Sound Transmission Class (STC) Calculator
Fit 1/3-octave sound transmission loss (TL) data against the ASTM E413 reference curve, enforce 8 dB maximum single deficiency rules, and determine single-number STC ratings.
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Sound Transmission Class (STC) & ASTM E413 Contour Fitting
The Sound Transmission Class (STC) is an integer rating used in North America to quantify how effectively an interior building partition (wall, floor/ceiling, door, or window) attenuates airborne human speech sound. Testing is conducted across sixteen 1/3-octave frequency bands ranging from 125 Hz to 4,000 Hz per ASTM E90, and the resulting Transmission Loss (TL) curve is fitted against a standardized contour per ASTM E413.
ASTM E413 Fitting Rules
The standard reference contour is shifted vertically in 1 dB increments until two conditions are satisfied:
- 32 dB Sum Rule: The sum of all negative deficiencies (where measured TL falls below the reference contour) must not exceed 32 dB across all 16 frequency bands.
- 8 dB Maximum Single Band Rule: The negative deficiency at any single one-third octave band must not exceed 8 dB.
- STC Assignment: When the contour is placed at the highest integer level satisfying both rules, the STC rating equals the transmission loss value of the contour at 500 Hz.
Frequently Asked Questions
Why does a standard drywall wall perform poorly at low bass frequencies?
The STC rating curve only tests down to 125 Hz, completely ignoring low-frequency bass rumble from subwoofers, heavy trucks, and drum kits (30 to 100 Hz). Standard drywall walls experience a mass-air-mass resonance at 60-90 Hz where low-frequency bass passes straight through with almost zero attenuation.
What is the "coincidence dip" in drywall partitions?
Every rigid sheet material exhibits a critical coincidence frequency where the bending wavelength of the panel matches the wavelength of sound in air. For standard 1/2" or 5/8" gypsum board, this coincidence dip occurs between 2,000 and 3,000 Hz, causing a noticeable 6 to 10 dB drop in sound transmission loss.
What STC rating is required by the International Building Code (IBC)?
IBC Section 1206 mandates that separating walls and floor/ceiling assemblies between adjacent dwelling units (apartments, condominiums, hotel guest rooms) must achieve a laboratory test rating of at least STC 50 (or field test FSTC 45).
How does STC compare to European ISO Rw ratings?
The European Weighted Sound Reduction Index (Rw per ISO 717-1) tests from 100 Hz to 3,150 Hz. For most standard drywall and masonry partitions, Rw is within 1 to 2 dB of the North American STC rating (Rw ≈ STC - 1 dB).