BUILDING PHYSICS 7 Structure-borne sound / footfall sound Structure-borne sound is induced in a component through mechanical stimulation (see Figure 2). Footfall sound is a structure-borne sound that is caused, for example, by children jumping around or knocking. The disruptive sound is mechanically induced directly into the ceiling and deflected to the neighbouring rooms. The insulation of a ceiling is marked by the rated standard footfall sound level L’nT,w [dB]. Consideration of the construction situation including the secondary sound paths is indicated here by the line. For the measurement of footfall sound, the ceiling in the transmitting room is excited by a standard hammer mill and the sound level generated is measured in the receiving room. The lower the level, the better the rating of the ceiling for insulating footfall sound. The assembly to be selected decisively depends on • the dynamic stiffness s’ of the sound insulation panels, • the masses of the floor screed or unfinished ceiling, • the reinforcement of the unfinished ceiling. The weaker the dynamic stiffness s’ the better the footfall sound insulation (the permissible load of the footfall sound insulation must be observed). It is essentially attempted to prevent or minimise the induction of footfall sound into the structure and its transfer and deflection in the form of airborne noise. The deflection to the receiving room can be reduced by means of facing formwork. Figure 2 – Reduction of structure-borne sound Source: HFA planning brochure “Ceiling structures for multi-storey timber construction”, 2009 Flanking transmission / secondary sound paths Besides the separating component, also all flanking building parts are involved in the sound insulation between two rooms. The separating component is just one of the many transmission paths. For separating components with high sound insulation, the sound is transmitted for the most part via the flanking ceilings, roofs, interior and exterior walls. To optimise the sound insulation of components, it must be aimed for the lowest possible transmission via secondary paths. The extent of the transmission via secondary paths depends on the concrete construction situation. The forwarding of the sound is structurally prevented by a bearing on elastic interim layers (see Figure 3). Figure 3 – On the left, Rothoblaas XYLOFON and on the right, Getzner Sylodyn By planning in facing formwork and suspended ceiling structures, these additional measures can be reduced and, in part, they can even be omitted entirely. Source: Planning brochure of Holzforschung Austria The behaviour of solid timber structures is very different from solid mineral construction. Forecast models existing so far do not reflect the actual behaviour of solid timber structures. To be able to reliably fulfil the requirements for sound insulation and suitability for use, the components are frequently overdimensioned through substitute models and simplified conservative approaches and thereby become inefficient. Within the scope of the project “Vibro-acoustics in the planning process for timber structures” that is supported by binderholz and SaintGobain Rigips Austria among others, comprehensive measurements of the sound transmission via flanking components have been conducted (see Figure 4). A prediction model according to DIN EN ISO 12354 was developed, which considers the diverse transmission paths in the construction situation and nonetheless remains applicable for the construction practitioner. The model is being integrated in the new DIN 4109. Reduktion von Körperschall Floor screed TSD boards Filling CLT BBS unfinished ceiling Intermediate layer CLT BBS wall elastic planking Induction Insulation Insulation Deflection Insulation © Rothoblaas © Getzner Werkstoffe
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