Acta Polytechnica CTU Proceedings https://doi.org/10.14311/APP.2022.38.0241 Acta Polytechnica CTU Proceedings 38:241–246, 2022 © 2022 The Author(s). Licensed under a CC-BY 4.0 licence Published by the Czech Technical University in Prague ACOUSTICAL ASPECTS OF REPLACING TRADITIONAL MATERIALS IN BUILDING ELEMENTS WITH RENEWABLE AND RECYCLED ONES Jiří Nováčeka,b,∗, Jaroslav Hejla a Czech Technical University in Prague, University Centre for Energy Efficient Buildings, Třinecká 1024, 273 43 Buštěhrad, Czech Republic b Czech Technical University in Prague, Faculty of Civil Engineering, Department of Architectural Engineering, Thákurova 7, 166 29 Prague 6, Czech Republic ∗ corresponding author: jiri.novacek@fsv.cvut.cz Abstract. Building elements, especially partitions, floors and external walls significantly affect indoor acoustic comfort. Their ability to reduce noise transmission from neighbouring rooms or from outdoors depends on the element composition and the building materials used. In Central Europe, the heavyweight masonry or concrete walls and slabs are typical elements both for family and residential buildings. However, increasing popularity of lightweight multi-layered structures is noticeable. This creates new opportunities for the gradual replacement of traditional materials with renewable and recycled ones, both for load-bearing components and for fillings and other layers of building elements. This paper introduces such design changes in relation to acoustics, particularly airborne sound insulation. The greatest attention is paid to the replacement of masonry and mineral wool insulation with timber and wood fibres. The overview is supplemented by examples of low-energy house external wall and timber wall with recycled infill whose sound insulation has been determined by measurements in the acoustic laboratory. Keywords: Building acoustics, sound insulation, sound reduction index, building elements. 1. Introduction In building acoustics, as in other fields of building design, there has been an effort in recent years to gradually replace traditional materials with more en- vironmentally friendly ones. Regarding this, multi- layered lightweight building elements provide more opportunities. Due to their complex character, ma- terial substitution can be made either in the load- bearing construction, sheathing or filler layers. Since the requirements for sound insulation are constantly increasing, such changes should not impair the acous- tic properties. This can be verified by comparative laboratory tests of original and new solutions. This paper focuses on airborne sound insulation of interior and exterior walls. For interior wall, the aim was to improve poor sound insulation at low frequen- cies which is typical for lightweight structures and to increase the overall weighted sound reduction index with regard to new acoustical requirements described in Section 2.1. The composition of exterior wall is usually driven by reaching the target value of the heat transfer coefficient. The sound insulation is often not so determining because the weakest parts of building envelopes are windows. Therefore, only the overall ef- fect of material substitution on Rw value is of interest in this paper. 2. Sound insulation requirements for dwellings Different descriptors are used to express requirements for sound insulation in buildings in various EU coun- tries. Therefore, great efforts have been made to harmonize them in the form of a new classification scheme in recent years, see Section 2.2. However, for laboratory measurements determining the airborne sound insulation of building elements, it is common to use the same quantities throughout Europe, the weighted sound reduction index Rw and relevant spec- trum adaptation terms C, Ctr, C50−3150, etc. 2.1. National requirements according to new ČSN 73 0532 In December 2020, after ten years of using the older version, the new Czech standard specifying require- ments for sound insulation in buildings was issued. The new requirements, listed in Table 1, are 0–2 dB higher for walls and floors between apartments, but 2 dB lower for walls between habitable rooms within the same flat. However, this second requirement, which is not typical for other EU countries, has been extended to all habitable rooms in the apartment (not just one as it was before). The requirements for sound insulation of building envelopes depend on external noise levels and range between 30 dB and 48 dB in extreme cases. The quan- tity R′ w is used for individual parts (e.g. windows or 241 https://doi.org/10.14311/APP.2022.38.0241 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en Jiří Nováček, Jaroslav Hejl Acta Polytechnica CTU Proceedings Floors Walls Type of space R′ w, DnT,w L′ n,w, L′ nT,w R′ w, DnT,w Between all habitable rooms within the same flat ≥ 47 ≤ 58 ≥ 40 Between habitable room and all rooms of other apartments ≥ 54 ≤ 53 ≥ 53 Table 1. Required sound insulation between habitable rooms in family houses and residential buildings [1]. Type of space Class A Class B Class C Class D Class E Class F Between habitable rooms in dwellings and other dwellings DnT,50 ≥ 58 DnT,50 ≥ 54 DnT,A ≥ 52 DnT,A ≥ 48 DnT,A ≥ 44 DnT,A ≥ 40 Table 2. Classes for airborne sound insulation [2]. walls) and DnT,w for building envelope as a whole. 2.2. Classification scheme according to ISO/TS 19488:2021 In 2021, ISO/TS 19488:2021 introducing new acoustic classification scheme for dwellings was issued. This new technical specification is based on the work of COST Action TU0901 “Integrating and Harmoniz- ing Sound Insulation Aspects in Sustainable Urban Housing Constructions” between 2009 and 2013, sum- marized in [3]. This new document uses six quality classes A to F (the best acoustic comfort ensures class A) based on the evaluation of various acoustic aspects (the airborne sound insulation is one of them), for details see Table 2. Classification scheme for airborne sound insulation uses weighted standardized level differences DnT,A and DnT,50 (DnT,A = DnT,w + C and DnT,50 = DnT,w + C50−3150). Since this paper is focused on acoustical behavior of building elements (in labora- tory conditions), corresponding quantities Rw + C and Rw + C50−3150 are used instead. Acoustic clas- sification of façades is based on descriptor DnT,A,tr (DnT,A,tr = DnT,w + Ctr). From the same reasons as for internal walls and floors, Rw + Ctr is used instead in this paper. 3. Timber wall with crushed brick rubble infill The original idea was to use the recycled loose-fill in a timber frame interior wall and to compare the acous- tical properties of such structure with the same wall with mineral wool infill and with a traditional hollow brick partition of approximately the same thickness and weight. The goal was to achieve equal or better weighted sound reduction index and to improve poor sound insulation at low frequencies, which is typical for lightweight building elements. Schemes of all three walls are shown in Figure 1. The tested timber wall with crushed brick rubble infill had following composition (thickness 140 mm, mass per unit area ca. 134 kg/m2): • structural plasterboard 12.5 mm (11.5 kg/m2), Figure 1. Schemes of tested walls: masonry (left), timber with recycled infill (center), timber with min- eral wool (right). • timber frame from KVH profiles (spruce wood) 60× 100 mm, axial distance of studs 625 mm, filled with recycled brick rubble 4–8 mm, • oriented strand board 15 mm (9 kg/m2), • structural plasterboard 12.5 mm (11.5 kg/m2). The process of filling the wall with crushed brick rubble is shown in Figures 2 and 3. For the lightweight variant, mineral wool boards, 100 mm thickness (≥ 15 kg/m3 and ≥ 5 kPa · s/m2) were used instead of rubble infill. The approximate total weight for this wall was 43.5 kg/m2. The hollow block masonry partition was 145 mm thick and weighted ca. 137 kg/m2. The measured laboratory sound reduction index vs. frequency is shown in Figures 4 and 5. There are several interesting observations coming from the frequency response: • the sound reduction index of timber wall with rubble infill is higher compared to the other variants almost in all frequency bands below 1 600 Hz, • sound insulation is well improved especially in low frequency region (in some 1/3-octave bands by more than 10 dB), • the acoustical behaviour of the wall is rather com- 242 vol. 38/2022 Acoustical aspects of replacing traditional materials . . . Figure 2. Crushed brick rubble in the cavity between sheathing and studs (photo by K. Staněk). Figure 3. Filling the wall with crushed brick rubble (photo by K. Staněk). plex and the wall has probably two critical frequen- cies; the first around 315 Hz is possibly caused by bending stiffness of the wall as a whole (it is higher than the critical frequency of single hollow brick wall approx. 160 Hz); the second around 2 500 Hz, associated with plasterboard sheathing, is the same for both dry wall variants, it comes from the fact that plasterboards are not rigidly fixed with the core (recycled infill). For the overall rating of walls the single number quantities presented in Table 3 are the most important. Based on the Rw values, wall with recycled infill is by 2 dB better than the other walls. Compared to the national requirement R′ w ≥ 40 dB on walls between all habitable rooms within the same flat, the laboratory value Rw = 44 dB is high enough even with regard to correction for flanking sound transmission. The parameter Rw + C50−3150 is by 3 dB better compared to dry wall with mineral wool. Figure 4. Sound reduction index of walls: timber with recycled infill – the solid blue curve, timber with mineral wool – the dashed red curve. Figure 5. Sound reduction index of walls: timber with recycled infill – the solid blue curve, masonry – the dashed red curve. 243 Jiří Nováček, Jaroslav Hejl Acta Polytechnica CTU Proceedings Type of wall Rw [dB] C [dB] Ctr [dB] C50−3150 [dB] Ctr,50−3150 [dB] Recycled infill 44 -1 -3 -1 -3 Mineral wool 42 -2 -3 -2 -8 Solid 42 -1 -3 -1 -4 Table 3. Single number results for internal walls. Type of wall Rw [dB] C [dB] Ctr [dB] C50−3150 [dB] Ctr,50−3150 [dB] Wood fibre – basic element 40 -2 -6 -2 -10 Mineral wool – basic element 44 -1 -2 -3 -13 Wood fibre – with ETICS 42 -1 -5 -1 -7 Mineral wool – with ETICS 51 -1 -6 -5 -17 Table 4. Single number results for external walls. Figure 6. Schemes of tested external walls: basic part of the wall (left), basic part with ETICS (right). 4. External wall for low energy houses The external wall with pre-insulated I-joists is an en- vironmentally friendly building system for low energy and passive houses. The heat transfer coefficient varies between ca. 0.15 and 0.18 W/(m2 · K), depending on the thickness of thermal insulation. The external wall shown in Figure 6 was tested with two types of ther- mal insulation: wood fibre boards and mineral wool of the same thickness. The composition of wall with wood fibre insulation was as follows (thickness 255 mm, mass per unit area ca. 55.8 kg/m2), the layers of the basic element (thick- ness 190 mm, mass per unit area ca. 32.4 kg/m2) are in italics: • 5 mm base coat with reinforcing fibre mesh (7.5 kg/m2), • 60 mm thermal insulation wood fibre boards 265 kg/m3 fastened with anchors (15.9 kg/m2), • vapour-permeable wood fibre board 15 mm (9 kg/m2 ), Figure 7. Wood fibre boards between I-joists STE- ICO wall. • pre-insulated I-joists STEICO wall 60/160 mm, stud centres 625 mm, with 2 × 80 mm wood fibre boards 60 kg/m3 and ≥ 5 kPa · s/m2 (14.4 kg/m2 ), • oriented strand board 15 mm (9 kg/m2 ). The composition of wall with mineral wool was following (thickness 252.5 mm, mass per unit area ca. 46.5 kg/m2), the layers of the basic element (thick- ness 187.5 mm, mass per unit area ca. 30 kg/m2) are in italics: • 5 mm base coat with reinforcing fibre mesh (7.5 kg/m2), • 60 mm thermal insulation mineral wool boards 150 kg/m3 fastened with anchors (9 kg/m2), • vapour-permeable wood fibre board 15 mm (9 kg/m2 ), • pre-insulated I-joists STEICO wall 60/160 mm, stud centres 625 mm, with 160 mm mineral wool 20 kg/m3 and ≥ 8 kPa · s/m2 (8 kg/m2 ), • plasterboard 12.5 mm (13 kg/m2 ). The measurement results are listed in Table 4. The test elements are shown in Figures 7 and 8. 244 vol. 38/2022 Acoustical aspects of replacing traditional materials . . . Figure 8. Mineral wool boards of ETICS fastened with anchors. Figure 9 shows that the sound reduction index of the basic element with mineral wool is higher across the frequency spectrum compared to the element with wood fibres. Since the structural parts of the wall were not changed (except OSB replaced with plaster- board), it can be assumed that sound transmission via studs is almost the same in both cases and the difference in the sound reduction index is therefore caused by increased sound transmission through the cavity for wall with wood fibres. This is also indicated by different slope of R between 100 Hz and 1 000 Hz (for wood fibres 8 dB/octave and for mineral wool 4 dB/octave). Smaller slope is typical for the effect of sound bridges (wooden studs) while steeper slope is common for cavity effect (usually due to weakly attenuated cavity). The difference between walls is also significant if we look at the weighted sound reduction index Rw which differs by 4 dB. The difference for Rw + Ctr (recommended descriptor for traffic noise spectrum) is even 8 dB. In contrast, when the low frequencies below 100 Hz are taken into account, the difference for Rw + Ctr,50−3150 is only 1 dB. This is due to the absence of a drop in SRI curve for the wall with wood fibres. For walls with ETICS the observations are almost the same with even greater differences, see Figure 10 and Table 4. For masonry external walls, typical values of Rw are between 45 dB and 50 dB. This is similar to the wall with mineral wool. Taking into account the spectrum adaptation term Ctr, quantity Rw + Ctr for masonry wall will be approximately 5 dB higher. This can be compensated with independent interior plasterboard lining in case of timber wall, which is often used for fire protection and for electrical installations. 5. Conclusions The use of renewable and recycled materials in build- ing elements can significantly change their acoustical properties. It was shown in two examples that this Figure 9. Sound reduction index of basic element: wood fibre – the solid blue curve, mineral wool – the dashed red curve. Figure 10. Sound reduction index of walls with ETICS: wood fibre – the solid blue curve, mineral wool – the dashed red curve. 245 Jiří Nováček, Jaroslav Hejl Acta Polytechnica CTU Proceedings change can mean either improvement or deterioration of airborne sound insulation. The idea of using recycled crushed brick rubble infill for increasing the sound reduction index of walls, espe- cially at low frequencies, was found correct. Although the application to a vertical structure is probably not so acoustically efficient as in the case of floors (since the rubble infill increases bending stiffness of the wall), the measured weighted sound reduction index Rw = 44 dB was nevertheless higher than for the double wall with mineral wool or for the hollow brick partition. This value is fully sufficient also with regard to the Czech requirement for sound insulation between the habitable rooms of the same apartment. Outstanding sound reduction index at low frequencies predeter- mines the use of such a wall between bedrooms and livings rooms where sources with strong low frequency components are common (e.g. home cinema, repro- duced music) [4]. It can be also successfully used as a part of a wall between different dwellings if supple- mented by independent acoustic lining. Interpretation of achieved results for tested external wall is more complicated. In general, the wood fibre infill was found less acoustically efficient than mineral wool in studied case, probably because it provides less sound attenuation in the cavity. However, the observed differences in sound reduction index can be also affected by different type of structural boards used at one side of the wall, since the wood based boards are lighter than plasterboards or gypsum fibre boards. The total weight must be considered in acoustic design and if necessary, the wall should be provided with independent lining on interior side. Acknowledgements This work has been supported by the Ministry of Edu- cation, Youth and Sports within National Sustainability Programme I, project No. LO1605. The measurements of interior timber wall with crushed brick rubble infill were realized within the NCK – MTS, TN01000056/08 – Centre for Advanced Materials and Efficient Buildings. References [1] ČSN 73 0532 Acoustics – Protection against noise in buildings and evaluation of acoustic properties of building elements – Requirements, 2020. [2] ISO/TS 19488:2021 Acoustics – Acoustic classification of dwellings. [3] B. Rasmussen, M. Machimbarrena (eds.). COST Action TU0901 – Building acoustics throughout Europe. Volume 1: Towards a common framework in building acoustics throughout Europe. DiScript Preimpresion, 2014. 257 p. ISBN 978-84-697-0158-4. https://www.cost.eu/uploads/2018/07/Part_I.pdf [4] J. H. Rindel. Sound Insulation in Buildings. CRC Press, 2017. 476 p. ISBN 978-1-4987-0041-2. 246 https://www.cost.eu/uploads/2018/07/Part_I.pdf Acta Polytechnica CTU Proceedings 38:241–246, 2022 1 Introduction 2 Sound insulation requirements for dwellings 2.1 National requirements according to new ČSN 73 0532 2.2 Classification scheme according to ISO/TS 19488:2021 3 Timber wall with crushed brick rubble infill 4 External wall for low energy houses 5 Conclusions Acknowledgements References