## 3 Results and Discussion  
### Impact Noise-Tapping Machine  
The impact sound pressure level results in 1/3 octave bands for bare CLT floors are shown in Figure 8.  
Interestingly, the impact sound pressure levels of the weighted index measured using the tapping machine as the generator provide very similar results for all the different horizontal partitions. This is very important since it demonstrates that all the further studies and noise reduction actions will have very similar influence on all bare floors. The retrieved differences depend on floor dimensions, receiving room shapes and volumes [37]. Accordingly, in the low frequency range, some differences are evidenced. In the middle frequencies (1000-2000 Hz), they tend to offer very similar results, while at higher ranges, again, some diversities are present. In Table 3, the 1/3 octave band average trend and the standard deviation are reported. It is evident how for most 1/3 octave frequency bands, the standard deviation falls within the \(\pm\)3 dB range. This permit to consider the average data reliable and thus a reference. When using Equation (4), the predicted trend is not similar to the measured one, as depicted in Figure 9.  
Figure 8: In situ measurements of normalized impact sound pressure level (tapping machine) for four bare CLT floors (configuration 1). \begin{table}
\begin{tabular}{c c c} \hline \hline
**Frequency [Hz]** & **IMPACT Noise Average (dB)** & **Standard Deviation (dB)** \\ \hline  
\hline \hline \end{tabular}
\end{table} 
Table 3: Average trend and standard deviation of averaged impact sound pressure levels (tapping machine). It is possible to highlight that for middle-low frequencies, the model more or less fits the measured values. In Table 4, the difference between measured (average) and calculated values are reported. It could be noticed that low and middle frequencies (125-800 Hz) present very good agreement. Accordingly, the prediction falls within a range of \(\pm\)3 dB. In contrast, from 1000 Hz on, the model is not able to correctly fit middle-high and high frequencies, mostly because the measurement of the structural reverberation time is measured using hard-surface sources (hammer). When the head of the hammer impinges the wood, it tends to present a resilient behavior (compared to concrete) on middle and high frequencies. For this reason, the measurement could not accurately determine this parameter, thus affecting acoustic performance predictions. When considering a suspended ceiling addition, no analytical model is present in literature or in standards. In Figure 10, the results are reported when the exciting source is the tapping machine. It can be seen how different configurations can act on the impact noise reduction when compared to the average impact noise of the bare CLT floor (configuration 1). Accordingly, \begin{table}
\begin{tabular}{c c c} \hline \hline
**Frequency [Hz]** & **Impact Noise Average (dB)** & **\(\Delta\) (dB)** \\ \hline
100 & 83.7 & 4.6 \\
125 & 80.4 & \(-\)0.3 \\
160 & 84.1 & 2.7 \\
200 & 84.6 & 0.7 \\
250 & 85.7 & 1.9 \\
315 & 86.2 & 1.1 \\
400 & 85.6 & 0.2 \\
500 & 85.8 & \(-\)0.5 \\
630 & 86.2 & \(-\)0.9 \\
800 & 85.4 & \(-\)2.5 \\
1000 & 83.6 & \(-\)4.5 \\
1250 & 82.6 & \(-\)6.2 \\
1600 & 78.2 & \(-\)11.4 \\
2000 & 72.7 & \(-\)17.4 \\
2500 & 68.1 & \(-\)22.2 \\
3150 & 65.5 & \(-\)22.8 \\
4000 & 64.0 & \(-\)23.2 \\
5000 & 59.9 & \(-\)26.5 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Average trend and difference between measured and calculated values (\(\Delta\)). Figure 9: Measured vs. predicted impact noise. when at the bare floor a suspended ceiling is added (configuration 2), a significant noise reduction both in frequency and weighted index is verified. In particular, this reduction follows the trend of the average bare floor almost constantly.