🎯 What Is STC?
STC (Sound Transmission Class) is a single-number rating that indicates how well a partition blocks airborne sound — voices, music, TV, traffic noise. Higher STC = better sound isolation.
- STC 25: Normal speech easily understood through wall
- STC 35: Loud speech can be heard but not understood
- STC 45: Must strain to hear loud speech
- STC 50+: Loud speech barely audible; code minimum for dwellings
- STC 60+: Most sounds inaudible; professional studio level
The 10 dB rule: Each 10-point increase in STC roughly halves the perceived loudness of transmitted sound. Going from STC 35 to STC 45 makes noise seem about half as loud.
📊 What the code actually requires
One number in this area is a requirement, and the rest are conventions. IBC 2021 §1206.2 covers walls, partitions and floor-ceiling assemblies separating dwelling units and sleeping units from each other or from public and service areas:
The 2022 California Building Code §1206.2 carries this text unchanged, so Title 24 is not stricter than the IBC on airborne sound between units.
Everything else is a convention, not a minimum. The figures widely quoted for home offices, conference rooms, medical exam rooms, studios and home theatres — STC 40, 45, 55 — are design targets that circulate in the trade. We could not attribute any of them to a code or a published standard, so this guide will not print them in a column headed “minimum”. If your project needs one, it comes from the specification or the client, and that is where to go and read it. Designing to a number nobody can source is how a wall ends up over-built in one place and under-built in another.
Lab vs. field: STC is a laboratory rating. Field performance is generally lower, because flanking paths and workmanship are real and a laboratory suppresses both. That is why the code names a separate field metric (NNIC) with its own, lower number, rather than asking a field measurement to meet the laboratory figure. Treat any single STC as carrying a spread of a few points before you have measured it — NRC rebuilt the same wall six times and got STC 45 to 46, and a two-layer version 56 to 59.
🔬 How STC Is Calculated
Transmission Loss (TL)
STC is derived from Transmission Loss measurements at 16 frequencies from 125 Hz to 4000 Hz. TL is the dB reduction at each frequency.
The STC Contour Method
A standard contour is shifted up until:
- No single TL value is more than 8 dB below the contour
- The sum of all deficiencies doesn't exceed 32 dB total
The STC rating is where the contour crosses 500 Hz when these conditions are met.
What TL Data Tells You
- Low TL at low frequencies: Wall lacks mass or has resonance problems
- Low TL at mid frequencies: Structural transmission or coincidence effect
- Relatively flat TL: Well-designed wall with good balance
🛠️ Key Soundproofing Principles
1. Mass
Heavier walls block more sound. Mass Law: every doubling of mass adds ~6 dB of TL.
- Add layers of drywall (5/8" Type X is denser than 1/2")
- Use mass-loaded vinyl (MLV) as an added layer
- Concrete and masonry are naturally high-mass
2. Decoupling
Breaking the vibration path prevents sound from traveling through the structure.
- Resilient channel: Metal strips that hold drywall away from studs
- Staggered studs: Alternating studs so drywall layers don't share studs
- Double walls: Completely separate frames with air gap
3. Absorption
Cavity insulation absorbs sound energy bouncing inside the wall.
- Mineral wool or fiberglass batts in cavity
- Doesn't add much by itself — works best with decoupling
- Dense batts (3+ pcf) work slightly better than fluffy insulation
4. Sealing
Gaps and leaks destroy STC. Sound finds the path of least resistance.
- Seal all penetrations (outlets, pipes, ducts)
- Acoustical caulk at perimeter (drywall-to-floor, ceiling, adjacent walls)
- Solid core doors with proper seals
The 1% Rule: A 1% opening in a wall can reduce effective STC by 10+ points. A gap under a door or unsealed outlet box can negate an expensive wall assembly.
⚠️ Common Mistakes
- Ignoring flanking paths: Sound travels through floors, ceilings, ductwork, and around walls — not just through them
- Resilient channel errors: RC must be installed correctly (drywall screws that hit studs = "short circuit")
- Shared outlets: Back-to-back outlets create a direct sound path
- HVAC connections: Ducts connecting rooms transmit sound; need lined duct and silencers
- Expecting too much from mass alone: Without decoupling, adding drywall layers has diminishing returns
📱 How to Get TL Data
From Manufacturer Testing
Many wall and floor assemblies have been lab-tested per ASTM E90. Look for:
- Gypsum Association GA-600 (drywall assemblies)
- UL Fire Resistance Directory
- USG, CertainTeed, National Gypsum technical guides
From Field Testing
ASTM E336 (ASTC) measures actual installed performance. Usually 3-7 points lower than lab STC.
Estimation
Use the calculator's Mass Law mode for quick estimates based on surface weight. Note: real assemblies outperform or underperform mass law depending on construction details.
📈 Comparing Wall Assemblies
Use the TL Curve Comparison feature to visually compare up to 6 different wall assemblies side-by-side:
- Add to Chart: Click "Add Current to Chart" to save the current assembly's TL curve
- Compare Options: Select different presets or enter custom TL data, then add each to build your comparison
- Toggle Visibility: Click the eye icon to show/hide individual curves
- Analyze Differences: See how walls perform at different frequencies — a wall might have a higher STC but weak low-frequency performance that won't block bass
Beyond STC: Two walls with identical STC ratings can have very different TL curves. Comparing curves helps you identify which wall better addresses your specific noise problems (low-frequency bass, mid-range speech, etc.).