Sound
Sound Transmission Class
STC
A single-number rating of how much airborne sound an assembly blocks, derived from laboratory measurements between 125 and 4000 Hz. It describes nothing below 125 Hz, where most complaints originate.
In more detail
STC compresses an assembly's airborne sound isolation into one number. A laboratory measures transmission loss across sixteen third-octave bands from 125 to 4000 Hz, and the resulting curve is fitted against a standard reference contour under ASTM E413. The single number is where that contour lands.
Because it is a curve fit, two assemblies with the same rating can behave differently — one strong in the mid range and weak at the low end, the other flatter. The rating also stops at 125 Hz, and most of what people complain about in houses has substantial energy below that: a subwoofer, a mechanical room, road traffic, footfall carried through a floor.
Laboratory and field results are different quantities. A lab test isolates the assembly. A house adds framing continuity, ductwork, outlets and gaps, and the field-measured equivalent — ASTC, under ASTM E336 — is routinely lower than the lab figure for nominally identical construction.
Why it matters
STC is the number written into most specifications for interior partitions, which makes it the number most likely to be read as a promise. A wall built to an STC 55 detail will not deliver STC 55 if drywall runs continuously into the adjacent room, if the electrical boxes are back to back, or if the assembly stops at a dropped ceiling. Specifying the rating and detailing the continuity are two separate pieces of work, and only one of them appears on the drawing.
The common misunderstanding
That a higher rating means the room will be quieter for the sound actually bothering someone. STC says nothing below 125 Hz, so it is close to silent on bass — the range behind most residential complaints. A wall can gain ten STC points and sound essentially identical to a person listening to music through it.
Related terms
On Compound
Where this term does real work — and where the decisions get made.
Sources
Last reviewed July 2026 by Compound Editorial. Definitions are educational and not project-specific engineering or medical advice.