How to Determine STC Rating of a Wall, Drywall and Windows


It does not take long when shopping for a soundproofing project to come across a reference to a Sound Transmission Class rating. Almost always shortened to STC, it is the standard way to measure just how well certain materials are in regards to blocking sound.

A lot of companies include STC rating numbers, so a person does not have to determine the rating themselves. However, it is still important to know how to determine the STC rating of a wall at home before and after a soundproofing technique is added.

STC Rating Meaning

STC ratings first popped up in the 1960s, as more and more people were looking to reduce sound transmission as much as possible between rooms at home, businesses, and more. This provided a uniform way to compare different types of materials as easy as possible.

It roughly measures the reduction in decibels to noise thanks to the material added. A higher number is better. For people not familiar with the rating, it might be best to break it down by what a typical family might here at home, depending on the STC rating.

STC Rating Chart

STC ratings are pretty much the only way to compare and contrast different products out there accurately. Since every product uses the same numbers and scale, it will allow people to understand the exact impact as much as possible.

Is the average amount of noise stopped at a total of 18 frequencies so people get an understanding of both high and low-frequency blocking?

STC Rating Chart

Determining STC Rating of Wall, Drywall, and Windows

STC Rating of Drywall

Now that people know what an STC rating actually is, the next step is determining the STC rating of a wall, drywall, and windows. This might seem a little difficult to do, but as long as a person has somewhat of an idea on how the ratings work, it is a pretty straightforward process.

It is common knowledge that no two walls are exactly the same. An extremely thin wall is usually going to do a pretty poor job of blocking sound and therefore have a low STC rating.

Thicker walls have more mass, and that leads to a higher STC rating. This is a pretty straightforward process, but it becomes trickier when walls are pretty much the same in thickness, but different material.

Measuring the STC rating of drywall first means looking at the specifics. Does the wall already have a resilient channel, or does one need to be added?

Is the wall made of drywall, concrete, or something else? What is the spacing like with the studs on the wall, and what are they made of?

Only people who know the specifics can then take the next step towards picking out the right soundproofing material. In some cases, there might be limited options that actually work.

In other cases, there is so little soundproofing done already that the STC rating is very low. That can be good news for people who want to take care of everything themselves because they can virtually start from scratch.

The measuring process is fairly easy for people who want to do things on their own instead of going off of estimates. After all, real-life performance is usually why people worry about it in the home or office.

All that is really needed is a device that measures in decibels to take readings into different spots. The first reading is the sound in the same room as the sound source. For the most accurate reading, take the measurement right up against the wall.

The next step is to go to the other room and measure on the side of the wall. Make sure that the sound remains at the same frequency, and stays in the same place. Simply subtract the second number from the first number to get transmission loss.

Here’s a video describing the difference between 1/2″ and 5/8″ drywall. The different thickness of the drywall will certainly raise the STC rating of the drywall.

What Changes a Wall’s STC Rating?

During the examination process, there are three important factors to keep in mind when determining the STC rating of a wall.

Wall Material

The material and thickness of the actual wall have a huge impact on the STC rating. Ratings will differ quite a bit between walls made of concrete or drywall, and even between concrete and drywall options of different thicknesses.

Vulnerable Areas

A wall might mostly be soundproof, but it just takes a weak point or two for the STC rating to go down. Airborne sounds can travel through pretty small areas that are vulnerable.

Think about windows and doors for sure, but also smaller things like air ducts and ventilation systems. Even the little space between a door and the floor can impact the overall rating.

Frequency

Different sounds are blocked more effectively than others. To determine the STC rating of a wall in a lab, they can test out frequencies from 125 Hz to 4000 Hz. This gives some varied answers for sure, which is what is used as part of an average to give an official STC rating.

If only one or two types of frequencies are predominant in an area, the overall STC rating might not matter as much. That is why in some cases, people like doing their own real-world tests instead of solely going off of STC ratings.

Comparing STC Ratings to Real Performance

STC Rating Walls

People might be a little bit satisfied with results after doing some soundproofing based on STC ratings. Keep in mind that real-life performance is usually a little worse than what STC ratings tend to suggest. This mostly has to do with the fact that STC ratings are measured in a lab.

Everything is controlled as much as possible in this scenario. In a real-life test, people are not going to have everything nearly as perfect.

It is usually not that big of a difference, so most people should be just fine using STC ratings as a solid base. It is, however, recommended to soundproof a little bit more than the suggested amount, to factor in any deficiencies once real-world measurements are examined.

The Most Basic Ways to Improve a Wall’s STC Rating

If measuring the STC rating of a wall is not satisfying enough, the next step is to increase it to a level that is. Adding mass is perhaps the easiest way to make quick improvements to a wall.

Something like mass loaded vinyl, soundproofing panels, and more are all pretty cheap to purchase, and capable of full insulation without any professional help.

Increasing air space in between two walls is another way to improve the STC rating. With more airspace, that allows more opportunity for sound waves to reduce significantly. This is sometimes tricky, so do not try to take on a project like this without looking for professional assistance first.

Finally, setting up a true soundproofing system is a more permanent solution for people who are just not satisfied with the wall’s STC rating at all. This involves more than just slapping on some panels that add mass.

If professionally done, soundproofing systems 100% work, and are used in many professional set up. Just keep in mind that the cost of a soundproofing system, and the amount of time it takes to do it properly, might be more of an investment than some people want.

Martin Poirier

For the last number of years, I've Been dedicating a lot of time in soundproofing and helping people be able to soundproof their home, business, and vehicles. I also have a YouTube channel by the same name Soundproof Guide.

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