STC (Sound Transmission Class) is a single number that describes how well a wall, floor, door or window stops airborne sound passing through it. It is measured in a lab under ASTM E90 and calculated with ASTM E413. Higher is better: each 10 points roughly halves how loud the sound seems on the other side.
| STC | What you hear through the wall | Typical use |
|---|---|---|
| 25 | Normal speech clearly understood | Hollow-core door |
| 30 | Loud speech understood | Single glass, poor partition |
| 35 | Loud speech audible, hard to follow | Basic office partition |
| 40 | Loud speech heard as a murmur | Insulated stud wall |
| 45 | Loud speech barely audible | Good office or bedroom wall |
| 50 | Loud music faint; speech inaudible | Code minimum between apartments (IBC) |
| 55 | Loud music barely audible | Theaters, studios, hotel rooms |
| 60+ | Most sound inaudible | Recording studios, cinemas |
What STC covers, and what it does not
- It covers airborne sound in the speech range, about 125 to 4,000 Hz.
- It does not describe low bass well, which is why music and home theaters often need more than the STC suggests.
- It does not cover footsteps and impacts: that is IIC.
- It is a lab number. Real buildings usually measure 5–10 points lower because of flanking paths.
Acoustic panels are not rated for STC because they do not block sound; they absorb it inside the room. See STC vs NRC.