Controlling sound is essential to creating functional, comfortable spaces — whether ensuring speech privacy in an office, protecting a studio, or preventing sound from traveling between apartments. The primary metric used in the United States is the Sound Transmission Class (STC) rating: a single number describing how well a partition reduces airborne sound transmission. A higher STC means better sound insulation.
How is an STC rating measured?
STC is determined in a controlled laboratory per ASTM E90. A sample partition is built between an isolated source room and receiving room; a source generates noise across 16 one-third-octave bands from 125 Hz to 4000 Hz; microphones measure the Transmission Loss (TL) at each frequency; and the resulting curve is fitted to a reference contour per ASTM E413. The STC rating is the value on the fitted contour at 500 Hz.
What do different STC ratings mean?
| STC | Audibility of speech | Example |
|---|---|---|
| 25–30 | Normal speech easily understood | Basic interior wall |
| 35–40 | Loud speech audible, not intelligible | Standard office/residential wall |
| 45–50 | Loud speech not audible — privacy begins | IBC apartment walls, conference rooms |
| 55–60+ | Very loud sounds faintly heard | Luxury condos, studios, sensitive labs |
The International Building Code requires a minimum STC of 50 for assemblies separating dwelling units. For sensitive spaces like executive offices, STC 55+ is often specified.
Limitations of the STC system
The most significant limitation is frequency range: STC only measures down to 125 Hz, so it does not account for low-frequency noise such as home-theater bass, heavy machinery rumble, or traffic and aircraft. For those situations, the Outdoor-Indoor Transmission Class (OITC) — which extends down to 80 Hz — is more appropriate. STC is also a laboratory rating; real installations are subject to flanking paths (ductwork, ceiling plenums, gaps around outlets) that reduce real-world performance.
How to improve an assembly's STC rating
- Add mass. Thicker or extra layers of drywall, or mass-loaded vinyl, make partitions harder for sound to vibrate.
- Improve absorption. Filling a stud cavity with fiberglass or mineral wool dissipates sound energy.
- Decouple the structure. Double-stud walls, staggered studs, resilient channels, or isolation clips break the vibration path.
- Seal all gaps. Acoustic sealant at joints and penetrations prevents small leaks from ruining an otherwise strong assembly.
For a truly effective design, look beyond the single number — understand the limits of STC, consider OITC where low-frequency noise matters, and treat the entire assembly, from materials to construction quality to sealing.