What Is a Good STC Rating for Windows?

If you are shopping for soundproof windows, you will see one number everywhere: the STC rating. Here is what it means and which rating you actually need.

What is STC?

STC stands for Sound Transmission Class. It measures how well a window or wall blocks airborne sound. A higher number means less noise gets through.

So what is a good STC rating?

For blocking street noise, STC 40 or higher is a good target, and 45 or above is very good. These ranges are approximate, because real results depend on the window design, the glass, and how well the frame is sealed.

- STC 25 to 30: typical single-pane windows. Loud speech and most street noise come through clearly.
- STC 30 to 35: typical double-pane windows. Some improvement, but traffic is still easy to hear.
- STC 35 to 40: laminated or acoustic glass. Noticeably quieter.
- STC 40 to 45: good. Traffic fades to a dull background sound.
- STC 45 and above: very good. Most street noise is hard to hear indoors.

What matters beyond the number

STC is measured mostly on mid-range sounds such as speech. Low-frequency noise like truck engines, train rumble and bass is harder to block, so even a high rating can let some of it through.

Gaps matter too. A window with a great rating but poor seals will perform worse than the number suggests.

Adding a secondary insert inside your existing frame creates an extra layer and air gap, which can raise the combined rating a lot. Always ask for the tested rating of the combined setup, not just the insert alone.

A simple guide

- Quiet suburban street: STC 30 to 35 may be enough.
- Busy road, sirens or nightlife: aim for 40 to 45.
- Trains, airports or heavy traffic: aim for 45 or above, and ask about low-frequency performance.

Not sure what you need? Measure your windows, note the main noise you hear, and ask a specialist for a recommendation based on your situation.

Contact SoundproofUSA or Call today at (617) 299-0026.