Explaining STC: Sound Transmission Classification

stc rating

What is an STC rating? 

Just as NRC is the standard rating for how well a material absorbs sound, the sound transmission class  (STC) is a single-number rating that gives a rough idea of how much sound a specific type of material might stop.

An industry standard when it comes to calculating sound transmission through barriers, STC is the most common sound reduction measurement currently in use. An STC rating can help you determine how much sound transmission is being reduced and the frequencies of sound being affected.

Understanding STC (Sound Transmission Class)

Sound Transmission Class

Simply explained, the Sound Transmission Class, or STC rating for a material tells you about how much sound that material can block.

An STC rating of 20 is considered very poor while a rating of 60 is considered excellent . For example, if you were in a room where a wall joining an adjacent room had an STC rating of 20 you would be able to hear someone speaking at a low volume through that wall. If the same wall had an STC rating of 60 then you would not be hearing that conversation.

Nowadays, many building codes require condos and townhouses to meet an STC of 50, while luxury hotels and condos should reach a rating of 60 or higher. For example, a standard 2×4 wood stud wall with ½” drywall has an STC of 32.

What is a Good STC Rating?

Most building codes for hotels and apartments require an STC 50 or higher. Anything lower and these types of projects will likely receive a significant amount of noise complaints and possibly even litigation from guests and tenants. It’s important to be sure you’ll be able to hit an STC of 50 or higher prior to construction, because once a wall is built it’s very costly and labor-involved to soundproof it after the fact.

What Each STC Rating Sounds Like – An STC Rating Chart:

  • STC 25: Soft Speech Clearly Understood
  • STC 30: Loud Speech Understood
  • STC 35: Loud Speech Audible, Not Understood
  • STC 40: Loud Speech Faintly Audible
  • STC 45: Loud Speech Inaudible
  • STC 50: Loud TV Audible
  • STC 55: Loud Music Audible
  • STC 60: Loud Music Faintly Audible
  • STC 65 and Above: Sound Transmission Class is no longer an accurate measure

When STC Products Work

An important point to keep in mind is that the STC only tests on frequency ranges from 125 to 4000 Hz and one common problem with this type of classification is that a lot of common noise issues can be attributed to sounds that often falling below or above this range. Examples of this include bass drums, deep male voice and trucks—all of these can potentially start at 50 Hz; high-pitched female voices,  the shrill ringing of phone and car horns are good examples of sounds that reach above the 4000 Hz range.

This will explain why many big projects—even after having paid tens of thousands of dollars on expert acoustical engineers and getting thousands of pages of tested soundproofing materials with lab testing of STC of 50 and above—still receive complaints for poor acoustical performance.

Soundproof Curtain

Our soundproof curtains are an STC rated product that block outside noises from getting into your home. We also make some that go over doors to help stop sound transfer between rooms.

Materials with STC Ratings

Material STC Rating
Hollow- Core Door 17
Double Pane Windows 26
Soundproof Curtains 26

Knowing if Your Walls and Windows Have Poor Ratings

If you live in a home with poor STC rated materials, chances are you hear noises coming from outside and from different rooms in your house. My bedroom in my house shares a wall with the living room so I can usually hear my family talking and watching TV if I’m in my room and they’re out there. Although I don’t know for sure, I’m sure that the wall between my room and the living room doesn’t have a very good STC rating.

Another issue I have is with my windows. The A/C unit is right outside my room so I can usually hear it turn on if I’m in my room when it happens. I’m sure my single pane windows don’t have a great rating either compared to some of the double-pane windows out there.

There are a couple of solutions I could use to improve the noise problems in my house. For the wall that joins my wall and the living room, I could add a layer of Wall Blokker Pro, or heavy duty mass loaded vinyl, between my dry wall and studs to increase the STC. For my windows, all I need to do is order some soundproof curtains from Residential Acoustics to block out any outside noises that keep me awake.

STC Rating for Soundproof Curtains

Fortunately, there are several different ways to soundproof your living space!

At Residential Acoustics, we pride ourselves on helping you reduce the sound in your bedroom, living room and office with quality, state-of-the-art products such as the AcousticCurtain™ and AcoustiDoor™—an easy and affordable way to soundproof your home! Our curtains have a rating of 26 and block out 60%-90% of outside noise whether it’s coming through a window or a door.

When considering the ideal soundproof curtains for your apartment or home, be sure that the product has a lab-tested STC rating.

To ensure you get the absolute best results, call TODAY for a FREE over-the-phone consultation! One of our soundproofing experts will assist you in diagnosing your issues and offer a solution—the best approach to fix them.