STC (Sound Transmission Class)

Sound Transmission Class (STC) is a single-number rating of the airborne sound insulation performance of a building element (wall, floor, door, window), defined in ASTM E413. The rating is determined by fitting a reference contour to the measured transmission loss curve across 16 third-octave bands from 125 Hz to 4000 Hz. Higher STC indicates better insulation. The contour is shifted upward until deficiencies (measured TL below the contour) total no more than 32 dB, with no single band exceeding 8 dB. Typical values include STC 33 for single-pane glass, STC 45–50 for standard stud walls, and STC 55–65 for double-stud walls. STC is widely used in North American building codes (IBC 2021 requires STC 50 between dwellings). The international equivalent is Rw per ISO 717-1. STC does not evaluate frequencies below 125 Hz, a significant limitation for low-frequency noise sources like bass music and HVAC rumble.