## 3 Results
### Airborne Sound Insulation Comparison  
The airborne sound insulation performance increases from simple cases to multilayered configurations, as depicted in Figure 1. This situation simply follows a generic rule: insulation ability increases as mass is added to a system (see Table 2) [2; 5; 13]. The reference CLT floor demonstrates the lowest \(R_{w}\) in this study and the simplest configuration (floor 1 with vibration mat and a gypsum layer) shows an initial increase in reduction with \(R(f)\) in 1/3 octave band values ranging from 1.4 dB (at 80 Hz) to 16.5 dB (at 1250 Hz). The improvements in insulation performance \(\Delta R(f)\) for all cases against the reference floor are depicted in Figure 2.  
Figure 1: Airborne sound reduction index \(R\) curves of the test floors (laboratory measurements). Figure 2: Improvements provided in terms of airborne sound reduction index \(\Delta R\) of the test floors relative to the reference CLT floor. However, improvements for the bare CLT floor apply only from 160 Hz and higher as a constant trend (Figures 1 and 2). A few cases show no reduction at 125 Hz (floors 1-4) or lower at 50-100 Hz (floors 7,10), but instead amplify the transmitted noise in certain bands. The first eigenfrequency of the combined mechanical systems of all layers (vibration mat, gypsum, and plywood) causes that distinct resonance around 125 Hz. Floor 7 exhibits those first eigenfrequencies even lower at 80-100 Hz, similarly to floors 11 and 12. Floor 10 shows some interactions owing to the suspended ceiling, which, as a layer, helps the overall insulation to increase, but resonates with airborne noise at the lowest frequency bands of 50-63 Hz. Generally, most \(R(f)\) curves show low values at 2000 Hz, which relate to critical frequencies, where a plate-type structure radiates sound with the highest intensity [39]. The cases with fewer layers and mass (floors 1-4) show greater sensitivity to the critical frequency effect, which leads to reduced insulation around 2 kHz. Only floor 10 compensates with high reduction against this phenomenon. Noticeably, floors 1 and 2 have a single vibration mat, while most configurations rather have two vibration mat layers (floors 3-6) or a vibration mat on top of glass wool insulation (floors 7-10). For cases with a double vibration mat, negative effects at 125 Hz can be observed (Figure 2). Further, floor 5, which has also two gypsum layers and two plywood layers on top of the isolation interlayers, would be expected to be the best in this cluster instead of floor 6 (two gypsum layers on top, the same type of vibration mat in both interlayers), which comes first. The cases with a vibration mat and a glass wool layer (floors 7-10) provide the highest levels of airborne sound reduction alongside floors 5 and 6 (top 6 curves in Figure 1). They all have \(R_{w}\) values of at least 50 dB. Firstly, floors 6 and 7 show that replacing a vibration mat layer with glass wool may provide a similar outcome, even if the material properties are different. They have \(R_{w}(C)\) values of 51 (\(-2\)) and 50 (\(-2\)) dB, respectively. Floors 8 and 9 provide a higher reduction than floors 6 and 7 as wet solutions. Floor 9 with a 60 mm concrete floating layer performs better than floor 8 (30 mm concrete layer) in most frequencies (e.g., up to 3.9 dB improved \(R(f)\) values at 500 Hz). Floor 10 is the only test structure with enhanced insulation and extra layers below the slab, namely, 25 mm of air gap and two gypsum layers for suspended ceiling. It provides \(R_{w,\,100}\) = 57 dB and \(R_{w,\,50}\) = 54 dB. Floors 11 and 12 are also exceptions, as they have only concrete screwed layers on CLT without any resilient layers for insulation, providing moderate results. Figure 2 shows the differences in airborne sound reduction index \(\Delta R(f)\) levels relative to the reference CLT curve. It depicts a clear trend of insulation improvements above 160 Hz and dips at 125 Hz or below.