Sound insulation ratings

Frequently asked questions

Can an STC figure be converted to an Rw?

Not by arithmetic, and the fact that they usually land within a point or two of each other is what makes the assumption dangerous. Both compress a measured curve of sound reduction against frequency into one number by sliding a reference contour until the deviations meet a rule — but the contours differ, the frequency bands included differ, and the rules about how much deviation is allowed differ. For a simple partition the two often agree closely; for constructions with an unusual curve — a lightweight panel with a pronounced dip, a double-glazed unit, a resiliently mounted lining — they can diverge by several points. So a product tested to one system has to be quoted in that system, and a specification asking for Rw is not satisfied by an STC figure however close it looks.

Why is the figure measured in a finished building always lower?

Because a laboratory measures the ELEMENT and a building measures the RESULT, and most of the difference is flanking. A lab test isolates a wall between two rooms that share nothing else; a real wall sits in a building with a floor running continuously under it, a ceiling void above it, a screed crossing it, services penetrating it and junctions at every edge. Sound takes all of those routes. That is why British regulation specifies DnT,w + Ctr measured on site rather than accepting a wall specification, why Canada introduced ASTC alongside STC, and why remedial work on noise between dwellings so often has to address the floor or the junctions rather than the wall everybody is complaining about. A wall achieving its lab figure in a building is the exception.

What is Ctr, and why does it change the answer so much?

It is a correction that re-weights the rating toward LOW frequencies, and it is included precisely because the single-figure rating without it flatters constructions that are poor at the bass end. The unweighted rating is dominated by mid frequencies — speech — while the noise that actually causes complaints between dwellings is traffic, music and footfall, all of which have most of their energy low down. Ctr is always negative and is larger for lightweight constructions, because mass is what stops low frequencies and a light wall has little. The practical consequence is that a lightweight partition can post a respectable headline figure and a poor one once Ctr is applied, which is exactly the case where occupants report that they can hear the neighbour's television through a wall that passed on paper.