If you do not know what STC is then you have come to the right place. For those new to the world of acoustics, here is my simple interpretation of STC to help you better understand.
What is STC rating? It’s the measurement of how much noise is reduced as it passes through a medium. The majority of common STC ratings fall within a range of 20 to 60. An STC rating of 20 is considered very poor soundproofing quality whereas an STC rating of 60 is considered excellent soundproofing quality.
STC relates to how sound transmits from one room to the other. Using this rating will enable you to determine the overall effectiveness of soundproofing material.
What does STC stand for?
STC = Sound Transmission Class
How is STC rating calculated?
STC rating is more or less the reduction in decibel level as noise travels from one room to another. Simply put, it measures how much sound a panel or wall blocks from reaching the other side.
To keep it simple, let’s say there is a 60dB noise on one side of a panel. As it travels through the panel to the opposite side, it is reduced to 25 dB. The panel has displayed an STC of 35.
Now lets a take a deeper look at what makes up this number.
Transmission Loss
What is the transmission loss? It is the difference in decibel levels as it travels from one room through a medium (i.e. a wall) into another room .
Here is a simple formula:
Transmission Loss Value = Ni – Nr
Where (Ni) is the initial noise level that travels through a medium and enters into another room at a reduced decibel level. This is the reduced noise level (Nr).
Transmission loss plays a role in a medium’s STC rating. In order to get the actual STC number, the transmission loss value is tested multiple times between 125 Hz – 4000 Hz.
This testing results in a plotted curve on a graph. These results are then compared to the appropriate standard STC logarithmic curve to finalize the STC rating.
It is important to note the major limitation of STC. This measurement is not applied evenly along the specified range of frequencies.
Only the most common frequencies in this specific range are used. This causes high-frequency noises to be reduced more efficiently than lower frequencies.
Can you add the STC ratings of different mediums?
No. You can not add STC ratings together in the way that you think you might be able to.
For example, let’s say you double up on acoustic panels that each have an STC of 30. You now have increased the STC from 30 to 60 right? Wrong.
Adding a second acoustic panel with an STC of 30 does not instantly increase the overall STC to 60. As mentioned before, STC is determined using logarithms, not addition.
What is a good STC rating?
A good STC rating is about 40 – 50. This is a typical standard for most commercial buildings.
Higher STC ratings are even better. Therefore, a product’s soundproofing ability for higher frequencies (greater than 125 Hz) is directly proportional to its STC rating.
However, I want to stress again that STC only measures sounds over 125 Hz. Low-frequency sounds cannot be calculated. Therefore, mediums with high STC ratings may not be effective if you have low-frequency noise problem.
Why the STC Rating Numbers are so Close in Value
The reason why STC numbers are close in value is due to the logarithmic scale being used for measurement. Even though an increase from one number to the next is small, the actual sound reduction result can be quite large.
What STC Ratings do Windows Have?
The STC ratings of windows vary by glass thickness and overall design structure. Typically, single pane windows have an STC rating around 27. Single pane and double pane windows alone will not stop sound entirely.
You will need to install soundproof windows if want to effectively reduce sound from entering your home through your windows. Make sure to check out this STC rating window chart if our want more information on windows and soundproof window STC ratings. Note that soundproof windows are not 100% effective.
What is a decibel?
A decibel is a measurement of sound. It gives it a number to help determine the level of loudness.
Decibels are abbreviated as dB. On a scale of 0dB to 150dB, 0 is very low while 150 dB is extremely loud. To help you gauge the loudness of decibels, the average household measures around 40 dB.
What is frequency?
Frequency is the measurement of the speed of a vibration. It is typically used to measure very high sounds such as musical instruments.
Frequency is measured in Hertz (Hz). To give you a point of reference, humans can typically hear a range of sounds between 20 Hz and 20,000 Hz.
How to improve STC of a Wall
In order to improve the soundproofing efficiency of a wall, you need to implement decoupling. You can do this using wall studs and drywall. This can greatly increase the STC rating and effectively reduce low-frequency noises.
What STC is needed for musical instruments?
To soundproof a room for musical instruments you will need a material with an STC rating of at least 55. This will greatly reduce noise so your neighbors will not be disturbed.
What STC is required for homes?
Residential homes typically require an STC rating between 50 and 80. You should pay attention to noise sensitive areas such as home theaters, recording rooms, and possibly your home gym.
How to compare noise reduction
You can compare noise reduction quality based on STC or NRC. If a product has a higher STC or higher NRC number, that means the product should perform better in reducing noise.
How to improve STC rating
Improving the STC rating is ideal for better soundproofing a room. There are a few ways you can do this. I have them some below:
- Add insulation – Adding insulation or soundproof material to the medium will increase the overall STC rating. An example of this is stuffing insulation in between wall studs or inside a hollow core door.
- Add air – Placing a gap of air within a medium can actually create decoupling by creating two separate entities. This will raise the STC rating.
- Add mass – Increasing the density of the medium will help to absorb more sound. This increases overall density thus reinforcing the soundproof barrier resulting in a much higher STC rating. For windows, you can add an extra layer of glass or window film.
What is the best strategy for soundproofing?
The best strategy is to soundproof evenly and give extra attention to the weakest points. A sound will always take the path of least resistance, which means penetrating any tiny openings. That is why doors and windows are usually the weakest links in a building structure.
If you have a door with an STC of 20 and wall with STC of 40 then the doorway would be the path of least resistance. The sound would be reduced by 20 dB instead of 40dB.
In this case, the best soundproofing strategy is to soundproof both of these until they have an equivalent STC rating. That’s why it is important to install a solid core door.
