STC Ratings for Walls: STC 45, 50, 55 and Code Minimums

Direct answer

Sound Transmission Class condenses a partition's measured transmission loss across sixteen one-third-octave bands into one number, using laboratory data from ASTM E90 and the rating procedure in ASTM E413. Higher is better, and the scale is not linear in perceived terms: an increase of about 10 points is heard as roughly halving the loudness coming through. Three levers move the number — adding mass, decoupling the two faces so vibration cannot cross the cavity, and filling the cavity with absorption — and decoupling is by far the most efficient of them. Model codes commonly require STC 50 for laboratory-tested assemblies between dwelling units, or a normalized field rating (NNIC) of 45 where field-tested, and the difference between those figures is the whole problem: flanking paths through the floor, ceiling plenum, back-to-back outlets and unsealed perimeters routinely cost a built wall points its tested design never lost.

Content and cited inputs reviewed by Harth team · Published 2026-08-14 · Updated 2026-08-14.

What does this term mean?

Sound Transmission Class (STC) is a single-number rating of a partition's airborne sound insulation, calculated under ASTM E413 from transmission-loss data measured to ASTM E90. It describes the assembly as tested in a laboratory, not as built on site.

What are the key takeaways?

• What STC measures: Airborne sound loss
• Test standard: ASTM E90 / E413
• Multifamily party wall (code): STC 50 lab / NNIC 45 field
• Biggest gains: Decoupling + mass + insulation

Which tested configuration and limitations apply?

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.

What STC 45, 50, 55 and 60 sound like

The number is easier to specify once you know what each step lets through. A widely used industry scale — this one from the Walls & Ceilings Center's technical bulletin on steel-stud walls — describes lab-rated partitions roughly as follows:

STC 55 is not a step of its own on this scale; it falls between the STC 50 and 60+ descriptions. Treat the scale as a guide to lab ratings — the as-built wall will usually land lower.

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 wood-stud walls top out in the mid-40s no matter how much gypsum you add, because the studs still conduct. Light-gauge steel studs flex and do better (see the tested values below), but to go reliably past the mid-50s you have to decouple — stagger or fully separate the studs.

Tested STC values: wood studs versus steel studs

The illustrative ranges above blur one of the biggest variables — the stud itself. Light-gauge steel studs flex more than wood studs and pass less vibration from face to face, so the same board and insulation score noticeably higher on steel. Laboratory results compiled in Owens Corning's Wall & Floor Assembly Guide (NRC Canada and other lab tests, Type X gypsum board on both faces, studs at 16 or 24 in. o.c.):

The Walls & Ceilings Center bulletin reports the same pattern for 25-gauge steel studs at 24 in. o.c.: 3-5/8 in. studs with one layer of 5/8 in. Type X each side and batts rate STC 50, and two layers each side with batts rate STC 58. The practical reading for multifamily: a single wood-stud wall does not reach STC 50 by adding board alone — it needs resilient channel, staggered studs or a double-stud wall — while a single row of nonbearing steel studs can. The steel figures are for nonbearing light-gauge studs; don't assume they carry over to load-bearing steel framing without a test for that gauge.

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.

The IBC minimum and how STC is measured

IBC Section 1206: dwelling and sleeping units

Section 1206 covers walls, partitions and floor-ceiling assemblies between dwelling or sleeping units, and between those units and public or service areas. The field-test metric changed after the 2018 edition:

Three details sit in the same section. The rating may instead be shown by engineering analysis comparing the design with ASTM E90-tested assemblies, and the 2024 edition requires that analysis to be done by a registered design professional. Penetrations for piping, electrical devices, recessed cabinets, bathtubs, soffits and ducts must be sealed, lined or insulated to keep the rating; entrance doors are exempt but must fit tightly to frame and sill. And the 2021 IBC (§1206.2.1) lets the STC of concrete and clay masonry be calculated under TMS 302 as an alternative to testing.

Lab versus field: STC, ASTC, NIC and NNIC

• STC comes from a laboratory test to ASTM E90, built so the specimen is the only meaningful sound path, with the one-third-octave results reduced to a single number by ASTM E413.
• ASTC (apparent STC) is measured in a finished building under ASTM E336 and describes the partition's apparent performance with flanking paths included.
• NIC and NNIC also come from ASTM E336 but describe room-to-room isolation; NNIC normalises the result to a furnished room of at least 25 m³, which is why the 2021 and 2024 IBC use it as the field metric.

So "how do I find a wall's STC?" has three honest answers: cite an ASTM E90 test report for the same assembly (UL, Gypsum Association or a manufacturer's acoustical data), submit an engineering comparison to tested assemblies, or — for masonry — calculate it. A field test on the finished wall returns ASTC or NNIC, not STC.

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.

STC Ratings for Walls: STC 45, 50, 55 and Code Minimums: tested-assembly evidence record

Stop condition: a component description, rule of thumb or generated diagram is not evidence of a fire, acoustic or thermal rating. Use the complete current listing and verify every departure.

Where can you explore related guidance?

• Explore all assemblies guidance
• Fire-Rated Wall Assemblies: 1-, 2- & 3-Hour Walls Explained
• Understanding R-Value and U-Factor
• Typical Exterior Wall Assemblies (Layer by Layer)

Sources and 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. Official references: UL Product iQ; Gypsum Association; 2021 IBC Chapter 12; 2024 IBC Chapter 12; ASTM E413; ASTM E336; Owens Corning Wall & Floor Assembly Guide; Walls & Ceilings Center Technical Bulletin 5.001.

Source links

• ul.com

• gypsum.org

• codes.iccsafe.org

• codes.iccsafe.org

• astm.org

• astm.org

• insulation.owenscorning.ca

• wccinfo.org