STC Ratings: Understanding STC Ratings for Wall and Stud Framed Walls
You’ve found your dream apartment, and it’s in your budget.
The only thing you’re a little concerned about is the level of background noise, but you’re sure you can find ways to fix that.
To be honest, you’re not sure what the realtor meant when she told you about the STC ratings for walls in your apartment.
Now you need to find out all about STC (Sound Transmission Class) ratings, including:
- What affects the STC rating, such as the thickness of walls and what they are made of
- Whether space between walls makes any difference to the level of noise, you can hear
- What can you do to reduce sound and improve your STC rating
We have all those answers and more, so stay with us to learn everything you want to know about STC ratings.
What are STCs, and Why are They Important?
When particles vibrate, this creates a wave that your brain recognizes as sound.
Sound is measured in decibels (dB), which the human ear can detect between 0dB – 130dB, although anything above 90dB can damage your hearing.
This is different from frequency, which is measured in hertz (Hz): humans can detect sound from 20 Hz to 20 kHz (kilohertz).
STC ratings for walls are given according to the amount of sound they eliminate.
They are important as it gives you an indication of the level of noise you can expect to penetrate your walls.
STC Rating
Various factors can affect STC rating for walls — the higher the rating, the less sound penetration there is.
- STC ratings for walls differ according to:
what materials are used to construct them e.g., masonry, drywall, or wood - how thick are they?
- whether they are hollow
STC Rating Chart
There is a difference between STC and FSTC (Field Sound Transmission Class) ratings.
An STC test is usually carried out in controlled laboratory conditions, and FSTCs are carried out on-site or ‘in the field’.
The STC wall rating chart measures between 45 – 80.
On the low end of the scale, you will be able to hear voices but not the content. At around 55, loud speech is audible, and at the top end, only very loud music is audible.
How do you measure STC?
STC is calculated by testing 16 standard frequencies between 125 Hz – 4000 Hz and plotting the Transmission Loss (TL) values on a graph.
Transmission Loss is the difference in volume on either side of a wall. For example, you could have music playing in a room at 100dB. The volume is measured in the room next door and measures 75dB. The Transmission Loss is, therefore, 25dB.
Comparing STC Ratings to Real Performance
STC ratings carried out in a laboratory cannot take all other noise factors into account in real-life situations
A wall may have a high STC rating, but if you have hollow doors or cracks between your windows and the wall, the noise will penetrate.
How to Determine the STC Rating of Walls, Drywall, and Windows
The Uniform Building Code (UBC) classes apartments in Group R, which require an STC rating of 50 or an FSTC rating of 45.
As discussed, STC ratings vary according to a number of factors, but simply put, the thicker the structure, the more effective it will block out sound.
Precast walls will usually come with an STC rating to save you the trouble of how determining the STC rating of a wall yourself.
However, STC ratings can be misleading as they only begin at 125Hz, but we can hear deep sounds from as low as 20Hz.
Understanding STC Ratings for Drywall
Understanding STC ratings for drywall can be complex.
An STC graph is one drawn up with 16 Transmission Loss values evaluated at 16 standard frequencies, from 125 Hz – 4000 Hz.
You compare your measured curve to the graph to obtain your STC rating.
What affects STC Ratings for Walls?
Numerous factors can affect STC ratings, which have to be considered as a whole to determine the STC rating of a wall.
What are examples of these factors?
Wall thickness
Walls that are of the same thickness but made of different materials can have different STC ratings.
This is because the denser the material, the more sound-absorbent it is.
Walls constructed of concrete blocks will be much more effective at blocking sound waves than single-layer drywall.
Materials
Wall frames, or stud walls, can be constructed of wood, steel, or aluminum, which can be clad with drywall, sheetrock, timber, fiberboard, etc.
Once fixed in place, many of these outer materials can be painted or papered over.
Weak points
An STC wall rating can be affected by areas that allow sounds in, such as air vents, gaps under doors, hollow doors, single-glazed windows, cracks in walls, or any place that allows air and, therefore, sound waves in.
Noise frequency
As STC wall ratings only begin at 125Hz: noise with lower frequencies is excluded.
You also need to consider noise pitch.
Frequency measures the rate of a wave cycle, while pitch is whether the sound is low (e.g., thunder) or high (e.g., a whistle).
This means that the higher the frequency, the higher the pitch.
How to Improve an STC Drywall Rating
There are many ways to soundproof and increase an STC drywall rating.
A rigid expanse of material is highly effective at reflecting sound.
The first thing is to determine the sound source before taking appropriate steps to reduce the noise levels.
Gaps and leaks
If you have gaps between your window frames and walls, sound waves will have no trouble inviting themselves in.
To eliminate or reduce noise, you can fill these with commercially available acoustic caulk.
With a little DIY experience, you can easily fill any gaps you have between molding and drywall with drywall compound.
If you have a burst water pipe inside your wall, you will probably need to call in professionals to fix the leak and repair any damage.
Green glue
Green glue is a commercially available product designed to improve soundproofing.
It is a sticky elastic compound that is painted on the two internal sides of the drywall.
It is designed to absorb sound, and once fully cured, which takes about 30 days, it is highly effective.
Increase the depth of studs
Using lighter gauge studs in drywall will increase its ability to deflect sound and therefore increase its STC rating. Using 25-gauge studs makes the wall more flexible than using heavier studs.
Inserting studs to a greater depth will also assist in sound blocking, e.g., 6” 25-gauge studs are more effective than 3 ⅝” studs.
Spacing the studs at irregular intervals will also help to make the wall surface less of a sounding board.
Space between walls
Maintaining an air gap between sheets of drywall helps to trap any sound waves. The sound waves bounce between the inner walls until they lose their momentum and thus do not escape through the wall.
Install a soundproofing system
It is almost impossible to completely soundproof a room, but you can take steps to dampen noise.
You could mount framed foam panels on the walls or drop wall hangings from the ceiling.
Any thick material, whether solid or flexible, will absorb sound waves to some degree.
Related Questions
What is the STC rating of ½” drywall?
If the drywall is constructed on hollow wood studs, the STC rating will be about 33dB.
Which wall has the highest STC rating?
The commercial product QuietRock has an STC rating of 56 when installed as a single stud wall.
Is drywall a good sound insulator?
Drywall without any other insulation, such as foam, is not a good insulator in itself as the sound waves travel right through it.
Does double drywall reduce sound?
Mass increases sound absorption, so a double layer of drywall is better than a single layer. The double layer is less likely to vibrate. Therefore, sound waves will not bounce off it so easily.
Is drywall good for sound absorption?
A single layer of drywall is not good at sound absorption as it does not have much mass, which is needed to absorb sound waves.
Why does my house have two layers of drywall?
Two layers of drywall are better at absorbing sound. If the sheets have been installed at 90o to each other so that the studs are not in uniform lines, this will increase its effectiveness.
Which is a better insulator – drywall or plywood?
Wood is a very good sound conductor, so the plywood is less effective than drywall as a sound insulator. Drywall comprises a combination of gypsum, paper, and additives such as resin or clay, making it denser.
Conclusion
When considering moving homes, it is highly advisable to ask your contractor or local municipality for your STC ratings for walls.
This will be especially relevant if you live on a busy road with constant traffic or near an airport.
Continuous loud noise can be detrimental to both your physical and mental health, so it’s advisable to take precautions before they become a problem.