STC, OITC, and what a rating doesn't tell you.
Every soundproofing company advertises an STC number — and it's the wrong metric for the noise their customers actually complain about. Including ours.
Nearly every company selling window soundproofing, ours included, leads with an STC number. Here's the uncomfortable part: STC is the wrong metric for the noise most of those customers are actually trying to stop. That isn't a dig at a rival — it's a measurement problem baked into the rating itself, and it applies to our claims as much as anyone's. This page explains what STC does and doesn't measure, why OITC is the more honest number for outdoor noise, and why even the right window number runs into a ceiling most pages never mention: the wall it sits in.
Before any of that, the admission this page has to make to be worth reading: we have no lab-measured STC or OITC for Insulatx panels. Every figure here is a published range for a product or assembly category, not a test of our specific product. A page that attacks unsupported numbers can't lean on one of its own — so where you see a figure, it's an industry range, and we'll say so.
Built to answer
an indoor question.
STC — Sound Transmission Class — was introduced in 1961, and it does one job well. It rates a partition across the one-third-octave bands from 125 Hz to 4000 Hz and reduces them to a single number by fitting a reference contour to the measured curve, tuned toward the frequencies of human speech. That focus isn't arbitrary: STC was built to answer an interior question — can I hear my neighbour's conversation through this wall? For predicting speech privacy between rooms, it works.
The problem is what it doesn't look at. STC's lowest band is 125 Hz; below that, it captures essentially nothing. And outdoor noise sources — road traffic, rail, aircraft, and mechanical equipment like AC condensers — carry far more of their energy in the low frequencies than speech does, including a substantial amount below the 125 Hz floor where STC simply stops measuring. The deep part of a truck passing, a jet overhead, or a heavy diesel idling is exactly the part STC can't see.
So an STC figure on an exterior window is describing performance against a frequency range that isn't where the outdoor problem lives. This isn't a fringe observation: specifying STC alone for a building envelope is a well-documented cause of low-frequency noise complaints in otherwise finished, code-compliant buildings — the walls and windows hit their STC targets, and the occupants still hear the highway. (If your windows are newer double-pane units and you're wondering why they're loud anyway, guide two is the specific answer.)
OITC exists, and almost
nobody publishes it.
There's a rating built for exactly this situation. OITC — Outdoor-Indoor Transmission Class — was introduced in 1990, specifically for building facades and transportation noise. It reaches down to 80 Hz — two one-third-octave bands below STC's 125 Hz floor — and instead of being tuned to speech, it is weighted against a reference spectrum of real road and aircraft noise, so it actually responds to the low end.
Here's the detail that should change how you read a spec sheet: OITC and STC come from the same test. Both are derived from sound transmission loss measured under ASTM E90. They differ only in how that measured data is scored — STC by fitting a reference contour to the curve (with limits on how far any band may dip below it), OITC by weighting the values against its transportation-noise reference spectrum. STC is classified per ASTM E413; OITC is calculated per ASTM E1332. No second test, no extra lab time — a manufacturer that already holds STC data for a product has everything needed to publish its OITC, and most simply don't.
| STC | OITC | |
|---|---|---|
| Introduced | 1961 | 1990 |
| Frequency range | 125–4000 Hz | 80–4000 Hz |
| Built for | interior partitions, speech privacy | facades, transportation noise |
| Method | contour fitted to the curve | weighted vs a noise spectrum |
| Lab test | ASTM E90 | ASTM E90 (same) |
| Rated per | ASTM E413 | ASTM E1332 |
Why does it matter which one you're quoted? Because for the same assembly, OITC typically comes out 4 to 8 points lower than STC. As a benchmark, a genuinely good exterior window is around OITC 30 or above; down at OITC 20 to 27, highway traffic is still clearly audible indoors. So when a product is advertised at, say, STC 35, the honest number for the road and aircraft noise you care about is several points below that — and it usually isn't printed.
To be fair about where the industry stands: at least one company in the soundproof-window industry states publicly that OITC is likely the more appropriate measure for exterior applications, then recommends STC anyway because that's what the market specifies. That's the whole situation in a sentence. The better metric exists, it's free to report, people who could report it have said it's better, and it mostly stays off the page because nobody is required to print it.
This cuts at
our marketing too.
Since this page is calling out marketing numbers, it has to include ours. Our own “up to 70% less noise” language describes high-frequency performance — the traffic whine, voices, and AC-fan noise an acrylic panel genuinely does cut the most. A low-frequency-weighted number, the OITC way of looking at it, would be less flattering to us too, because deep rumble is the hard case for our product exactly as it is for everyone else's. We think the honest move is to tell you that directly rather than pick the number that sounds best. That concession is the point of this page: the argument only works if it cuts both ways, and it does.
The number nobody
mentions — the wall.
Suppose you set STC and OITC aside and just chase the highest window number you can buy. There's still a ceiling, and it's the wall the window sits in. Sound through a wall-plus-window is area-weighted: each part contributes in proportion to its area, so the weakest element dominates the result without solely determining it. (Guide one works the composite math if you want the arithmetic.) The practical version is a comparison of ranges:
| Element | Typical STC |
|---|---|
| Single-pane window | 18–28 |
| Standard double-pane window | 26–33 |
| Acrylic insert over an existing window | ~42 |
| Laminated-glass insert over an existing window | ~48 |
| Interior secondary window systems (category) | 48–54 |
| Frame-and-siding wall | ~40 |
| Brick veneer over frame | between 40 and 50 |
| Full masonry wall | ~50 |
Read the two groups against each other. A standard double-pane window at 26–33 sits well below a typical frame-and-siding wall around 40 — the window is clearly the weak path. An acrylic insert over that window, around STC 42 as a category (here's how the panels physically work), moves it from below the wall to at or a little above it in frame construction. That's the pivot: once the window is no longer the weakest path, the wall becomes the limiting one. In a frame-walled house, the difference between a 42 window and a 48 window matters less inside the room than six points suggests, because something else — the wall — is now the bound.
Read that carefully, because there's a way to misread it. This is not an argument that a laminated-glass insert is no better than an acrylic one. It is better, measurably — 48 versus 42 is a real difference, and in a brick-veneer or masonry house, where the wall ceiling is higher than 40, there's more headroom above 42 for that difference to show up in the room. The point is narrower and more honest: window performance has diminishing returns against a ceiling set by your construction — not that all inserts are equal. In a frame house you may reach the wall's limit sooner; in a masonry house you won't.
Which reframes the question worth asking. Not “what's your STC.” The useful questions are: what does the rest of this room rate, and where is the weakest path once the window is handled — because that's what sets what you'll actually hear.
The questions
worth asking.
- Ask for OITC, not just STC. If a manufacturer has tested a product — and any STC number means they have — the OITC is calculable from the same data. If they can't or won't give it, that tells you something.
- Treat STC as an optimistic ceiling, not a prediction. For outdoor noise the real-world figure is lower, often by 4 to 8 points.
- Ask what the surrounding wall rates, and where the weak path will be after the window is treated. A great window in an average wall is still in an average wall.
- Be sceptical of any number quoted without its test conditions. A figure with no standard, no assembly description, and no lab attached isn't a measurement — it's a claim.
And our position, to close where we started: we would rather tell you plainly that high-frequency noise drops the most and deep low-frequency rumble is reduced, not eliminated, than hand you a single number that implies more than it delivers. That isn't a line written for this page — it's the same honest limit stated on every page of this site, from the aircraft-noise guide (where military jets are the clearest case of what STC can't see — low, heavy, and right in the blind spot) to the traffic-noise guide. The consistency is the argument: a company willing to state its own limit in the same words everywhere is one you can check.
Common questions,
answered.
Still wondering something? Call (945) 395-9424 — we'll tell you straight what we can do for your home.
What's the difference between STC and OITC?
Both rate how well something blocks sound and come from the same lab test, but STC (1961) covers 125–4000 Hz and is tuned to speech, while OITC (1990) reaches down to 80 Hz and is weighted for the low frequencies that dominate traffic and aircraft noise. For outdoor noise, OITC is the more honest number.
Why do companies advertise STC and not OITC?
Mostly because the industry specifies STC and nobody's required to print OITC — even though it's calculable from the same test data. For the same window, OITC usually runs 4–8 points lower, so it's the less flattering figure.
A window is advertised at STC 35 — is that its real noise rating?
Against speech-range sound, roughly. Against road or aircraft noise, the honest figure is several points lower, closer to its (usually unpublished) OITC. Treat STC as an optimistic ceiling, not a prediction.
Does Insulatx have STC or OITC numbers?
No. We don't have lab-measured STC or OITC for our panels, and we won't quote numbers we don't have. Every figure on this page is a published range for a product or assembly category, not a test of our product.
If I buy the highest-rated window, will the noise stop?
Only down to the wall around it. Once the window matches the wall, the wall becomes the limiting path — and that depends on your construction. The right question is what the rest of the room rates.
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