STC vs OITC: Key Differences and When to Use Each

STC vs OITC comes down to where the noise starts. STC rates how well an interior wall blocks speech and other indoor sound. OITC rates how well an exterior wall or window blocks aircraft and road noise.

A wall can post a strong STC and still let jet rumble through, because STC stops scoring at 125 Hz and misses the low-frequency energy where jet rumble concentrates; OITC reaches down to 80 Hz to account for more of it.

Our temporary walls carry ASTM E90 lab data, and we’ve seen contractors build behind them in live hospitals and airports. You’ll learn what each rating measures and which one governs the walls inside an occupied building.

STC vs OITC: At a Glance

What Is STC?

Sound Transmission Class (STC) is a single-number rating of how well a partition stops airborne sound. A lab measures transmission loss through the assembly under ASTM E90, then ASTM E413 fits a reference contour to the curve and sets the STC rating. Higher numbers stop more.

STC describes a whole assembly, so framing and cavity insulation both move the number. The Gypsum Association puts insulation’s share at 2 to 4 points in a wood-framed wall.

What Is OITC?

Outdoor-Indoor Transmission Class (OITC) rates how well a building envelope cuts noise that starts outside. ASTM E1332 builds its reference spectrum from an average of three transportation sources:

• Aircraft takeoff

• Road traffic

• Diesel locomotive

That spectrum leans low, so glass and thin cladding take the hit.

Picture a terminal with a glass curtain wall facing the apron. Takeoff noise hits that glazing directly, so the glass usually sets the facade’s OITC.

OITC joins STC and other acoustic ratings on a spec, each scoring a different noise path.

STC vs OITC: Key Differences

These differences explain why one wall earns two scores and how that changes your spec.

Low-Frequency Coverage

Transportation noise concentrates in the low frequencies, so OITC starts scoring at 80 Hz. ASTM E1332 cites studies showing that band is necessary for acceptable correlation, and STC skips both the 80 Hz and 100 Hz bands.

AcousPlan puts numbers on that skipped band. An STC 55 wall might deliver only 35 dB of transmission loss at 63 Hz, well below anything STC scores. A high STC describes how a wall handles voices, and it says little about rumble.

Score Difference

Both ratings start from the same ASTM E90 transmission loss curve, so one test report can feed both, but the weighting changes the result. STC listens for voices, and OITC listens for engines.

Lightweight stud walls show the widest spread. In ROCKWOOL’s tests, OITC trails STC by 6 to 25 points, with most pairs 13 to 20 apart.

CMHA’s STC and OITC equations put 8-inch hollow block about 6 apart, and Viracon’s glass units land 2 to 6 apart.

A lightweight window with strong mid- and high-frequency loss but weak low-frequency loss scores well on STC and worse on OITC. Two specimens with the same rating can still sound different indoors.

Keep comparisons inside one rating, because an OITC of 34 and an STC of 34 describe different noise.

Typical STC and OITC Values

The chart pairs STC and OITC for common assemblies. The concrete-block values are weight-based estimates, and the glass values are glazing only, so check a product’s own test report before specifying a finished window.

Figures from ROCKWOOL’s Acoustic Wall Assemblies Catalog, CMHA’s TEK 13-01D and TEK 13-04A, and Viracon’s Acoustic Performance Data Tables.

Test Method

ASTM E90 supplies the transmission loss data behind both ratings. Labs mount the specimen between two calibrated rooms and record how much broadband sound gets through.

E90 now requires labs to report 80 Hz data on exterior facade elements, so OITC can come straight from the E90 report.

ASTM E1332 tells you to use single-number ratings with caution. Its standard spectrum can ask for more low-frequency loss than a site needs, as with tire-dominated freeways or train horns.

Code Reach

IBC §1206.2 requires STC 50 (lab-tested) or NNIC 45 (field-tested) for walls separating dwelling units or sleeping units from each other or from public or service areas.

Section 1206 applies only to those spaces, so a temporary wall’s STC target comes from the project spec.

Confirm any OITC requirement with the authority having jurisdiction before you issue the spec.

How to Choose STC vs OITC

Trace the sound from its source to the listener. A path through a partition calls for STC, and a path through the facade calls for OITC.

Use STC when:

• A wall separates a corridor from a patient room, where speech privacy and patient rest set the target.

• Office wall partitions separate tenant spaces, so seal them to the ceiling grid before chasing a higher number.

• A temporary wall stands between a work zone and occupied space.

Use OITC when:

• A patient room’s window faces a busy street or flight path.

• An office tower’s curtain wall faces a highway.

• A project spec or local noise rule names OITC.

A project with exterior glass and interior partitions needs both ratings, each with its own minimum. Sarasota-Bradenton International Airport shows the split: the terminal facade answers to OITC, while inside, STARC’s RealWall™ panels contained the escalator-replacement noise on the interior walls that STC governs.

Reading a Test Report

Ask for these four items before you accept a rating:

• Band-by-band transmission loss data

• Specimen size and construction details

• Whether the tested assembly included a door or glazing

• Lab name and test date

Valley Acoustics shows a hollow door dragging an STC 55 wall down to about STC 28. Check the door rating before you trust the wall rating.

STC vs OITC for Occupied Renovations with STARC

On an occupied renovation, the design team sets OITC for the exterior glass, while you control the temporary wall that stands between your crew and the occupants.

RealWall installs 100 feet in an hour with two workers, and its foam core cuts jobsite noise. Intertek verified RealWall at ASTM E84 Class A, and it’s non-fire-rated.

Noise ranks among the toughest jobsite challenges in an occupied building. At Fresenius Medical Care’s Waltham headquarters, J Calnan & Associates controlled it with RealWall across a fast-track, three-phase build-out. John Noone, the firm’s VP of Field Operations, said the walls kept the site quiet and the dust out while employees stayed at work nearby.

At Brigham and Women’s Hospital, Suffolk Construction also ran STARC’s ICRA barriers through a postpartum renovation and noise dropped by up to 50%, with patient-satisfaction scores improving through the work.

FireblockWall™ holds a one-hour rating under ASTM E119 and an STC of 40. Pick it when the code requires a rated wall.

Not sure which wall your job calls for? Our Wall Wizard asks about your environment and work zone, then recommends a system from the four-system lineup. Run your room through it, then request a quote for the system it picks.

Frequently Asked Questions

Is OITC the same as STC?

No, OITC and STC rate different noise. OITC scores outdoor aircraft and road noise from 80 to 4000 Hz, and STC scores indoor speech-range noise from 125 to 4000 Hz. Compare scores only within one rating.

Does OITC convert to STC?

No, OITC doesn’t convert to STC, because the two ratings weigh different frequency ranges against different reference spectra. In tested stud walls OITC trails STC by 6 to 25 points, while masonry and glass sit closer, so the offset varies by assembly.

Is a higher OITC rating better?

Yes, a higher OITC rating is better, because the assembly blocks more outdoor transportation noise. Check the glazing first, since glass usually sets a facade’s rating.

What is a good OITC rating for windows?

A good OITC rating for windows depends on the site’s noise level, so an acoustical consultant should set the target. For reference, Viracon’s insulating glass units range from OITC 30 for a standard unit to 35 for a laminated unit.

What is an STC rating in construction?

An STC rating in construction is a single-number score of how well a wall blocks airborne sound from 125 to 4000 Hz. Higher scores block more, and an uninsulated wood-stud wall with single-layer drywall scores about 30–33, or about 38 with insulation.

Is a higher STC rating better?

Yes, a higher STC rating is better, because the partition blocks more airborne sound. A 10-point jump roughly halves perceived loudness, but doors and other openings can pull a wall’s rating down.

What is the difference between IIC and STC?

The difference between IIC and STC is the type of noise each rates, impact versus airborne. IIC covers footsteps through floor-ceiling assemblies, tested under ASTM E492 with a tapping machine, while STC covers voices through walls.