STC Ratings
25-70+
Sound blocking scale
NRC Values
0.0-1.0
Absorption scale
Frequency Range
20Hz-20kHz
Human hearing
01 — Sound Isolation
STC Ratings
Explained
Sound Transmission Class (STC) quantifies how effectively a building partition attenuates airborne sound. The STC scale ranges from approximately 25 (normal speech easily understood through the barrier) to 70+ (essentially soundproof). This metric is fundamental to residential acoustic design and code compliance.
According to ASTM International standards, STC is calculated by measuring sound transmission loss at 16 frequencies from 125 Hz to 4000 Hz, then comparing results to a standard reference contour. Higher STC ratings indicate better sound blocking performance.
Code Requirements
CHFA 2026 Multifamily Standards require:
- • STC 52 between units and corridors
- • STC 55 near noisy public spaces
- • STC 34-40 within units depending on construction
STC Rating Scale
Normal speech audible
Standard interior door
Loud speech audible
Standard interior wall
Loud speech faint
Improved wall construction
Speech barely audible
Code minimum between units
Essentially soundproof
High-performance construction
02 — Sound Absorption
NRC Values & Material Performance
Noise Reduction Coefficient (NRC) measures a material's ability to absorb sound energy across the frequency spectrum most relevant to human speech (250 Hz to 2000 Hz). NRC values range from 0.0 (perfect reflection) to 1.0 (perfect absorption).
Materials commonly used in residential acoustic treatment span this spectrum: painted concrete walls might achieve NRC 0.05, while specialized acoustic panels can exceed NRC 0.90. The ASTM C423 standard establishes testing protocols for determining NRC ratings, ensuring consistency in material specifications.
Understanding NRC
NRC is an average of absorption coefficients at four frequencies: 250 Hz, 500 Hz, 1000 Hz, and 2000 Hz. A material with NRC 0.80 absorbs 80% of sound energy and reflects 20%. Higher NRC values indicate better absorption performance.
Common Material NRC Values
| Material | NRC | Application |
|---|---|---|
| Painted concrete | 0.05 | Reflective surface |
| Carpet | 0.30 | Floor treatment |
| Curtains (heavy) | 0.50 | Window treatment |
| Acoustic foam (2") | 0.70 | Studio treatment |
| Fiberglass panels | 0.85 | Wall/ceiling |
| High-performance panels | 0.95+ | Professional studios |
03 — Treatment Systems
Absorption, Diffusion & Bass Control
Absorption
Converts sound energy to heat through porous materials. Reduces echo and reverberation for clarity.
- • Acoustic panels
- • Ceiling clouds
- • Fabric-wrapped fiberglass
- • Foam treatments
Diffusion
Scatters sound waves evenly, avoiding directional focus for immersive, natural sound.
- • Quadratic diffusers
- • Skyline panels
- • Poly-cylindrical
- • Binary arrays
Bass Traps
Target low-frequency buildup in corners and wall junctions. Essential for accurate bass response.
- • Corner traps
- • Broadband absorbers
- • Diaphragmatic traps
- • VPR (tuned) traps
04 — Construction
Isolation Techniques
Sound isolation requires different techniques than absorption. While absorption improves sound quality within a room, isolation prevents sound transmission between spaces. The most effective approaches combine multiple strategies: mass, damping, decoupling, and sealing.
ASC Iso-Wall System
Developed by Art Noxon, the Iso-Wall System uses resilient channels (flexible "Z"-shaped metal springs) between studs and drywall to "float" inner layers, reducing vibration transfer. WallDamp viscoelastic material converts low-frequency energy to heat via constrained-layer damping.
Key Principles
Mass
Heavier materials block more sound. Double drywall layers significantly improve performance.
Decoupling
Separating wall layers prevents vibration transmission. Resilient channels and isolation clips achieve this.
Damping
Viscoelastic materials convert vibration energy to heat, reducing resonance.
Sealing
Even small gaps dramatically reduce isolation. Acoustic sealant is essential at all joints.
Wall Assembly Comparison
2x4 studs, single drywall each side, fiberglass insulation
2x6 studs, double drywall one side, safe insulation, sealed outlets
Staggered studs or resilient channels, double drywall both sides, damping compound, full sealing