Why STC and OITC Are Not the Same Number

Every soundproof window spec sheet carries two acoustic ratings, and buyers routinely confuse them. STC (Sound Transmission Class) is a single-number rating derived from laboratory tests per ASTM E90 and classified per ASTM E413. It weights frequencies from 125 Hz to 4,000 Hz — roughly the human speech range. OITC (Outdoor–Indoor Transmission Class), classified per ASTM E1332, extends the weighting down to 80 Hz and up to 4,000 Hz, which captures the low-frequency rumble of road traffic, trains, and aircraft.

For an aluminum glass soundproof window destined for an airport-adjacent hotel or a highway-side apartment, OITC is the more honest specification. STC can look flattering while the window still lets through the deep drone that keeps occupants awake. As a rule of thumb, OITC runs 4 to 8 points below STC on the same product — a window rated STC 40 will typically show OITC 32 to 36.

"We specified STC 40 based on the developer's brief. The corridor traffic noise never went away because the real requirement was OITC — the low-frequency component was the problem." — Acoustic consultant, Brisbane airport hotel project

What Actually Blocks Sound: Mass, Damping, and Air Tightness

Acoustic performance is governed by three physical mechanisms, and each maps to a specific part of the window design:

This is why an aluminum glass soundproof window cannot be judged on glass alone. The frame must carry a continuous thermal break (PA66-GF25), the sash must press a triple EPDM gasket line against the frame, and hardware must pull the sash tight enough to maintain uniform gasket compression. The same glass installed in a poorly sealed frame can lose 5 to 8 points of STC.

Typical Acoustic Ratings by Glass Configuration

Values are indicative laboratory ratings for the glazing unit under ASTM E90/E1332; whole-window ratings run 3-5 points lower depending on frame, size, and installation. Always request the test report for your exact configuration.

Asymmetric Build-Ups and the Coincidence Dip

Equal-thickness panes share the same coincidence frequency, creating a resonant dip that swallows performance in a narrow band. Specifying unequal panes — for example 8mm outer and 6mm inner — staggers the coincidence dips so each pane masks the other's weakness. This is a low-cost trick that consistently buys 2-4 points of STC on an aluminum glass soundproof window without changing the total glass budget much.

The cavity width matters too. Narrow 6-9mm air spaces couple the panes acoustically; 12mm is the practical sweet spot for export profiles, and widening to 16-20mm with a reinforced frame yields diminishing but real gains at low frequency — exactly where OITC lives.

Whole-Window vs Glazing-Only Ratings: What to Ask the Supplier

1. Is the STC/OITC figure for the glazing unit or the assembled window? Many Chinese suppliers quote the glass lab value. The assembled window — with frame, hardware, and seals — is the number that matters for your building.
2. What test standard was used? STC per ASTM E413 requires ASTM E90 test data; OITC per ASTM E1332. ISO 10140-2 / ISO 717-1 (R_w) and AS/NZS 1276.1 are the EU and AU/NZ equivalents. The same window will produce different numbers under different standards — never compare across them without converting.
3. Is the cavity gas-filled and how is it sealed? Argon fill has minimal acoustic effect but a properly sealed unit prevents moisture ingress that degrades both thermal and acoustic performance over time. Request the unit manufacturer and spacer type.
4. Can you test my exact size? Acoustic performance changes with sash dimensions and hardware layout. A supplier who can run or commission a test on your configuration — rather than only citing a catalogue table — is a supplier you can trust.

Standard-to-Product Index: Which Rating Applies Where

From Spec to Installed Performance: The Verification Chain

Rogenilan Windows Doors ships aluminum glass soundproof windows with full acoustic documentation: ASTM E90 test reports, ISO 10140-2 data where required, and per-batch seal and hardware inspection records. The company's factory in Foshan — a major aluminum door and window production hub in China — manufactures 6063-T5/T6 profiles with PA66-GF25 thermal breaks, 1.4-2.0mm wall thickness, Low-E laminated glazing, and SUS304 hardware from SIEGENIA, ROTO, or HOPO, depending on the acoustic specification.

2026 Indicative FOB Prices from Foshan Factories

Prices are indicative 2026 factory FOB ranges for typical project volumes (200+ m²) and vary with sash size, hardware brand, finish, and certification documents. Contact Rogenilan Windows Doors for a quotation on your exact specification.

Soundproof Window FAQ

What STC rating do I need for a hotel near an airport?

For airport-adjacent rooms, target a whole-window STC of 38-40 with an OITC of at least 32-35. A 6.38+12A+6 laminated IGU in a tightly sealed frame is the entry point; for direct flight-path exposure, step up to double laminated glass and verify with a project-specific test report.

Is OITC always lower than STC?

Almost always, by 4-8 points, because OITC includes the 80-125 Hz band where windows are weakest. A large gap between the two numbers is normal; an unexpectedly small gap can indicate a glazing build-up that is unusually good at low frequencies — worth confirming with the test data.

Does Low-E glass reduce acoustic performance?

No meaningful acoustic penalty. Low-E coatings affect radiative heat transfer, not sound transmission. You can combine Low-E with laminated or asymmetric build-ups without sacrificing acoustic ratings.

Can a soundproof window also meet energy requirements?

Yes. Laminated glass with Low-E coating, a 12-16mm argon cavity, and a 24-35mm PA66-GF25 thermal break delivers whole-window U-values around 1.4-1.8 W/m²K while holding STC 38-40 — which is why this configuration is the default for green building facades in noise-exposed sites.