Sound Transmission Class (STC)
Definition
Sound Transmission Class (STC)
Sound Transmission Class (STC) is a single-number rating that quantifies a wall, floor, or ceiling partition's ability to block airborne sound transmission. Defined by ASTM E413, STC is derived by fitting a reference contour to 1/3 octave transmission loss data from 125 Hz to 4000 Hz, subject to an 8 dB single-band deficiency limit and 32 dB total deficiency limit. SonaVyx calculates STC from measured sound reduction data.
STC = reference contour value at 500 Hz after best-fit shifting per ASTM E413 rules
How It Is Measured
STC is measured per ASTM E90 by generating noise in a source room, measuring the SPL on both sides of the partition, and computing the transmission loss (TL) at each 1/3 octave band. SonaVyx computes STC from the measured TL data by implementing the ASTM E413 contour-shifting algorithm with both the 8 dB single-band deficiency rule and 32 dB total deficiency limit.
Practical Example
A building inspector measures sound levels on both sides of a hotel room partition wall. Source room SPL averages 95 dB and receiving room SPL averages 48 dB, with RT60 of 0.5s in the receiving room. SonaVyx standardizes the level difference and computes Rw = 49 (ISO 717-1) and STC 50 (ASTM E413), meeting the IBC minimum requirement of STC 50 for hotel room separations.
STC Ratings and Building Codes
The International Building Code (IBC) requires STC 50 for walls and floors between dwelling units. Higher ratings are recommended for premium construction: STC 55 provides noticeable improvement, STC 60 blocks most speech, and STC 65 is suitable for music studios. A standard single-stud drywall partition with insulation achieves STC 45 to 52 depending on construction details. Double-stud or staggered-stud walls reach STC 55 to 65.
STC vs Rw
STC is the North American standard (ASTM E413), while Rw is the international equivalent (ISO 717-1). Both use similar contour-matching methods but differ in frequency range (STC: 125-4000 Hz in 16 bands; Rw: 100-3150 Hz in 16 bands), deficiency rules, and reference contour shapes. The numerical values are typically within 1 to 2 points of each other for most partition types. SonaVyx calculates both ratings from the same measurement data.
Low-Frequency Limitation
STC evaluates transmission loss from 125 Hz upward, ignoring frequencies below where many noise complaints originate (bass from music, traffic rumble, mechanical equipment). The Rw system addresses this with spectrum adaptation terms C and Ctr: Rw + C rates performance for pink noise (higher frequencies), while Rw + Ctr rates performance for traffic noise (emphasizing low frequencies). SonaVyx reports both C and Ctr values for complete characterization.
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