Sound Transmission Class
Also known as: STC, sound transmission loss, acoustic isolation
Sound Transmission Class (STC) is a single-number rating used to describe how well building elements (walls, doors, windows) reduce sound transmission between adjacent spaces. Standardized by ASTM International and ISO, STC is calculated from sound transmission loss (STL) measurements across the speech frequency range (125 Hz–4 kHz). It is the primary metric used in building codes, product specifications, and acoustic design to ensure privacy, noise control, and occupant comfort.
Key highlights
- Simple, universally standardized single-number rating; widely recognized and used in building codes, product labels, and specifications.
- Based on the speech intelligibility frequency range (125 Hz–4 kHz), making it directly relevant to privacy and occupant comfort.
- Well-established correlation with subjective perception of sound isolation; published data on STC ranges and associated privacy levels guide design.
- Fast to calculate from measured STL data; allows rapid comparison of material and partition performance.
- Supported by extensive product databases and historical performance data, facilitating design decisions.
Intuition
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How it works
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When to use it
Use STC ratings when specifying building partitions for acoustic privacy (bedrooms, offices, conference rooms), designing noise barriers (highway, industrial), or meeting building code acoustic requirements. STC is particularly useful for speech and general background noise. Note that STC is less appropriate for low-frequency noise (HVAC, traffic bass); use weighted descriptors (C, Ctr) or full-spectrum analysis for comprehensive assessment.
Strengths & limitations
- Simple, universally standardized single-number rating; widely recognized and used in building codes, product labels, and specifications.
- Based on the speech intelligibility frequency range (125 Hz–4 kHz), making it directly relevant to privacy and occupant comfort.
- Well-established correlation with subjective perception of sound isolation; published data on STC ranges and associated privacy levels guide design.
- Fast to calculate from measured STL data; allows rapid comparison of material and partition performance.
- Supported by extensive product databases and historical performance data, facilitating design decisions.
- Single-number rating masks frequency-dependent performance; materials may isolate well at mid-frequencies but poorly at low or high frequencies.
- Poor prediction of low-frequency noise isolation; STC does not weight bass frequencies, yet traffic and mechanical noise often dominate low-frequency complaints.
- Dependent on test facility (reverberation chamber vs. field measurements); large variations in measured STL can arise from facility differences, making comparison problematic.
- Assumes diffuse sound field; does not account for flanking paths (sound traveling around the partition through HVAC ducts, structural connections), which significantly reduce in-situ performance.
- Increasingly criticized for its limitations in low-frequency prediction; newer metrics (NIC, NR, CI metrics) are being adopted in some regions.
Common pitfalls
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Applications
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Frequently asked
What does an STC rating of 50 mean in practical terms?
STC 50 means loud music or shouting is reduced to a murmur; normal speech is faintly heard. STC 50 is adequate for most residential and office applications. Speech privacy is reasonable but not complete; confidential conversations may still be overheard in quiet conditions.
Why is low-frequency noise a problem for STC?
STC is weighted toward speech frequencies (125 Hz–4 kHz) and does not reflect low-frequency performance. Bass from traffic, mechanical systems (HVAC), and subwoofers often penetrates even high-STC partitions. Use additional metrics (C, Ctr) or full STL analysis for low-frequency assessment.
How does installation affect STC performance?
Laboratory STC ratings assume ideal installation with no air leaks, properly sealed edges, and no flanking paths. Real-world performance is typically 5–10 dB lower if installation is poor. Caulking, gaskets, resilient channels, and careful sealing are critical to achieving rated STC.
What is the difference between STC and Rw (ISO rating)?
STC (ASTM E413) and Rw (ISO 717-1) are similar single-number acoustic ratings based on sound transmission loss. Minor differences in the reference contour and calculation method exist, so STC and Rw values are not directly interchangeable; typically Rw ≈ STC – 1 or 2 dB.
Can I add two STC ratings together?
No. Combining two partitions (e.g., a wall with STC 50 plus a window with STC 30) does not yield STC 80. The acoustic performance is dominated by the weakest element. Use STL curves and energy summation to combine multiple barriers.
Sources
- 1.ASTM E413-16 (2016). Classification for Rating Sound Insulation. American Society for Testing and Materials.
- 2.ISO 717-1 (1996). Rating of Sound Insulation for Buildings and Building Elements. International Organization for Standardization.
- 3.ASTM E2179-16 (2016). Standard Test Method for Measuring Sound Transmission Loss of Building Partitions by the Reverberation Method. American Society for Testing and Materials.
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Cite this page
ScholarGate. (2026, June 3). Sound Transmission Class. ScholarGate. https://scholargate.app/acoustics/sound-transmission-class