Airborne sound (R'w) and impact sound (L'n,w) between dwellings are mandatory requirements in BR18 and DS 490, and they are met with carpentry decisions: double wall, offset framing, floating floor and decoupled ceiling. Here are the requirements, principles and the seals where sound otherwise escapes.
Introduction
Sound between dwellings is one of the places where the Building Code sets binding requirements, and where the carpenter's construction choice determines whether the requirement is met. The neighbour's television (airborne sound) and footsteps above (impact sound) are two different problems solved in different ways — but common to them is that the smallest rigid connection or leak destroys the effort.
Two types of sound — two target values
Airborne sound is sound that propagates through the air (speech, music, TV) and must be stopped by the wall's or ceiling's ability to dampen it. Impact sound is footstep noise that arises when someone walks or drops something on the floor above, and propagates as vibration in the structure itself. The two metrics are:
- 01R'w — vægtet luftlydisolation (dB), målt i den færdige bygning. Højere tal = bedre luftlydisolation. Mærket ' betyder, at det er den reelle bygningsværdi inklusive omvejstransmission (flanketransmission), ikke kun laboratorieværdien Rw.
- 02L'n,w — vægtet trinlydniveau (dB), målt i rummet under. Lavere tal = mindre trinlyd, altså det modsatte af luftlyd.
The binding requirements — BR18 and DS 490
Building Regulations BR18 (chapter 17, sound conditions) establish that dwellings must have satisfactory sound conditions and refer to the sound classes in DS 490. New dwellings must as a minimum meet sound class C. DS 490 has four classes (A is best, D most lenient), so you can negotiate better sound than the minimum.
How to stop airborne sound
Airborne sound insulation is built on three principles: mass (a heavy construction dampens sound better), separation (two layers not rigidly connected work much better than one thick one) and damping (insulation in the cavity that absorbs sound vibrations). In light timber construction, where mass is limited, separation is the strongest card.
- 01Double boarding: two layers of plaster on each side with staggered joints give more mass and close the cracks.
- 02Offset or double skeleton: two separate rows of posts, so the two wall sides do not share a post — sound cannot run stiffly through the frame. The simplest, major improvement in a lightweight wall.
- 03Cavity insulation: mineral wool in the cavity damps air vibrations; an empty double wall rings like a drum.
- 04Wider cavities: greater distance between the two wall sides improves especially the low sound (bass), which is hardest to stop.
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