STC Sound Ratings for Walls

STC Sound Ratings for Walls

Direct answer

What STC (Sound Transmission Class) measures, typical STC of single-stud, staggered-stud, and double-stud partitions, and the STC targets for offices, hotels, and multifamily party walls.

Key facts

What STC measures

Sound Transmission Class (STC) is a single-number rating of how well a partition blocks airborne sound — voices, music, television. It's derived from the wall's sound transmission loss measured across standard frequency bands in a laboratory (ASTM E90) and reduced to one number by fitting a reference contour (ASTM E413). Higher STC means more sound blocked: each ~10-point increase is subjectively about half as loud through the wall.

Two important limits:

• STC weights mid and high frequencies (roughly speech range) and de-emphasizes low bass, so it under-represents rumble, mechanical noise, and home theaters. For those, look at low-frequency transmission loss or OITC.
• STC is airborne only. Footfall and impact noise through floors is a different rating, IIC.

And lab STC is not field STC: real walls lose points to flanking (sound going around the wall via floors, ceilings, and shared structure) and workmanship. Field-measured ASTC / FSTC typically runs ~5 points below the lab number.

The three levers: mass, decoupling, absorption

Every high-STC wall pulls some combination of three levers:

1. Mass — heavier layers (extra gypsum) resist being driven by sound. Doubling mass adds only a few STC points on its own — useful but subject to diminishing returns.
2. Decoupling — separating the two faces so vibration can't pass straight through the framing. This is the biggest lever: double-stud walls, staggered studs, resilient channel, or sound-isolation clips.
3. Absorption — cavity insulation (mineral wool or fiberglass) damps the air cavity, which matters most once the faces are decoupled.

Typical STC of common partitions

These are typical, illustrative ranges — the governing value for a project comes from a tested assembly or a manufacturer's acoustical report.

The pattern: single-stud walls top out in the high-40s/low-50s no matter how much gypsum you add, because the studs still conduct. To reach the mid-50s and beyond you have to decouple — stagger or fully separate the studs.

STC targets by occupancy

For multifamily and hotels, the IBC sets a minimum airborne rating for separations between dwelling units and between units and public/service areas — commonly STC 50 (lab) or 45 (field). Many owners and hotel brands specify higher for marketability, and party walls between owned units (condos) are frequently pushed to STC 55-60.

Designing to a target

Because STC belongs to the assembly and is easily undone by flanking and penetrations, acoustic wall types should carry their target STC and tested design number the same way rated walls carry a fire design number — and outlets, back-to-back boxes, and door undercuts have to be detailed so they don't leak the rating away. Tracking wall types and their acoustic targets alongside fire ratings in one model keeps both intents coordinated as the plan changes.

Frequently asked questions

What is STC and what does it not tell me?

STC (Sound Transmission Class) is a single-number rating of how well a wall or floor blocks airborne sound — speech, TV, music — derived from lab measurements across frequencies per ASTM E90 and rated per ASTM E413. Higher is better. What it doesn't capture well is low-frequency sound (bass, mechanical rumble, home theaters), because the standard curve de-emphasizes low frequencies; for those, look at low-frequency data or metrics like OITC. STC also doesn't cover impact/footfall noise on floors — that's IIC.

How much STC do I actually need?

It depends on adjacency and privacy expectations, but typical targets are: STC 45-50 for office demising and normal speech privacy, STC 50 (lab) for code-minimum multifamily and hotel dwelling-unit separations, and STC 55-60+ for high-end hotels, party walls over noisy spaces, and condos where confidential-speech privacy is expected. Field performance (ASTC/FSTC) usually runs ~5 points below the lab STC because of flanking and workmanship, which is why codes pair an STC 50 lab requirement with an STC 45 field requirement.

What's the single most effective way to raise a wall's STC?

Decoupling the two faces so vibration can't travel straight through the studs — using a double-stud wall, a staggered-stud wall, or resilient channel / sound clips. Decoupling, combined with added mass (extra gypsum layers) and cavity insulation, is far more effective than any one of those alone. A single 2x4 stud wall might be around STC 33-40; the same wall decoupled and insulated can reach the mid-50s or higher.

Related

• Fire-Rated Wall Assemblies Explained (1-Hour vs 2-Hour)
• Understanding R-Value and U-Factor
• Typical Exterior Wall Assemblies (Layer by Layer)

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Source & method: STC per ASTM E90 (lab test) and ASTM E413 (rating); field performance per ASTM E336 / ASTC. IBC requires a minimum airborne rating for dwelling-unit separations (commonly STC 50 lab / 45 field). STC values shown are typical, illustrative ranges; the governing value comes from a tested assembly (UL/GA) or a manufacturer's acoustical test report.