Sound Transmission Class (STC) Rating Explained

Sound Transmission Class STC Rating

Your sound transmission class rating tells you in a simplistic, easy to understand manner exactly how much noise a building fixture (be it a wall, floor, ceiling, or window) will block. Specifically, an STC rating refers to the integer value of how much an airborne sound is dampened or muffled by a structure. 

Ideally, you want to be looking for a product with as high of a Sound Transmission Class Rating as possible. For example, a professionally soundproofed music studio will have an incredibly high STC rating, alongside further interior sound dampening. 

In the US, this rating is more commonly applied to rating interior partitions but can also be used to describe nearly any element of building construction. 

There are various elements to bear in mind that can affect the sound transmission class of any given product or partition. The acoustic medium of the transmitted sound is incredibly important here.

When I say, “acoustic medium”, I am not talking about guitar-strumming psychics, I am talking about both the air and partition material the sound must pass through.

This can affect the STC in numerous ways – any holes or airways in the partition (including, for example, a hollow space in the structure) will decrease the STC rating, and any loosely connected parts of the structure (alongside its material) will have an impact. 

Sound absorption and any damping materials applied to the surface can significantly increase STC, and the stiffness or poor framing of the structure can have a negative impact on the rating.

Unsurprisingly, the mass of your partition will also increase your STC rating. If you have a thick wall, then less sound will pass through it. Any bank heist film will tell you this – Ocean’s 11 would have been much less tense if the guards could hear all the thievery happening in the vault next to them. 

You can also improve the STC of any given partition by applying acoustic joint tape or caulking to any holes or gaps in the surface.  

So, what is Sound Transmission Class?

Further to the brief definition we have already settled upon, Sound Transmission Class has a touch more science behind it. 

This rating is also known as the Sound Transmission Coefficient, to give it its more scientific name, and is calculated by taking sixteen measurements of different frequencies above 125 hertz and below 4000 hertz. 

It should be noted that normally due to the uncertainty and instability of measuring sound outside of test conditions, the results of each given partition measured do not directly inform its exact STC rating. Normally, a graph is plotted of the partition’s STC, and then compared to similar graphical plots of STCs measured under carefully controlled conditions.

Testers then check and see which STC standard rating the partition aligns with, and that is then assigned to the product (or that is what is used in any apartment or building checks). 

The key component that defines STC, and the measurement that is recorded 16 times to assign this rating is something called Transmission Loss.

What is Transmission Loss?

Put simply, it is how much of any given transmission is lost. For STC purposes, this refers to the sound in a transmission passing from one room to another – using the mediums of the air and any partitions in the way. 

The way transmission loss is calculated is the real lynchpin that makes Sound Transmission Class a useful rating. The number you see next to an “STC rating” literally tells you how many decibels are cut from any given transmitted sound that tries to pass through a wall or partition. 

For example, let us say you have a neighbor with a particularly upsetting drum practice habit. Unfortunately, their jam sessions take place in a room that borders your own by one simple wall.

On average, a snare drum will generate between 90 and 120 decibels of noise – equaling a passing subway train or an ongoing rock concert. Have you ever sat next to a drum kit in band? No? There’s a reason that drummers plug their ears. If your wall has a particularly flimsy construction and concurrent STC rating, let’s say a Sound Transmission Class of 35.

In that case, you will hear your drumming neighbor practicing in the next room over to you at a volume of 55 to 85 decibels (this lower end being equivalent to a loud dishwasher, and the upper limit being harmful to your ears if you listen for extended periods).

So, when you are looking at STC levels of different partitions, you want to consider the volume of the sounds you are trying to block with a certain partition. The STC rating of the structure you choose will be how much it reduces that noise by, with some luxury items possibly being able to eliminate such noises entirely!

Why does it matter?

You might wonder why a number like this really matters. Well, if you are particularly invested in your ability to hear properly for a long time, it might become important. Unfortunately, we cannot choose how loud our neighbors will be (unless you own the entire building or block, in which case it is still pertinent information). 

However, most people can control how high the STC of their neighboring walls are. This will basically limit the unpredictable nature of potential neighbor noise – if your bordering walls have a high STC you can be sure that you won’t be able to hear huge amounts of noise from nearby domiciles.

Another way this might be important is if you are a parent or roommate to someone who practices a lot of music. Or if, like me, you do practice music to an exceptionally amateur level and do not want to inflict in the people around you until you get good. 

For example, my guitar produces on average between 60 and 80 decibels of noise (depending on how hard I strum Country Roads). If I have a wall with a 60 STC rating, the likelihood is that my long-suffering housemate will hear little to nothing of my frequent and fraught practices, which is ideal.

What is a good Sound Transmission Class Rating?

Ideally, you want the highest Sound Transmission Class rating possible to eliminate excess noise. Of course, the classic tradeoff is that the higher the STC of a partition, the more “luxury” it is considered, and therefore the more expensive.

This is the classic dichotomy one needs to consider when purchasing partitions based on STC rating.

To give you some insight on what minimum specifications you might want to consider, here is what top audiologists think an STC of any given rating might block:

STC RatingWhat you can hear through the partition
25Regulated, normal speech can be completely discerned.
30Louder speech can be heard AND understood. 
35Louder speech can be heard but is not intelligible.
40Louder speech can be heard as a whispering/murmur.
45Really loud talking can be slightly heard, but not discerned as speech.
50Very loud sounds are barely audible.
60+Considered excellent soundproofing, most sounds bar the loudest screams or amplified music won’t disturb neighbors.
Table – STC Rating

As you can see, a rating of 60+ would be ideal for your partitions. In general, this is thought of as a “luxury” figure for a wall or partition. 

US Building Codes generally require an absolute minimum STC Rating of 35, barring most speech and rendering loud speech unintelligible. This makes sense for the sake of privacy and general notions of personal space and the ability to relax. 

Typical abodes have interior partitions made of a combination of wood and drywall. This has an STC rating of around 30 to 34 as standard, so must be carefully contained and sometimes reinforced to meet the building codes and standards. 

What should you bear in mind when using Sound Transmission Rating?

You might have a reasonable question in mind having read the previous section: if most places are meant to have an STC rating of at least 35, why can a lot of people hear their neighbors? 

The answer is that a Sound Transmission Class Rating is not a perfect or completely accurate measure of the amount of sound that passes through partitions. Firstly, it is not a direct rate derived from the partition in the wild, and more of a comparative rating to experiments done in lab conditions.

Furthermore, while the walls themselves provided in different domiciles might well be within STC building code standards – no wall in a house I have lived in has been in perfect, good-as-new condition. Any time a wall is knocked, dented, drilled through or in some way altered – the STC rating will suffer. It is not a perfect, completely accurate rating for how a partition will function forever. 

It is, however, still a useful tool if used in an informed way as one factor in your purchasing process. 

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