Acoustics
Improvement from adding a board layer (mass law)
- where
- improvement in sound reduction (dB)
- existing leaf surface mass (kg/m²)
- added layer surface mass (kg/m²)
- new surface mass (kg/m²)
Explain this equation
Estimates what adding a board layer to a single leaf does to its sound reduction, using the mass law.
m1 is the surface mass of the existing leaf and m2 that of the added layer, both in kg per m². mt is the new total. ΔR is the gain in decibels.
In the mass law the sound reduction goes with 20 log of the mass, so the frequency and constant cancel when you compare before and after, leaving only the mass ratio (m1 + m2)/m1.
Sense check: doubling the mass gives 20 log 2, about 6 dB. Adding a 12.5 mm standard board to a dense acoustic board gives less than that because the ratio is smaller.
Watch out: the real gain on a framed wall depends on the system and on sealing gaps; air leaks can cost more than an extra layer gains.
Mass-law estimate of what a second (or heavier) layer of board adds to a single leaf.
Enter your values to see the result.
Good to know
Doubling the mass of a leaf adds about 6 dB in the mass-controlled range; adding a quarter more adds under 2 dB. That is why upgrades often use a dense board (SoundBloc about 11–14 kg/m²) rather than another standard board, and why sealing gaps and adding cavity insulation or resilient bars can do more than extra mass. Surface masses come from the board data sheets.
Common mistake
Expecting two extra layers to double the sound insulation in dB. Each doubling of mass gives about 6 dB, so the returns fall off quickly.
Limits
Applies to layers fixed solidly together on one leaf, below coincidence. Real gains on a framed wall depend on the system; use tested Rw figures for compliance.
Sources
- NRC Canada, Canadian Building Digest CBD-239: Sound transmission through walls (TL = 20 log(m·f) − 48, m in kg/m², f in Hz; +6 dB per doubling of mass or frequency; coincidence dips)
- D. W. Herrin, University of Kentucky Vibro-Acoustics Consortium, Empirical models: normal-incidence mass law TL = 20 log(ρs·f) − 42 dB; field incidence TL = normal − 5 dB
- Gyproc (Saint-Gobain) Plasterboard Selector Guide: board weights, e.g. WallBoard 12.5 mm 8.45, 15 mm 10.5; SoundBloc 12.5 mm 11.4, 15 mm 14; FireLine 12.5 mm 12, 15 mm 14; Plank 19 mm 16.6 kg/m²
Results are calculations from the published methods and manufacturer figures shown, and depend on the data you enter. Always follow the manufacturers’ system and product data, BS 8212, BS EN 13914 and Building Regulations. Coverage rates vary by product, background and thickness. Check prices, VAT and CIS rules with your own records and HMRC.