Rw Ratings Explained: Reading Acoustic Numbers Honestly
Every acoustic glazing quote carries a dB figure and almost nobody explains what it is measured against. Understanding three terms is enough to compare quotes properly and to spot a number that has been flattered.
Rw, and why it can mislead on traffic
Rw is a single-figure weighted sound reduction index derived from lab testing across sixteen frequency bands. The weighting curve emphasises mid frequencies, where speech lives. Road and rail noise carries far more low-frequency energy than the Rw curve assumes, so a window can post a strong Rw and still let the rumble through.
Ctr is the number to ask for
Ctr is a correction term, always negative, that adapts the rating to traffic-type spectra. An assembly rated Rw 45 with Ctr −6 performs like 39 against road noise. If a supplier quotes only Rw for a main-road property, ask for the Ctr figure. A reputable one will have it.
- Rw: general weighted index
- Rw + Ctr: realistic for road and rail
- 3 dB is a just-noticeable change; 10 dB reads as roughly half as loud
Lab figures versus your room
Lab tests use a perfectly sealed sample in a rigid test aperture. Your reveal is not perfect, your walls have their own transmission path, and your loft hatch, letterbox and adjacent trickle vents all flank the window. Real-world outcomes typically land a few decibels below the lab figure. Anyone promising the lab number in your bedroom is not being straight with you.
Frequently asked questions
For a main-road bedroom, an assembly in the low-to-mid 40s Rw with a modest Ctr penalty transforms the room. For a quiet street, the mid 30s is plenty.
Ready for a free, no-obligation quote?
South Yorkshire's specialists in secondary glazing — survey, measure, manufacture and install.