How to Determine STC Rating of a Wall
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Understanding the Importance of Soundproofing
You value your privacy and the privacy of others, so you are considering soundproofing your apartment so you will not be able to hear your neighbors, or disable them to hear you.
In some modern buildings, the walls between the apartments are so thin (read: low STC), that you can literally hear what your neighbors are talking about if you just give it a try.
When you are looking to soundproof your apartment, home theater, an office, or a music recording studio there are various factors that you should take into account.
First, you need to examine your needs.
If you are looking to create a nice and quiet home environment so that traffic and sounds from your neighbors' home do not bother you, you will choose one thing.
On the other hand, if you are adapting your basement for your teenage child's band practice, you will need something completely different.
The next thing you will do is pick out the soundproofing material that fits those needs and there is a significant number of different soundproofing materials to choose from.
In this process, you will have to take into account many things, one of which is the STC rating, so let's see what that is and how to choose.
What is STC?
STC is an abbreviation form of Sound Transmission Class.
Its definition: an integer rating of how well a building partition attenuates airborne sound.
Well, what does that mean in laymen's terms? Basically, it is a rating of how well a material or assembly blocks out the sound.
The sound in question is airborne as opposed to other types of sound transmission, for example, structure-borne vibration which is not measured by STC.
It is mostly related to inner walls, doors, and windows.
The general rule is that the higher the STC the better the barrier.
So, determining the STC of your walls will determine your soundproofing needs.
If the wall's STC is high, and you are not planning on playing the drums until late at night, or are not living near the railway, you will need no, or very basic soundproofing system.
Investing in a super-strong soundproofing system can be a waste of money if your walls already have an STC relatively high to what your environment requirements are.
It goes the other way around as well if you only want to eliminate some faraway traffic sound pollution and you decide to purchase the lightest soundproofing system (based on analyzing the source of noise pollution) and the STC of your walls is low, the combination might not do the trick.
To find a good solution to your soundproofing needs you need to consider all factors.
Types of STC
A typical interior wall without added insulation has an STC rating of 35. In that case, the sound passes through the wall pretty easily.
Conversation can still be heard with some straining in a way that loud talk can be heard but not easily understood. You would still lack privacy and quiet.
A wall with STC of 45 provides better sound blocking in which the conversation is not heard through the walls yet the sound of music and heavy traffic will be transmitted either from the inside-out or from the outside-in.
At an STC of 60, a wall will be able to block out the majority of traffic and music sounds, although low-frequency sounds will still be heard (below 125Hz).
In real life, this translates to the fact that the sound produced by a drum, bass, home theater subwoofer or any power equipment such as industrial pumps will emit a thumping, buzzing sound that can still be an issue and will account for noise pollution in your environment.
The basic STC needs of a normal household are between 50 and 80.
What affects the STC?
There are three important things that are important in determining a wall's STC.
First of all, it is the material and the thickness of the wall itself.
There will be a difference if you compare concrete or drywall walls, the studs, the thickness of the concrete and so on.
Another aspect you need to examine are the weak points i.e. all those points that might reduce the STC value.
Because the sound in question here is airborne it can travel through ventilation systems, windows or doors (if there is a gap between your door and your floor the sound will travel through there and the STC value of your wall will not come into much of an account concerning the noise reduction and pollution.)
The third thing is the frequency of the sound you want to block. The STC of a wall is tested in a laboratory at 16 of the standard frequencies and they go from 125Hz to 4000Hz.
At each of these 16 frequencies, the STC will differ.
Some walls will have a higher STC at 2000 Hz while others will be the best for 85 Hz to 255 Hz which is the frequency range for the human voice (male and female).
The STC value of the examined wall is then compared to an STC standard and it gets the approximate value (e.g. 35 or 60).
Note: The structure-borne vibrations and the noise pollution they cause are not taken into account in testing so the walls' effectiveness in blocking those low-frequency sounds will not depend on the STC level.
Real-life performance
To avoid being dissatisfied with sound reduction, make sure to check your needs.
If you are living next to a practice studio, a gym or an industrial area, you might want to take other factors into account.
Also, the real-life performance of the walls is usually slightly worse than it is said.
This is because the STC is tested in a lab where all the environmental factors are controlled (e.g. there are no ventilation ducts and other weak points).
How to Determine the STC rating of a wall?
The STC of a wall is measured by the Transmission Loss between rooms.
The transmission loss is the reduction of noise in the decibel reduction (dB).
Decibels measure how loud something is where 0dB is the quietest a human ear can hear and 130dB is loudness which can cause physical pain.
To see the decibel scale of the most common sounds we hear, click here.
If a sound is at 80dB when it is measured in the room where the sound source is, and it is measured 65dB in the next room separated by a wall we want to test, we say that there is a 15dB transmission loss.
This is where frequency comes into action because at various frequencies the Transmission loss varies.
To calculate the STC value you can use the table below. There are two conditions that have to be met:
1. No single frequency band may have more than 8 deficiencies
2. The total deficiencies may not exceed 32
The highest number that satisfies both conditions is the measured STC.
Source: https://www.soundproofingcompany.com/soundproofing_101/understanding-stc-and-stc-ratings
There is also an online calculator that can help you determine STC of your walls.
How can you improve the wall's STC?
A wall's STC can be increased by
1. Adding mass
2. Increasing Air Space
3. Adding absorptive material (soundproofing systems)
Conclusion: Determining the STC Rating of a Wall
The STC is measured by Transmission loss of the loudness of the sound (in dB) and at various frequencies.
It depends on the material and the quality of the wall itself.
You might also like: STC Ratings for Windows