Sound Rated Wall Calculator — Part E System Selection

Approved Document E specifies airborne sound insulation for separating walls as DnT,w + Ctr (not bare DnT,w). New-build separating walls between dwellings must achieve a minimum of 45 dB DnT,w + Ctr. Walls in dwellings formed by a material change of use (e.g. converting a house into flats), and walls in rooms for residential purposes, must achieve at least 43 dB DnT,w + Ctr. Ctr is the low-frequency spectrum adaptation term — it is a negative number (typically −4 to −16 dB) that penalises low-frequency (traffic/bass) performance, so it is the binding figure for lightweight stud-and-plasterboard walls. These are minimum values measured on site, so design with headroom for workmanship and flanking transmission.

Rw is the laboratory single-figure sound reduction index of the wall system itself, measured on the element alone. DnT,w + Ctr is the on-site weighted level difference with the low-frequency adaptation term, measured between the completed rooms, so it also carries room volume, partition area, reverberation time, flanking at every junction and workmanship. There is no arithmetic conversion between the two, and Approved Document E gives none, so no laboratory figure can be turned into a predicted site result. Compliance is shown by a pre-completion sound test under Regulation 41. For new-build houses and buildings containing flats only, Regulation 41 can instead be disapplied by notifying a Robust Detail before work starts on site (AD E para 0.2, Annex E). That exemption is not available for a material change of use or for rooms for residential purposes, because Annex E condition (a) requires the building work to be the erection of a new dwelling-house or a building containing flats, and Regulation 2 defines a room for residential purposes as a room which is not a dwelling-house or a flat. Note also that a Robust Detail exempts the work from testing; Annex E states it is not a deemed-to-satisfy condition, so the duty to comply with E1 remains either way.

It is a trade-off between isolation, floor space and cost, and the ordering is the useful part. A resilient bar with a single board is the least isolating of the four here and adds the least depth; a second board adds mass; a twin stud frame separates the two linings onto their own frames; an independent free-standing leaf is the most isolating and needs a clear gap from the existing wall, so it takes the most space. No decibel value is quoted for any of them on this page, because a laboratory Rw belongs to one specific tested build and these are generic descriptions: take the figure from your chosen manufacturer's tested system. All of them depend on sealing every perimeter and junction, because flanking transmission is included in the site measurement and excluded from the laboratory one.

Yes. Acoustic mineral wool (typically 100mm thick) is essential in all acoustic wall systems — it absorbs sound energy within the cavity. Without it, the cavity acts as a resonance chamber and can amplify certain frequencies. For stud walls the wool's job is cavity absorption, so its presence and the correct thickness matter more than precise density. Acoustic-grade products span roughly 10-60 kg/m³: Rockwool RWA45 is about 45 kg/m³, while lighter acoustic partition rolls such as Isover APR 1200 are around 10-12 kg/m³ and are also acceptable. Use acoustic-grade wool from brands like Rockwool, Knauf or Isover; denser slabs such as RWA45 are popular for friction-fit between studs.

Acoustic sealant is a permanently flexible, non-setting mastic applied at all junctions where the plasterboard meets the floor, ceiling and adjacent walls. It seals air gaps that would otherwise allow sound to flanking around the partition. Apply a continuous bead behind the board at all perimeter edges. Standard decorator's caulk is not an adequate substitute — use a proper acoustic sealant rated for the purpose.