Sound is defined as mechanical kinetic energy which is transmitted through elastic media by pressure fluctuations and molecular motion. After identifying the source of noise which induces vibrations in a component, acoustic design distinguishes between airborne sound and structure-borne sound.
- Airborne sound: air sound waves cause components to vibrate, and these vibrations are transmitted to adjacent rooms in the building. Sources of airborne sound include traffic, voices or music.
- Structure-borne sound: the sound of walking, banging, scraping furniture, etc. is transmitted to components and radiated as airborne sound into neighbouring rooms. Impact sound is particularly relevant to the acoustic design.
Improving the sound insulation of wall structures
The sound insulation of single-sided stud partitions is determined by their surface-related mass and deflection stiffness. The sound curve of an acoustic single-sided stud partition shows a peak at a rate of 6 dB per octave (an octave is a doubling in frequency), a drop in the coincidence cut-off frequency range, followed by another peak at a rate of 6 dB per octave. As doubling the mass increases sound insulation by 6 dB, to ensure good sound insulation (Rw > 50 dB), single-sided stud partitions require surface-related masses of greater than 250 kg/m².
For multi-layer panels with facing, greater sound insulation can be achieved with less mass. In such mass-spring systems, below the resonance frequency f0, the sound insulation increases at a rate of 6 dB per octave, however, above f0, it increases by 18 dB per octave. To achieve good sound insulation, the resonance must be as low in frequency as possible (≤ 100 Hz). Resonance frequency can be reduced by increasing the gaps between layers, increasing the vibrating mass and ensuring that facings are attached as flexibly as possible to the load-bearing wall. To avoid cavity resonance, the facing should be filled with sound-absorbing insulation material.
Improving the sound insulation of ceiling structures
The sound proofing of ceiling structures can be improved either by increasing the mass or by improving the decoupling. Adding mass by ballasting a raw ceiling or suspended ceiling reduces vibrations, causing less noise emissions. Above their resonance frequency, the transmission of component vibrations within the structure is reduced. Therefore, the resonance should be as low in frequency as possible (< 80 Hz).
In practice, this means installing relatively heavy screed (5–7 cm cement screed; note: the edge insulation strip is not cropped until the flooring has been laid) on a soft impact sound insulation board (s´ ≤ 10) with backfill to provide additional mass underneath. In the case of non-suspended ceilings, the thickness of the backfill must be increased to approx. 10 cm and, due to its high sound attenuation capacity, loose backfill should preferably be bonded. (The use of backfill should be discussed in advance with the screed installer). In terms of sound insulation, ceiling linings are most effective when decoupled (mounted on spring clips or hoops). Cavities should be insulated with mineral wool to prevent cavity resonance.
Soundproofing of CLT internal wall structures
Even if there are no specific soundproofing requirements for individual rooms within an apartment, sound insulation should still be borne in mind when planning buildings to ensure privacy and provide protection against noise. In addition to ensuring an acoustically favourable layout of rooms in the building, it also makes sense to incorporate individual structural components with high-quality sound insulation between noisy and quiet areas or between rooms used for different purposes (e.g between living areas and bedrooms). To support planners in selecting the appropriate structure, the sound insulation of a 100 mm-thick CLT wall with different types of cladding underwent a series of measurement tests in the laboratory for building physics at the Technical University of Graz.
Good sound insulation solutions can be found in our “Building physics catalogue” (see Technical folder CLT) and on the website www.dataholz.com, where you can discover a large number of popular layer structures with the respective sound insulation information. Sound insulation values from additional laboratory and site measurements are available on request or can be found in the building physics section of our Architects folder.