Soft Shoe Shuffle:
With the support of the Forest & Wood Products Research & Development Corporation, researchers have developed a new way to shield downstairs occupants from ‘thumps and bumps’ using engineering techniques that raise the acoustic performance of timber floors and ceilings to match that of a 150-millimetre thick concrete floor. The perception that timber floors have high-impact sound transmission in multi-storey residential apartments has limited their growth in Australasia, prompting a consortium of New Zealand building acoustics researchers and Australasian companies to tackle the problem. The project aim was to build on existing research by developing design recommendations for timber floor/ceiling systems with acoustic properties comparable to masonry floor constructions, while meeting acoustic requirements stipulated in the Australian and New Zealand building codes. Cost effectiveness and use of existing construction industry skills were also important factors. According to researcher Dr Grant Emms, of Scion, New Zealand, the study has produced significant findings in relation to the multi-residential housing market, especially low-rise buildings up to four storeys. “The impact of low-frequency noise holds back construction of multi-storey timber-frame housing, and for good reason,” he said. “Timber does have poor acoustics compared with concrete, even though it could potentially deliver better-value housing.” The project focused on the issue of low-frequency impact insulation – the cause of ‘bumps and thumps’ from above in multi-level residential buildings – and comprised three separate aspects: theoretical modelling, experimental testing and subjective testing. Theoretical modelling involved the development of an analytical model to describe the low-frequency impact insulation performance of timber floors and the effect rooms have on sounds produced by such floors. The model, which has been hailed as ‘novel’ in many respects and will be published in international journals, was used to predict the performance of a timber floor that had undergone certain parametric changes. Results will be used as a basis for design recommendations. The experimental testing program involved constructing and testing 25 floor designs in a purpose-built laboratory. The process consisted of low-frequency vibration measurements of the floor upper surface and ceiling, as well as higher-frequency measurements using standard procedures. Results of the low-frequency measurements were used to develop and validate the theoretical model. The subjective testing program consisted of recording the sounds made by various impacts on a selection of experimentally tested floors. These sounds, along with impact sounds from a 150mm thick concrete floor, were played back to 30 subjects in a listening room designed to simulate a living room. Subjects were asked which floor they would prefer to live with, based on the sounds. The approach was an innovative modification of previous tests. The results proved that it was possible for a timber floor to perform as well as a 150mm concrete floor. Design recommendations for production of timber floors with low-frequency performance comparable with concrete floors were developed based on the project outcomes. The key design recommendations – which can be used in isolation or together to produce a floor with effective low-frequency impact insulation – include:
- adding mass and vibration damping to the floor upper surface in the form of a granular material (consisting of a sand/sawdust mix);
- using independent ceiling joists to further insulate the ceiling from the rest of the floor; and
- making the ceiling heavier.“Industry partners such as CSR, Carter Holt Harvey and Winstone Wallboards (NZ) have been involved in this research and are now likely to take our findings and develop them further into a more complete building system,” Dr Emms said.
Key Points (break out box)
Timber framed floor/ceiling systems an perform acoustically as well as 150mm concrete floors.
Design alterations recommended are: adding mass and vibration damping to the floor’s upper surface, using independent ceiling joists and making the ceiling heavier.