How do I calculate composite STC for a wall with a door or window?

Convert each element's STC to a transmission coefficient, area-weight them, then convert back. Composite transmission tau = sum(Si times 10^(-STCi/10)) divided by sum(Si), and composite STC = -10 times log10(tau). Because transmission is energy-based, a small low-STC door or window can dominate the result and pull the whole wall's rating down.

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Free Acoustics Toolkit · Architectural

STC / NRC Material Selector & Composite-Wall Calculator

Q: What is a good STC rating?

STC (Sound Transmission Class) rates a partition's airborne sound isolation; higher is better. As a rough chart: a single-stud gypsum wall is about STC 34, a staggered-stud insulated wall about STC 50, a double-stud wall about STC 60, and 8-inch CMU about STC 48. STC 50 is a common code minimum between dwelling units; STC 55 to 60 is typical for good speech privacy.

Q: What are typical NRC values?

NRC (Noise Reduction Coefficient, per ASTM C423) is the average absorption from 0 to 1. Typical NRC values: painted concrete about 0.05, carpet about 0.30, acoustic ceiling tile about 0.55 to 0.70, and a 2-inch fiberglass panel about 0.90. STC measures sound blocked between spaces; NRC measures sound absorbed within a space. They solve different problems.

Look up typical STC and NRC values for common walls, doors, glazing and ceilings — then compute the real composite STC of a wall once a door or window penetrates it. A wall is only as good as its weakest opening, and this tool shows you exactly which element is controlling the result.

Method reviewed by CSTI's INCE Board-Certified engineers

Composite wall calculator

Enter the main wall, then the one door or window that penetrates it. The tool combines them by transmitted sound energy — not by a simple average.

Wall area (ft²)

Wall STC

Door area (ft²)

Door STC

Tip: use the reference chart below to grab a starting STC for the wall and the penetration.

Result

Wall + door, combined by area-weighted transmission

Composite STC

—STC

Dominant leak path

—%

STC & NRC reference chart

Typical lab-tested values for common assemblies. Real ratings vary with construction, sealing and flanking — use these to size the problem, not to certify a design.

Transmission loss · STC (ASTM E413)

Single-stud gypsum wallSTC 34

Staggered-stud, insulatedSTC 50

Double-stud wallSTC 60

8" CMU (concrete block)STC 48

Single glazing (¼" glass)STC 27

Insulated glazing unit (IGU)STC 35

Solid-core wood doorSTC 30

Absorption · NRC (ASTM C423)

2" fiberglass panelNRC 0.90

Acoustic ceiling tileNRC 0.55–0.70

Carpet on padNRC 0.30

Painted concrete / hard wallNRC 0.05

NRC rates how much sound a surface absorbs within a room (0–1). STC rates how much sound a partition blocks between rooms. Reverberation problems need NRC; privacy problems need STC.

The method (ASTM E413 · C423 · E90)

Composite transmission τ = Σ ( Sᵢ · 10^(−STCᵢ / 10) ) / Σ Sᵢ (area-weighted energy)

Composite STC = −10 · log₁₀( τ )

Worked example: a 100 ft² wall rated STC 50 with a 20 ft² door rated STC 30. τ = (100·10^(−5.0) + 20·10^(−3.0)) / 120 = (0.001 + 0.020) / 120 = 1.75×10⁻⁴. Composite STC = −10·log₁₀(1.75×10⁻⁴) = 37.6. The door is only 17% of the area but carries about 95% of the transmitted energy, dragging the STC 50 wall down to ≈38. That is why a partition is only as strong as its weakest penetration. STC values here are laboratory ratings per ASTM E90; the field-measured value (ASTM E336, reported as ASTC) is the defensible number of record. See ASTM E413 and ASTM C423.

Frequently asked

What is a good STC rating? (STC rating chart)

Higher STC means better airborne sound isolation. As a quick chart: single-stud gypsum ≈ STC 34, staggered-stud insulated ≈ STC 50, double-stud ≈ STC 60, 8" CMU ≈ STC 48, single glazing ≈ STC 27, an IGU ≈ STC 35, and a solid-core wood door ≈ STC 30. STC 50 is a common code minimum between dwelling units; STC 55–60 gives good speech privacy.

How do I use a composite STC calculator for a wall with a door or window?

You can't average STC ratings — transmission is energy-based, so you convert each element to a transmission coefficient, area-weight them, and convert back: τ = Σ(Sᵢ·10^(−STCᵢ/10)) / ΣSᵢ, then composite STC = −10·log₁₀(τ). A small, low-STC door or window can dominate the sum and pull the whole assembly's rating down, which is exactly what this calculator surfaces as the weak link.

What are typical NRC values, and how is NRC different from STC?

NRC (Noise Reduction Coefficient, ASTM C423) is average absorption from 0 to 1: painted concrete ≈ 0.05, carpet ≈ 0.30, acoustic ceiling tile ≈ 0.55–0.70, a 2" fiberglass panel ≈ 0.90. NRC measures sound absorbed within a room (it fixes echo and reverberation); STC measures sound blocked between rooms (it fixes privacy). They are not interchangeable.