## 3 Results
### Linear Regression Model Comparison
However, there were adverse results in the frequency bands 50-80 Hz and 1.6-5 kHz for impact sound only. This case demonstrates the potential of suspended ceilings with gypsum boards below CLT slabs, which are already utilized in timber buildings [2]. Similar results with positive acoustic effects due to suspended ceiling and floating floors on CLT slabs have been reported in previous studies [2; 8; 34]. Floors 11 and 12 comprise exceptions in the study cases, which are the only wet solutions without a floating floor setup; they have concrete screed layers directly on CLT without a vibration mat or other resilient layers. Consequently, their insulation abilities are limited, providing average results for airborne sound similar to the simple floating floor cases 1-4. For impact sound insulation, they perform poorly, even worse than the bare CLT when there is no parquet floor, comparing the descriptors \(R_{w}\) and \(L_{n,w}\) without correction spectra (Table 2). Such a technical solution is inefficient. Summarizing practical applications, the cases of floors 6 and 7 stand out as optimal among the dry solutions (floors 1-7), for their \(R_{w}(C)\) and \(L_{n,w}(C_{f})\) values. Two plasterboard layers on top of two vibration mats seem to have sufficient performance without extended total thickness. This is important as an easy economic setup during the construction stage. Then, floor 9 stands out from wet solutions, having a 60 mm concrete cast and parquet floor on top, with the best overall performance for airborne and impact sound insulation. The presented findings can be considered for further laboratory measurements, building construction options, and industrial applications. The linear regression comparison offers a few insights about the shifting of the trends in each case and the importance of the slope values. The impact sound model of the reference floor also demonstrated the necessity of using the full spectrum from 50 Hz to acquire a representative descriptor, which has already been suggested [12; 16; 17; 18]. This study has a few limitations to be addressed. It utilizes full configurations, as combined materials, hence specific material behavior is not analyzed extensively. Secondly, it employs a single CLT floor component, without testing other similar samples for deviations that are reported to be significant [21; 32]. Further, laboratory measurements record the insulation performance of the tested sample, while in situ measurements take into account flanking transmission and the interactions of the surrounding structure [13; 34; 38]. Consequently, laboratory measurements can be indicative, but not representative of a setup in a realized building. Finally, the issue of structure classification can be mentioned, although standards utilize in situ measurements, not laboratory values, in order to account for flanking transmission in structures. The Swedish regulations have minimum requirements (acceptable Class C/BBR) of apparent (in situ) acoustic descriptors \(L^{\prime}_{nT,w,50}=56\) dB and \(D_{nT,w,50}=52\) dB [44] for new buildings. The latest ISO 19488:2021 indicates acceptable thresholds of \(L^{\prime}_{nT,w,50}=58\) dB and \(D_{nT,w,50}=50\) dB [45] (Class D). Only floor cases 8-10 of this study have descriptor values close to the above requirements, thus they could provide acceptable solutions assuming negligible flanking transmission in a real setup. However, such a hypothesis can be only validated with field measurements, which is not the case in this study.