STC Ratings Explained – What It Means and Why It Matters
STC (Sound Transmission Class) is a single-number rating of how well a wall, floor/ceiling, door, or window reduces airborne sound. It’s calculated from transmission‑loss data measured in the lab (ASTM E90) and converted to STC using ASTM E413.
STC is great for speech frequencies but has limits below ~125 Hz; for traffic/aircraft noise see OITC. Building codes like the IBC typically require STC 50 (lab) or 45 (field) between dwelling units.
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What is STC?
STC (Sound Transmission Class) is a standardized way to compare the airborne sound isolation of partitions. Labs measure Transmission Loss in one‑third‑octave bands (per ASTM E90) and convert those data to a single STC number using ASTM E413. Internationally, the related metric is defined in ISO 717‑1.
What STC covers: mid/high speech bands. What it doesn’t: very low frequencies (sub‑125 Hz). For façade noise that is heavy in low frequencies (traffic, aircraft), consider OITC (rated via ASTM E1332).
Code requirements (residential multi‑family)
The International Building Code requires airborne sound control between dwelling/sleeping units of STC 50 (laboratory) or STC 45 (field). See IBC 2021 §1206 Sound Transmission.
STC rating chart: what the numbers feel like
| STC | Perceived performance |
|---|---|
| 25 | Very poor — normal speech easily understood |
| 30 | Poor — normal speech heard and understood |
| 35 | Fair — loud speech can be understood |
| 40 | Good — loud speech heard but not understood |
| 45 | Very good — loud speech barely audible |
| 50 | Excellent — very loud sounds faint |
| 60+ | Superior — shouting/hammering heard faintly |
Adapted from industry summaries and reference tables.
Typical STC ranges by element
- Windows: single‑pane ~27; double‑glazed upper‑20s to low‑30s; specialty “soundproof” windows can reach high‑40s/50s. See our guide to soundproofing windows.
- Common interior wall: 1/2″ drywall each side, wood studs, no insulation ≈ STC 33. Options to improve below.
- Doors: hollow core ≈ low‑20s; solid core ≈ 30–35; acoustic doors can reach 40–70+. See best soundproofing materials.
- Floors/ceilings: wood joist floor/ceiling ~40–43; 6″ concrete slab ~52–55.
What affects STC (and what to fix first)
- Mass: More layers of drywall, denser materials. See our primer on sound insulation vs regular insulation.
- Decoupling: Staggered/double studs, isolation clips + hat channel. Compare Green Glue vs resilient channel and staggered stud vs double wall.
- Damping: Constrained‑layer damping between sheets (e.g., Green Glue) converts vibration to heat.
- Sealing: Airtight perimeter + outlets sealed to stop flanking paths.
- Windows/doors: Use laminated/IGU windows and ensure tight seals; consider communicating doors. See window noise control.
How to raise STC (practical options)
- Add mass: Extra 5/8″ drywall layers. For masonry upgrades, see concrete wall soundproofing.
- Decouple one side: Isolation clips + hat channel (resilient systems compared).
- Create a double wall: Where space permits, a separated stud wall boosts TL; compare staggered vs double.
- Add damping: Use a viscoelastic compound between drywall sheets.
- Seal everything: Perimeter/backboxes/penetrations. For a budget‑first approach, see our cheap DIY soundproofing guide.
Important: Acoustic foam panels and décor absorbers reduce echo inside a room but do not materially raise STC of a partition. For their role, see Does soundproof foam work?
Frequently Asked Questions
What is a “good” STC rating?
For multi‑family separations, codes typically target STC 50 lab / 45 field (see IBC §1206). For bedrooms/home offices, many aim for STC 55+ to reduce intelligibility of loud speech.
Why does field STC differ from lab STC?
Real buildings have flanking paths and installation variability; field values (FSTC) are often lower than lab ratings. Lab tests follow ASTM E90, while ASTM E413 defines how to compute STC/FSTC from those measurements.
Does STC include low frequencies?
Not well. The standard contour focuses on 125–4,000 Hz. For low‑frequency‑heavy exterior noise, use OITC (per ASTM E1332).
Can acoustic panels raise STC?
Wall‑mounted absorbers tame echo in the room but don’t add mass or isolation, so they don’t meaningfully increase STC. To block sound, focus on mass, airtightness, damping, and decoupling.
Where can I see example STC values?
Reference tables (e.g., windows ~27–32, basic interior wall ~33) are compiled from lab data and industry summaries—see this overview and manufacturer data sheets.
