## 1 Introduction  
This publication concerns contemporary configurations of wooden floors for lightweight CLT-frame buildings. Their production is part of a growing trend in the construction industry [1; 2; 3; 4; 5]. The eco-friendly properties of wood as a building material and its structural properties have made CLT a promising material for sustainable buildings [6; 7]. This happens because the wood for CLT is renewable and has a considerably lower carbon footprint than concrete [6; 8]. The CLT components can be produced with extended prefabrication, implying design flexibility and high protection from moisture, and provide great thermal performance [3; 6; 9]. All of these characteristics make CLT appropriate for the sustainable design of modern structures [2; 5; 9; 10]. For instance, small- or large-scale high wooden buildings have been erected in Scandinavia [11], Canada [1], and Australia [5]. The construction of wooden multistory buildings was approved in Sweden in 1994 and has been developing constantly [11; 12]. Thanks to this development, acoustic issues regarding CLT as structural material have arisen [5; 9; 11; 12]. Sound insulation in CLT components can be insufficient owing to certain properties such as low mass, density, or stiffness [5; 13], as well as because of extended flanking transmission [2]. Measurements have shown that CLT components offer better sound insulation than traditional concrete structures at middle and high frequencies, but offer considerably worse sound insulation at low frequencies below 100 Hz [10; 12; 14]. Further, multistory buildings are common structures with multiple tenants who often complain about noise and the quality of the sound environment [15]. Early research for sound insulation and noise annoyance in Swedish CLT or other wooden buildings (Akulite, Acuwood projects) revealed shortcomings owing to low frequency sound propagation and disturbances for occupants [12; 16; 17]. Most complaints in CLT apartments concern impact sound, namely noise from neighbors stepping barefoot or walking on heels [18]. Acoustic comfort studies also found significant differences in sound environment perception between the users of heavyweight concrete and lightweight CLT dwellings [14]. To correlate acoustic descriptors to residents' noise annoyance, the use of low frequency measurement data from 50 Hz was suggested [12; 16; 17; 18]. Thus, the obligatory limit from 100 Hz in ISO 717 \(-\)1 and \(-\)2 [19; 20] is under debate. The same applies to regulations, threshold values, and the lack of harmonization [15; 21], while in Nordic countries, the extended frequency range from 50 Hz has been adopted [22]. Floor coverings and resilient layers with insulation materials are commonly used for acoustic problems in buildings; floating floor solutions are preferred for enhanced insulation and they offer reduced subjective annoyance due to impact sound [5; 10; 23]. Modifications with additional mass and damping on a floor system represents a successful strategy for insulation improvements [5; 10]. Some studies show measured data for wooden floors of the three usual types: wooden joist floors [24], CLT massive wooden slabs [2], or CLT timber volume elements (TVEs) [10]. However, limited measurement data for sound insulation of massive CLT floors have been published and no reliable prediction models for sound transmission in CLT structures have been found [2; 3; 5; 10; 25]. Although several approaches for modeling sound insulation of CLT floors have been developed, including analytical models [8; 11; 26], finite element methods [4; 21; 27; 28], wave propagation modeling [26], statistical energy analysis [25], and modeling with neural networks [29], to estimate the acoustic performance of CLT floors, acousticians still rely on field or laboratory measurements, which still provide only indications [3; 8; 10; 21]. Moreover, reference data are provided for concrete or wooden joist floors in ISO 10140-5 [30], but not for massive CLT floors, while this necessity has been reported [9; 31].