If you have ever shopped for a soundproofing product, you have seen the number. STC 35. STC 50. STC 60. It shows up on walls, doors, windows, and ceiling assemblies. But what is it really telling you? And more importantly, does it tell you what you actually need to know? This guide breaks it down in plain English.
STC in one sentence
STC stands for Sound Transmission Class. It is a single-number rating of how well a wall, door, window, or floor assembly blocks airborne sound, like voices, music, and TV noise. The higher the number, the more sound the assembly keeps from passing through.
The rating comes from lab testing. An assembly is built in a test chamber, sound is played on one side, and the reduction is measured on the other side across a range of frequencies. Those measurements get condensed into one number. That number is the STC rating.
What the numbers mean in real life
There is no official chart, and every situation is different, but here is the general sense most acousticians work with:
- STC 25 to 30: Typical interior hollow door. You can hear normal conversation through it easily.
- STC 35 to 40: A standard wall with a good seal. You can hear loud speech but usually not follow the words.
- STC 45 to 50: A well-built wall. Loud sounds are faint; normal speech is blocked.
- STC 55 and up: Serious isolation. This is the territory of studios and demanding multi-unit construction.
A useful rule of thumb: a change of about 10 points in STC is perceived as roughly cutting the noise in half. Going from STC 35 to STC 45 is a big, noticeable improvement.
What STC does not tell you
This is the part most product pages leave out, and it matters.
STC focuses on mid-frequency sound, the range where speech lives. It is far less informative about low-frequency sound: bass from a subwoofer, a diesel truck idling outside, a washing machine thumping through the floor. An assembly can earn a respectable STC rating and still let low rumble pass right through. If bass or mechanical noise is your problem, STC alone will not describe the fix you need.
STC is also measured in a lab, on a perfect assembly with perfect seals. Real buildings are not lab chambers. Sound flanks around walls through gaps, ducts, shared ceilings, and penetrations for plumbing and electrical. A wall rated STC 50 performs like STC 50 only if every gap is sealed and every edge is detailed. In the field, weak spots decide the result. That is why professionals talk about the whole assembly, not just one material.
How to actually use STC when you are choosing products
First, figure out what kind of noise you are fighting. If it is voices and TV from the next room, STC is a good guide. If it is bass, traffic rumble, or footsteps, ask about low-frequency performance and impact ratings too, not just STC.
Second, compare whole assemblies, not single materials. A panel with a high STC means little if it goes onto a wall full of gaps. Mass, separation (an air gap or decoupled frame), and airtight sealing do the heavy lifting together.
Third, be skeptical of big numbers with no context. An STC rating without a test report behind it is just a claim. Reputable products are tested to recognized lab standards, and the results describe the full assembly as tested, seals and all.
The bottom line
STC is a genuinely useful comparison tool once you know its limits. It tells you how an assembly handles airborne, mid-frequency sound in lab conditions. It does not cover bass, it does not cover impact, and it does not cover the flanking paths in your actual building. Use it as a starting point, not the whole answer.
If you are weighing products for a noise problem and the numbers are starting to blur together, that is exactly what a noise assessment is for. Send us a few photos and describe what you hear through our quote flow, and we will put together a plan for the right assembly for your space. And if you are planning a studio, an office build-out, or anything where quiet really matters, our consulting team can review the design before construction so the rating on paper is the rating you actually get. Reach out anytime at (716) 330-1147.