine are the highest of the three sources presented here. Figure 14 shows the radiation indices of the CLT for each exciting source (tapping machine, airborne, noise and rubber ball). It can be clearly seen that, between 50 Hz and 80 Hz, the radiation indices of the two impactures are almost superimposed, while after 80 Hz, the value of the tapping machine is far higher, not only than the radiation index of the rubber ball, but also than that of the dodecahedron. From this analysis, it is evident how each sound source produces a different type of excitation. It is, therefore, important that all three sources are considered, when analyzing the behavior of a CLT building element. Figure 14: Measurements of radiation index: tapping machine (50–5000 Hz), dodecahedron (50–5000 Hz) and rubber ball (50–630 Hz). Figure 13: Measurements of velocity level: tapping machine (50–5000 Hz), dodecahedron (50–5000 Hz) and rubber ball (50–630 Hz). Finally, by looking at the average of the nine measurements and at the variability of values for the nine measurements of the radiation indices, for each of the three sources (Figure 15), some considerations can follow. Firstly, looking at the tapping machine measurements, it is clear that, especially in the range between 50-80 Hz, all nine measurements are quite close to each other and to the average values. From 100 Hz to 200 Hz, they are rather scattered, then especially from 250 Hz onwards, the nine measurements seem quite different from each other, rarely overlapping the average curve. Then, when looking at the measurements performed with the dodecahedron, it can be noted how the trends of each of the nine positions are quite dissimilar for almost all the frequencies. A more similar behavior of the nine curves, on the other hand, can be observed in the proximity of high frequencies (3150-5000 Hz). Finally, by looking at the rubber ball trends, it is possible to note that (i) the trends are more similar to that of the averaged values at low frequencies (50-100 Hz) and that (ii) the differences between the nine positions, at high frequencies, are much more evident than at low frequencies. Using the vibration data as a baseline, it was possible to derive and study the radiation index caused by each individual source at different receiver positions. This resulted in mapping, useful to better understand how the entire CLT floor surface behaves (Appendix A). Looking at the maps produced by the impact of the tapping machine, some considerations can be made for the radiation index. The resonance peak of the normalized impact sound pressure level at 63 Hz is the same as the one of the corresponding radiation index. The negative peak at 80 Hz is also found in both \(L_{\mathrm{n}}\) and in \(L_{\varphi_{\mathrm{rad}}}\). As in the case of the tapping machine, and also for the airborne noise, the peak at 63 Hz is clearly shown in Figure 11. Then, in accordance with the trend found in Figures 14 and 15, the minimum is reached at 80 Hz. Conversely to the tapping machine case, a fluctuation between negative and positive values of the radiation index can be seen.