STC Rating Calculator
Sound Transmission Class (STC) is the standard single-number rating for how much airborne sound a wall, floor or door blocks. It is defined by ASTM E413 from measured transmission loss across one-third-octave bands. This calculator uses your partition's STC and a source level to estimate the noise level reaching the other side and the perceived loudness reduction. Because perceived loudness roughly halves for every 10 dB, even a modest STC increase makes a clear difference. Enter the STC and the source level to see the transmitted level and how much quieter it sounds.
STC estimate formula
Transmitted level = source level - STC
Perceived loudness fraction = 2^(-STC / 10)
(Speech-range engineering approximation)
STC closely approximates speech-range attenuation in dB. The perceived-loudness rule uses the 10 dB per halving of loudness convention. Both ignore frequency detail and flanking transmission.
STC privacy guidance
- STC 25: normal speech easily understood through the wall.
- STC 35: loud speech audible but largely unintelligible.
- STC 45: loud speech barely heard.
- STC 50+: good privacy, loud sounds only faintly heard; common code minimum for party walls.
- STC understates low-frequency noise; consider OITC for bass-heavy sources.
STC rating: frequently asked questions
What is Sound Transmission Class (STC)?
STC is a single-number rating of how well a building partition such as a wall, floor or door reduces airborne sound. It is derived per ASTM E413 by fitting a standard contour to the measured transmission loss across sixteen one-third-octave bands from 125 to 4000 Hz. A higher STC means more sound is blocked. A typical interior wall is around STC 35, while a high-performance party wall can exceed STC 60.
How much does each STC point reduce sound?
Roughly speaking, the STC number approximates the decibel reduction of speech-range sound through the partition. So a source at 70 dB on one side of an STC 45 wall arrives at about 25 dB on the other side, treated here as source level minus STC. This is an engineering approximation; exact attenuation varies with frequency and flanking paths.
Why does a 10 dB reduction sound like half as loud?
Human loudness perception follows roughly a power law: a 10 dB reduction in level is perceived as about half the loudness. This calculator estimates the perceived loudness fraction as 2^(-STC/10), so an STC 30 wall makes sound about one eighth as loud, and STC 50 makes it about 1/32 as loud, ignoring frequency detail.
What STC do I need for privacy?
As a guide, STC 25 lets normal speech be understood; STC 35 makes loud speech audible but mostly unintelligible; STC 45 means loud speech is barely heard; STC 50 and above provides good privacy where loud sounds are faint. Building codes specify minimum STC values for party walls and floor-ceiling assemblies, commonly STC 50.
Does STC account for low-frequency noise like bass?
No. STC is based on the 125 to 4000 Hz range and is weighted toward speech frequencies, so it understates how much low-frequency noise (home theater bass, machinery) gets through. For low-frequency-heavy sources, designers also consider the OITC rating and the partition's performance below 125 Hz.
Official sources
- ASTM International: Standards including E413 STC classification.
- NIST: National Institute of Standards and Technology.
Reviewed by the CalculatorHub team, edited by James Graham, 17 June 2026. See our methodology.