What is the weighted sound reduction index (Rw)?

The Rw value is the weighted sound reduction index of a building element, expressed in decibels (dB). The figure indicates how much airborne sound a wall, floor, door or window holds back. The value is measured in a laboratory and reduced to a single number according to the standard EN ISO 717-1. The higher the Rw, the better the insulation.

How to read the figure

• Every extra 10 dB is perceived by the ear as roughly halving the sound.
• Correction terms C and Ctr: the Rw is often followed by two figures, for example Rw (C; Ctr) = 38 (-1; -4). C applies to high and mid-frequency sounds such as voices; Ctr to low, droning sound such as traffic and bass. If road traffic bothers you, look at Rw + Ctr.
• A lab value is not a real-world value. In an actual building, sound also travels through adjoining walls, floors, gaps and pipes (flanking transmission). The insulation measured in the building (shown with an apostrophe, R’w, or as DnT) is therefore lower than the Rw in the brochure.

Applications

You use the Rw value to compare and choose building elements:

• Internal walls: a light metal stud wall with a single layer of plasterboard on each side insulates far less than the same wall with double boarding and mineral wool in the cavity.
• Windows: insulating glass with unequal pane thicknesses and laminated glass performs better than standard double glazing with two identical panes.
• Doors: a solid door with a drop-down seal and draught-proofing all round insulates much better than a hollow internal door.

Rules of thumb

• Mass helps: a heavier, solid wall insulates better. Doubling the mass gives, in theory, about 6 dB.
• Decoupling helps more: two separate leaves with a cavity and soft insulation between them perform better than one leaf of the same total mass.
• The weakest point determines the result. A gap of a few millimetres, sockets back to back in the same spot on both sides, or an open plenum above a suspended ceiling can largely cancel out a good wall.

Related terms

• Sound insulation
• Sound
• Noise nuisance
• Glass wool
• Sound absorption

Want to know more about building terms? Visit our knowledge base at fredsdiyplans.com.