STC vs OITC: Which Window Sound-Rating Evidence Matters?

Read the rating basis before comparing window numbers

Updated October 2026

Selecting an exterior window from a single STC number can fail to control low-frequency road or rail rumble and lead to costly glazing rework. Confirm the rating classification, the tested specimen boundary and the one-third-octave band transmission loss table before comparing two window submittals.

A higher rating on a product sheet does not guarantee an equivalent decibel drop inside a finished room. Use this guide when comparing exterior window documents against outdoor noise so that center-of-glass tables, laboratory window assemblies and field isolation metrics are kept on separate, verifiable lines. For interior room echo and surface absorption rather than exterior facade transmission loss, see acoustic wall treatments and what they actually do.

How does STC differ from OITC in noise spectrum and frequency range?

STC (Sound Transmission Class, ASTM E413-22) evaluates airborne sound attenuation from 125 to 4,000 Hz for speech and indoor building noise, whereas OITC (Outdoor-Indoor Transmission Class, ASTM E1332-22) evaluates 80 to 4,000 Hz against an A-weighted reference spectrum of outdoor transportation noise.

In the public Significance and Use section of ASTM E413-22 (updated May 2022), the classification states that its single-number ratings correlate in a general way with subjective impressions of speech, radio, television and similar sources in offices and buildings. Section 4.1 explicitly states that E413 is not appropriate for sources with significantly different spectra, including machinery, industrial processes, musical instruments and transportation noises such as motor vehicles, aircraft and trains, and directs facade comparisons to Classification E1332.

ASTM E1332-22 (updated August 2022) calculates a single-number rating from the difference between the overall A-weighted sound level of a reference source spectrum (Table 1 of the standard, which averages three transportation spectra: aircraft takeoff, road traffic and diesel locomotive) and the A-weighted spectrum remaining after subtracting the specimen’s transmission loss in each one-third-octave band.

Two standard boundaries affect how you read window submittals:

1. Check whether the 80 Hz band was actually measured. According to ASTM E1332-22 §4.3, laboratories historically omitted measurements below 100 Hz because of chamber volume accuracy limits in E90 and E966. After studies showed that the 80 Hz one-third-octave band is required for acceptable correlation with transportation noise, Test Method E90 (when testing facade elements, exterior doors or windows) and Guide E966 required reporting the 80 Hz band. Older glazing tables often start at 100 Hz and mark OITC* as an estimate.
2. Do not treat OITC as universal for every outdoor source. ASTM E1332-22 §4.1 and §1.4 warn that because the Table 1 reference spectrum is heavily weighted toward low frequencies, using OITC where the actual outdoor noise is dominated by higher-frequency tire whine on high-speed freeways, stage 3 jet aircraft, emergency sirens or train horns can over-specify low-frequency mass, while industrial sources peaking below 80 Hz fall outside E1332’s scope altogether. Where the site noise spectrum differs from the reference curve, E1332 §1.3 advises selecting facade elements using specific one-third-octave band transmission loss values alongside the project’s measured outdoor spectrum.

Takeaway: For exterior windows exposed to urban road, rail or aircraft corridors, require OITC alongside one-third-octave band data down to 80 Hz rather than relying on STC alone.

Does the report test center-of-glass only or the complete window assembly?

A center-of-glass insulated glass unit (IGU) table does not establish the STC or OITC of a complete window assembly; full-window qualification requires testing the glass inside its specific framing, sash, gasket and hardware system under ASTM E90 and ASTM E1425.

Glass manufacturers routinely publish acoustical data tables for unframed monolithic lites, laminated lites and IGUs. Those figures measure how airborne sound traverses the glass plies and airspace when sealed directly into a laboratory filler opening. Once that same IGU is glazed into an aluminum, vinyl, fiberglass or wood window frame, sound also interacts with frame chambers, glazing beads, mitered joints, operable sash perimeter seals, locking points and drainage weep slots.

In The Construction Specifier’s January 2021 review of FGIA standards, Fenestration and Glazing Industry Alliance standards director Steven Saffell notes that the AAMA Curtain Wall Manual (CWM) guide specification (§5.2.3.7) explicitly requires acoustical testing per ASTM E90 and ASTM E1425 on the specified glass rigidly supported in framing of the same product family, adding: “Glass-only test results shall not be acceptable.”

When reviewing a complete window laboratory report prepared under ASTM E1425-14(2023) (updated February 2023) and AAMA 1801 (Voluntary Specification for the Acoustical Rating of Windows, Doors, and Glazed Wall Sections), verify four test conditions documented in the public scopes:

• Concurrent air leakage, operating force and latching force (§1.1.1, §4.1–4.3): Tight compression seals and heavy cam locks can reduce cavity sound transmission, but they also increase the force needed to slide or latch an operable sash. ASTM E1425 requires concurrent measurement of air leakage, operating force and latching force so that an operable window is tested in a realistic functional state rather than taped or clamped beyond normal operation.
• Perimeter isolation and mastic limits: As summarized in The Construction Specifier (January 2021), ASTM E1425 places the window into a filler wall whose sound transmission loss exceeds the specimen by at least 10 dB at every test frequency. The specimen rests on a closed-cell foam isolator pad with a perimeter gap no greater than 10 mm (393 mils) (up to 9.5 mm / 3/8 in. at the top and sides). Dense mastic seals the interior and exterior perimeter but may not overlap more than 6 to 12 mm (1/4 to 1/2 in.) of the frame so artificial damping is not added, and weep holes or slots are left uncovered.
• Temperature control for laminated interlayers: Because polyvinyl butyral (PVB) and acoustic interlayers are temperature-sensitive, The Construction Specifier notes that ASTM E1425 holds test room and specimen surface temperatures within 22 ± 2 °C (71.6 ± 3.6 °F) for laminated glass (versus 22 ± 5 °C / 71.6 ± 9 °F for non-temperature-sensitive specimens).
• Specimen size and edge restraint effects: ASTM E1332-22 §4.4 warns that low-frequency transmission loss measurements—and therefore OITC—are sensitive to test specimen size and edge restraints, especially on small modular specimens such as windows and doors. Confirm that the tested unit dimensions and operation type (fixed lite vs casement, awning, hopper or sliding sash) match the schedule.

Why must you read one-third-octave band data alongside single-number ratings?

Two glazing assemblies with identical STC ratings can differ by 3 to 5 points in OITC and by 6 dB or more in individual low-frequency bands because STC allows deficiencies above 125 Hz and ignores 80 and 100 Hz entirely.

Both Vitro Architectural Glass’s acoustic design guide and General Glass International’s September 2020 acoustical data tables illustrate how glass build-up shifts the gap between STC and OITC. The examples below are published center-of-glass laboratory figures from those two sources; they demonstrate how glass configuration affects frequency response, not what a framed window will achieve in a building.

Compare the last three rows from General Glass International’s September 2020 table. All three IGUs carry an identical STC 40, yet their reported OITC* values range from 30 to 34, and their transmission loss at 125 Hz drops from 23 dB (1-3/8" asymmetric monolithic IGU) down to 17 dB (1-1/8" laminated IGU with a thinner 1/8" ply)—a 6 dB difference at a key low-frequency band. Moreover, the printed columns in that September 2020 PDF run from 100 Hz to 5,000 Hz, omitting the 80 Hz column required by ASTM E1332-22 §4.2.

When you inspect a one-third-octave band table, check two physical regions noted in the ASTM scopes:

• Low-frequency mass-air-mass resonance (80–250 Hz): Insulating glass units can dip in transmission loss in the low bands depending on pane mass and airspace depth. Adding a thin laminated ply without sufficient mass or airspace can raise mid-frequency damping (boosting STC) while leaving a deep dip at 100–160 Hz that depresses OITC.
• High-frequency coincidence dip (typically around 2,000–2,500 Hz for glass): As explained in ASTM E966-18a §5.3.1, the coincidence effect occurs when the wavelength of sound projected along the pane matches the bending wave in the glass, creating a localized transparency dip (often near 2,500 Hz in diffuse-field tests of glass and drywall). Asymmetric pane thicknesses and damping interlayers are commonly used to stagger or damp that dip.

How do laboratory ratings differ from field facade measurements and flanking limits?

Laboratory ratings under ASTM E90 represent an upper bound where the window is the sole transmission path in a diffuse sound field; installed windows face directional outdoor sound waves, perimeter shims and wall flanking paths governed by ASTM E966.

According to ASTM E90-23 §5.1–5.2 (updated January 2024), laboratory chambers are isolated so that sound travels only through the test specimen under an approximately diffuse sound field. Section 5.2 states explicitly that because buildings contain flanking paths, laboratory sound ratings do not relate directly to sound isolation in buildings; they represent an upper limit to what would be measured in a field test.

When field testing is specified on a completed building, keep interior room-to-room standards separate from exterior facade standards:

• Interior partitions between rooms (ASTM E336-25a + ASTM E413-22): ASTM E336-25a (updated February 2026) measures Noise Reduction (NR), Normalized Noise Reduction (NNR) and Apparent Transmission Loss (ATL) between enclosed interior rooms, which E413 converts into NIC, NNIC and ASTC. Those metrics depend on receiving-room absorption (unfurnished rooms typically yield lower NR than furnished rooms per E336-25a §5.3.1) and apply to interior space-dividing assemblies.
• Exterior facades and windows (ASTM E966-18a + ASTM E1332-22): ASTM E966-18a (updated November 2018) covers field measurements of Outdoor-Indoor Noise Reduction (OINR, rated via E1332 as OINIC) and angle-dependent outdoor-indoor transmission loss (OITL(θ) or AOITL(θ), rated via E1332 as FOITC(θ) or AOITC(θ)) using either calibrated loudspeakers or existing traffic/aircraft sources across six geometry methods.

ASTM E966-18a §5.3 highlights three reasons why field facade measurements will not match laboratory E90 window numbers:

1. Incidence angle θ shifts the coincidence dip (§5.3.1): In the outdoors, sound arrives as a directional free field rather than a room-averaged diffuse field. Near grazing incidence (θ close to 90°, such as street traffic below an upper-floor window), the coincidence frequency drops significantly below the diffuse-field value; at steep angles (θ of 30° or less), it shifts considerably above the diffuse-field frequency.
2. Directional surface intensity (§5.3.2): Free-field sound striking normal to a facade surface produces four times the intensity (a 6 dB term in E966 Equation 6) of diffuse-field sound at the same pressure level, while oblique angles scale by cos(θ). E966 §5.3.3 therefore states that field guide methods should not be used as a substitute for laboratory testing in accordance with Test Method E90.
3. Flanking and multi-surface exposure (§5.5–5.6): Unsealed perimeter rough-opening gaps, curtain wall mullion cavities, spandrel junctions, trickle vents and adjacent wall cladding transmit sound around the window unit. E966 §5.5 notes that where auxiliary flanking checks cannot be satisfied, only OINR and apparent loss AOITL(θ) are valid. Section 5.6 adds that when a room has multiple exterior surfaces, an unexposed wall or roof surface needs an OITC at least 10 points greater than the exposed window or facade element for its contribution to be negligible.

To coordinate the opaque wall assembly, drainage cavity and cladding transitions around the window opening, review our guides on types of facade systems, exterior wall cladding materials compared and ventilated vs direct-applied cladding.

Copy this window sound-rating evidence record

Use one row per candidate window submittal and attach the laboratory report and perimeter detail drawing. Do not infer full-window performance from a center-of-glass row, and leave unverified items marked Unknown — request report before selection.

How to read the worked comparison above: Candidate A appears 2 points higher in STC (39 vs 37) and 3 points higher in OITC (34 vs 31), but its numbers come from an unframed glass table that omits the 80 Hz band and all frame, gasket and weep-slot transmission paths. Candidate B’s lower numbers document a complete operable window tested with its seals and hardware. Until Candidate A supplies an ASTM E90 / E1425 full-assembly report for the actual frame and size—and until the designer verifies the perimeter rough-opening seal and adjacent wall OITC margin—the two options are not on an equal evidentiary basis.

When your project also needs exterior transition detailing after the window assembly is shortlisted, review how to combine facade materials or return to the facades and exterior design guide to coordinate glazing with wall drainage and cladding assemblies.

Method and limits: Sources were accessed between 11 October 2026 and 17 October 2026. This guide synthesizes the publicly available Scope and Significance and Use sections of ASTM E413-22, ASTM E1332-22, ASTM E966-18a, ASTM E90-23, ASTM E1425-14(2023) and ASTM E336-25a, alongside FGIA / AAMA 1801 testing summaries in The Construction Specifier (January 2021) and published center-of-glass tables from Vitro Architectural Glass and General Glass International (September 2020). Full paid ASTM standards and proprietary window manufacturer lab reports were not reviewed, and no independent acoustic testing was performed by this publication.