Sound Isolation and STC (Sound Transmission Class)

Posted by Auralex on 21st Jan 2026

STC (Sound Transmission Class) is a single rating number of how effectively a material or partition is at reducing airborne sound transmission between spaces. For example, ½” drywall has an SCT of 28.

The average commercial or residential room is constructed of ½” drywall and metal or wood 2”x4” studs and does not have internal wall insulation. This structure has an STC of 33.

Typical residential or commercial wall construction with an STC rating of 33

Sound Transmission Class (STC)

What it measures

STC applies to things like walls, doors, floors, ceilings, and windows
It measures how much airborne sound (speech, TV, music) is blocked
It does not measure impact noise (footsteps, vibration) or low-frequency bass very well

How the number works

STC is measured in a lab by testing how much sound is lost as it passes through a partition
The result is expressed as a single number—higher is better
Rough guideline:

STC 25 – Normal speech easily understood
STC 30 – Loud speech understood
STC 35 – Loud speech audible but not intelligible
STC 40 – Loud speech barely audible
STC 45–50 – Speech generally not heard
STC 55+ – Very good sound isolation (studios, high-end construction)

Chart showing different STC guidelines

What STC does not tell you

It downplays low-frequency sound (subwoofers, drums, traffic rumble)
It assumes perfect construction—real-world performance is often lower due to:

Gaps, cracks, or poor sealing
Flanking paths (sound traveling around walls)

For footsteps and structure-borne noise, IIC (Impact Insulation Class) is used. IIC is a measure of how well a floor-ceiling system blocks impact sounds, like footsteps or dropped items from your upstairs neighbors. One way it's measured is with a tapping machine. A standardized hammer taps the floor, and a measurement is taken in the room below.

Why STC matters

Architects and builders use STC to compare wall and floor assemblies
Studio designers use it as a baseline, but rely on more detailed data for isolation
Higher STC generally means better privacy and noise control

Quick examples