What is STC rating [2023]

You may want to learn more about STC rating when planning a construction project.  

Transmission Class ratings generally pop up in places where we either want to keep sounds in or out like a sound studio or a bedroom above the garage.  

People rely on the Sound Transmission Class or as STC rating of a product when evaluating building materials and acoustical products for sound reduction. 

By understanding what the STC rating of a product means, you will have an idea of how much sound will be blocked from passing through the material.

Some people may confuse STC ratings with NRC ratings because they are both used to rate materials. 

However, they are two slightly different rating systems. 

STC tells you how much sound is blocked from going through a product while NRC tells you how much sound is absorbed by the product.

To learn more about what an STC rating is, what it means and how to calculate it, keep reading.

What is STC rating?

Some transmission class is an integer rating that determines how well a product can reduce airborne sounds. 

STC sound rating was introduced in 1961 as the method of comparing various wall, ceiling, floor, door and window assemblies. 

STC is calculated by taking the Transmission Loss(TL) values tested at 16 standard frequencies over the range of 125 Hz to 4000 Hz.

In the US, STC is used to rate interior partitions, ceilings, floors, doors, windows and exterior wall configurations. 

STC gives you a rough estimation of how much sound a wall, for example, may stop.

STC is the most commonly used sound reduction measurement.

However, as common as it is, it is quite limited and you shouldn’t totally rely on it for real-world soundproofing expectations.

What is measuring STC good for?

Sound vibration decreases as it travels from one side of a wall to the other. 

You will need to quantify and measure this loss to fully understand the value of STC ratings. 

Understanding STC can be valuable but what matters most is whether your choice of soundproofing solution actually works. 

You’ll find that some expensive and exotic building materials don’t always help much even though they may look great and sound great on paper. 

For good soundproofing results every time trying to stick to the inexpensive yet effective basics like essential decoupling, mass added from drywall and a competent dampening compound. 

Sound Transmission Basics

Decibels 

A decibel (dB) is a logarithmic unit that is used to measure how loud a sound is. 

Think of decibel as a volume controller of a device.

A low dB is quiet and a higher dB is so loud it can cause physical pain.

To understand decibels a little better, take a look at the table below.

Transmission Loss

Transmission Loss is a measurement of the decibel or volume difference on both sides of a wall.

For instance, let’s say you have a wall with a 100 dB tone on one side and you measure the same tone on the other side of the wall but find that it has a dB of 75 on this side of the wall.

So you can safely say at this tone or pitch, the Transmission Loss is 25 dB .

This means that less than 25 dB of sound energy made it through the wall to the other side. 

Frequency 

Frequency is the measurement of the tone or musical note of a sound. 

It is written as Hertz or Hz. 

Frequency can have a high pitch like the sounds that a flute may make which are around 2000hHz or a low pitch like the sounds from a tuba that go as low as 29Hz. 

Most people are born with the ability to hear frequencies from as low as 20Hz to as high as 20,000Hz. 

However, that range shrinks as we grow older. 

Interestingly enough a test tone with a different pitch sent through the same wall in the example above may see a Transmission loss of only 4dB. 

The performance of the wall varies greatly when it comes to the tone or frequency of a sound.

Calculating STC

Calculating STC can look complicated but it’s not as tricky as you think. 

To determine the STC ratings for windows, walls and ceilings you can do it the same way as defined by the ASTM E90 standard. 

The first step in figuring out the STC ratings of materials is to determine the transmission loss of the material at various frequencies. 

Here’s an example. 

Let’s say you have two rooms with measurement equipment and they are divided by some type of material. 

One room is the source room and the author is the receiving room. 

The sound pressure levels in both rooms are compared at various 1/3 of the band frequencies which are from 125 Hz to 4000 Hz. 

The difference is used to calculate the transmission loss of the material at each of the frequency bands. 

Each transmission loss value is then plotted on a graph and compared to a standard set of STC contours. 

The closest matching contour determines the STC rating of the material. 

If your results look more like the curve for an STC rating of 50 then that would be the rating of the material you tested. 

If this seems like too much work, you can p[erform a search for an STC wall rating calculator in your favorite search engine.

What is a good STC rating?

We’ve all at some point in our lives been in a place where the walls were paper thin and you could literally hear every little thing that went on in the room next door. 

No doubt that the wall has a low STC rating. 

STC ratings need to be measured to calculate noise reduction capabilities on measurable facts, not individual opinions. 

This is because the perception of sound is unique to each person and is up to interpretation. 

So, what is considered to be a good STC rating? 

STC ratings are measured using a uniquely designed scale. 

The higher the number is, the greater the ability of the measured structure or material to reduce sound transmission. 

The scale used is sensitive and even small changes in ratings can result in dramatic differences in sound transmission. 

To answer the question of what a good STC rating is, you’ll need a bit more context. 

What may be good for one situation may not be good for the other especially if soundproofing is vital. 

A good and acceptable rating for walls may not be the same for floors, etc. 

With that in mind, an STC rating below 40 will allow you to hear most of what’s happening on the other side of the wall. 

For further help, check out the STC rating chart below.

STC rating chart 

Is a higher or lower STC rating better? 

The higher the STC rating, the more sound is blocked from going through the material. 

If a wall has a low STC rating you will hear almost everything that happens on the other side. 

You will even be able to understand soft speech coming from the other side. 

If the walls have a high rating, you can play music or speak fairly loudly without disturbing your neighbors. 

Check the STC rating chart above to have a better understanding of this. 

What does an STC rating of 45 mean?

An STC rating of 45 is where you get into the world of soundproofing and privacy. 

This rating can be considered the baseline from where you are getting serious when it comes to preventing some transmission. 

STC rating of 45 is the first level where conversations won’t be understood through the walls. 

A person in a quiet room on the other side may be able to hear a conversation happening but they would not be able to understand it. 

However, as helpful as an STC rating of 45 is, it would be insufficient for insulating townhouses and condos due to shared walls, floors and ceilings. 

What does an STC rating of 50 mean? 

The International Building Code requires an STC rating of 50 for multifamily construction. 

A rating of 50 is the point at which noise is reduced to a level that people feel like their homes are adequately insulated. 

This is because you cannot hear speech through the walls but may hear loud sounds faintly with a sound transmission class rating of 50. 

Recommended STC ratings

If you’re not sure of what rating would work for your project there are some standards you can go by. 

These will be able to help you determine your target range. 

A study on the importance of sound insulation by the National Research Council of Canada found that an STC rating of 55 is effective. 

Meanwhile, an STC rating of 60+ is ideal especially in rooms where there will be loud music. 

These ratings are enough to keep most sounds muffled but if you’re just trying to add privacy to your conversations, an STC of 45 should be enough 

What is the STC rating of drywall?

In this instance, the STC rating of drywall has to do with multiple variables.

These include brand, manufacturing process, the raw material mixture used in production, auxiliary materials and more.

Things like thickness and air space within the wall can have a greater impact on the rating. 

For example, a standard interior wall that consists of two sheets of ½ inch drywall separated by wood studs and no insulation has an STC rating of 33.

This provides almost no form of soundproofing. 

If you add standard fiberglass insulation to the wall, the STC increases to 39.

This won’t keep noises in the next room at bay but at least it’s a step in the right direction.

If you’re serious about soundproofing your interior walls, you can add a soundproofing backer to your drywall.

You can also fill the gaps with acoustic insulation.

Other things you need to know about STC rating

STC Ratings give you a rough estimate of how much sound is reduced in decibels. 

There is no exact correlation from STC rating to dB. 

However, materials do block sounds of varying frequencies. 

The rating works well for things such as speech and daily sounds associated with living or working in an enclosed space. 

However, it isn’t entirely accurate when evaluating music or heavy machinery since these sounds have lower frequencies and will vibrate through structures differently. 

When you’re adding material or changing the construction to improve STC ratings of a surface you need to think about the whole picture. 

You are usually defined by your weakest point when it comes to soundproofing. 

A high STC rating wall can be rendered ineffective if the room has a hollow core door or a single pane window. 

What is a good STC rating for windows? 

The STC rating of your windows is also important if you’re serious about soundproofing.

A standard single-pane window has an STC rating between 26-28.

Well-constructed double-pane windows will have a higher STC rating.

However, if you want to substantially reduce the amount of noise coming in from your windows, consider using a window plug.

These can be permanent or removable inserts that are placed over your existing windows.

They can significantly improve STC ratings and block sounds from the outside world. 

How to improve STC rating

The easiest way to improve STC ratings is by adding mass to your walls.

This increases the overall thickness and blocks out more sounds.

You can add insulation inside the walls which also stops sounds from passing through the wall.

Let’s look at an example.

 2 4-inch walls that are separated with a 2-inch gap would transmit less sound than a solid 8-inch wall.

Filling the 2-inch gap with STC-rated insulation will increase the STC rating even more.

If you add soundproofing treatments inside the room, they will stop the volume of noise in the room from increasing too much.  

However, making meaningful changes to your STC usually involves making structural changes.

Conclusion

Now that you have a better understanding of STC, you will be better able to decide what it means to you and your project.

As you know STC ratings can be affected by a wide range of independent factors.

If you’re dealing with any type of noise issue, there are a few simple things you can do to make things a little better.

In the end, it all comes down to the source of the noise and your tolerance level.If you’d like to know how to soundproof a room using the best soundproofing materials, click the links.