## 2 Materials and Methods
### Experimental Structures Characterization  
In order to investigate the influence of different layers on bare CLT, three different noise excitation sources were used: dodecahedral speaker for airborne noise generation, ISO tapping machine for heavyweight impact noise generation, and rubber ball for lightweight noise generation (Figure 7). Four different floors were tested for each configuration. For the sake of brevity, only average results are presented and discussed.  
The measurement methods of airborne and impact sound insulation were conducted in accordance with international standards ISO 16283 part 1 (airborne noise) [31] and part 2 (impact noise) [32]. In particular, part 2 of the standard has recently introduced the use of the rubber ball also at an international level, associating it to subjective perception evaluation. The indices are calculated in accordance with the procedures indicated in the ISO 717 standards part 1 (airborne noise) and part 2 (impact noise).  
The used tapping machine features the following characteristics:  
* five hammers placed in a line. The distance between centerlines of hammers is (\(100\pm 3\)) mm;
* the distance between the center of the supports of the tapping machine and the centerlines of neighboring hammers is at least 100 mm;
* height of fall of the hammers is \(40\pm 2\) mm;
* the mass of each hammer is \(500\pm 12\) g from which it follows that the velocity at impact should be 0.886 \(\pm\) 0.022 m/s. The tolerance limits of the velocity may be increased to a maximum of \(\pm 0.033\) m/s if it is ensured that the hammer mass lies within accordingly reduced limits of \(500\pm 6\) g;
* the falling direction of the hammers is perpendicular to the test surface to within \(\pm 0.5^{\circ}\).
* the part of the hammer carrying the impact surface is cylindrical with a diameter of \(30\pm 0.2\) mm;
* the impact surface shall be of hardened steel and is spherical with a curvature radius of \(500\pm 100\) mm;
* the mean time between impacts is \(100\pm 5\) ms. The time between successive impacts is \(100\pm 20\) ms;
* the time between impact and lift of the hammer is less than 80 ms;
* the weight of the tapping machine is less than 25 kg.  
The rubber ball generates the impact force exposure level LFE in each octave band shown in Table 2, when it is dropped vertically in a free fall from the height of \(100\) cm \(\pm 1\) cm, measured from the bottom of the rubber ball to the surface of the floor under test. The used rubber ball features the following characteristics:  
* hollow ball of 180 mm in diameter with 30 mm thickness;
* effective mass of (2.5 \(\pm\) 0.1) kg;
* coefficient of restitution of 0.8 \(\pm\) 0.1.  
Figure 7: Tapping machine and rubber ball used to test the impact noise of CLT floors.  
The impact force exposure level, \(\mathrm{L_{FE}}\), is expressed by Equation (1):  
\[\mathrm{L_{FE}}=10\mathrm{lg}\left[\frac{1}{\mathrm{T_{ref}}}\int_{\mathrm{t_{1} }}^{\mathrm{t_{2}}}\frac{\mathrm{F^{2}(t)}}{\mathrm{F^{2}_{0}}}\mathrm{dt} \right]\ (\mathrm{dB}) \tag{1}\]  
where F(t) is the instantaneous force acted on the floor under test when the rubber ball is dropped on the floor [N], \(\mathrm{F_{0}}=1\) N is the reference force, \(\mathrm{t_{2}}-\mathrm{t_{1}}\) is the time range of the impact force [s], and \(\mathrm{T_{ref}}=1\) s is the reference time interval. In Table 2, the standard rubber ball force is depicted.  
The dodecahedral source features 12 speaker units. All speaker units in the same cabinet radiate in phase. The directivity of loudspeakers is approximately uniform and omnidirectional.