STC stands for Sound Transmission Class and is a rating of how well a structure reduces the transmission of airborne sound (also known as attenuation). The higher the STC, the better the barrier is against sound.
When STC is talked about, it is mostly related to doors, walls, ceilings and windows. Calculating the STC is relatively straightforward as you simply need to measure the decibels on each side of the wall and calculate the transmission loss to then compare this to the Standard STC reference.
Knowing this rating makes all the difference when you are looking to soundproof your home as it lets you know what you are working with. Let’s look at how STC is determined and why it matters.
All products use the same STC rating scale so it is easy to understand the effectiveness of each product in blocking noise and you can directly compare different products.
The transmission loss is the reduction in noise or decibel reduction (dB) so the decibels are measured in the room of the source of the sound and then the decibels are measured in the next room along (separated by the wall that is being tested) and the difference in the sound is the transmission loss.
The transmission loss values are tested against different frequencies to create a curve on a graph. This curve is then compared to the STC reference curves and the closest resembled curve on the graph represents the STC rating of the wall.
If you are trying to calculate the STC of a wall yourself you will need to have a device that measures in decibels (such as a sound level meter) so you can take readings from different positions.
Follow these steps to calculate the STC rating of a wall:
Take a measure of the sound in the room where the source of the sound, such as a TV, is. (for best results, take the measurement very close to the wall).
Now, go to the other room and measure the sound on that side of the wall. The sound you are testing needs to remain the same frequency during both these measurements.
You can now subtract the second number from the first to get the transmission loss.
If you can test a range of frequencies this will give you a better end result. Test several different frequencies and calculate the transmission loss for each.
To accurately work out what the real STC rating is you should test at 16 different frequencies ranging from 125Hz to 4000Hz, however for most people this is not practical. The best way to get a STC rating number which is useful to your situation is to measure a sound that is the same frequency as whatever your usual sound problem may be.
Your average transmission loss number is your rough STC rating (an accurate STC rating would be carried out in a lab with carefully controlled variables but for msot of us that isn't an option).
STC ratings range from 20 to 65, the higher the rating the more sound that is blocked. Below is a summary of the common ratings and how soundproof they are.
As you can see, 25 and lower offers little to no soundproofing while 60+ offers very effective soundproofing with almost no sound passing through the wall.
To give you an idea of how these figures translate to the real world, a typical interior wall with no insulation offers an STC rating of 35 and a standard single-pane window usually has a rating of up to 27.
The basic STC needs of a normal household are between 50-80.
There are several factors that can impact the STC rating including:
STC matters but the best STC rating will depend on your purpose and how much sound you want to block for the room in question.
The best STC rating for a room in a residential home is likely going to be different from an office building or an apartment complex.
Soundproofing your wall will improve the STC rating. Here are a few easy ways you can improve the STC rating of the wall:
STC ratings are very useful but are often not fully understood. Knowing what STC ratings are and how they are calculated will go a long way in helping you effectively soundproof your rooms and stop noise traveling from one room to another.
Of course, there is a range of factors that can impact the STC rating of a wall but the rating will provide you with an average of the soundproofing performance across a range of frequencies.
The STC rating is important to consider before you begin installing soundproofing and hopefully, this guide has given you everything you need to get started.