The Sound Transmission Class (STC) is a numerical rating system that quantifies a physical structure’s effectiveness in blocking noise. It is widely used to assess the sound-blocking capabilities of walls, ceilings, doors, and windows.

Essentially, it indicates the extent to which a structure muffles or reduces sound transmission to the other side.

This article explains STC rating, discusses its calculation, identifies the factors involved, and compares it with other standards.

What makes a good STC rating?

STC RatingObserved Result
25Clear speech can be easily heard from the other side
30Loud speech is intelligible. Normal speech can be heard but not understood
35Loud speech is audible but not clear
40Good level of privacy. Loud speech can be heard as muffled sounds
45Loud speech is barely audible
50Thunderous speech can be faintly heard
Standard STC rating range and the observed results

To accurately determine the effectiveness of a material or structure in blocking sound, STC ratings must be measured objectively rather than relying on personal perceptions.

The STC scale, specifically designed for this purpose, usually ranges from 20-80, with higher numbers indicating better sound-blocking capabilities.

It is important to note that even minor variations in sound transmission can significantly impact the ratings.

To determine the appropriate STC rating for a given space, it’s essential to consider that space’s specific needs and requirements.

For instance, what may be suitable soundproofing for a business meeting room may not be necessary for a private living room.

Conversely, a lower STC rating that may be sufficient for a casual living space may not be adequate for a professional conference room where sound reduction is crucial.

High vs Low STC Rating – Which is Better?

The higher the STC rating, the better a material or assembly is at blocking sound.

For example, an STC rating of 45 would indicate that a material or assembly blocks about 45 dB of sound, which would be considered excellent sound insulation.

A rating of 25 would indicate that a material or assembly blocks about 25 decibels of sound, which would be considered poor sound insulation.

However, it’s important to note that the desired STC rating will depend on the specific sound reduction needs of a building or room.

It will vary based on the type of noise, the specific location of the noise, and the intended use of the space.

Consider consulting an acoustical engineer to determine the best STC rating for your space.

Generally, a higher STC rating is better as it indicates better sound insulation performance, but it’s also essential to consider the cost factor.

If an installation with an STC rating of 45 can block the noise you are dealing with, then installing a 60-rated setup will only drain up your pocket.

But, if you want to future-proof your space, it’ll be an excellent way to go, but keep an eye out for the installation’s approximate lifetime.

How to Find Out the STC Rating of an Assembly?

There are two ways to determine the STC rating of an assembly accurately. Either through lab or field tests.

When original drawings of a building are available, you can determine the intended STC rating by comparing it to data from lab tests that have already been conducted on similar designs.

This can serve as a starting point for determining the STC rating.

However, not all structures are built to their original specifications. Therefore, on-site testing is a more reliable method.

Field tests are conducted on the actual physical location. This testing method ensures accuracy, reliability, and repeatability by comparing the results to established standards.

It involves placing a loudspeaker in one room that generates noise across various frequencies.

The sound levels are then recorded from both the source and receiver rooms.

Then the reverberation time in the receiver room can be used to calculate sound absorption from these measurements.

When plotting the measurements on a graph, you’ll get a curve.

Airborne sound insulation data from NTi Audio based on ASTM E336-20, ASTM E413, etc.
Airborne sound insulation data from NTi Audio based on ASTM E336-20, ASTM E413, etc.

Then this curve is compared to the standard STC benchmark curves to find similarities.

The curve most closely resembles the one you have just measured is the STC rating for the test.

For example, if the curve that best represents the window you measured is most like a basic STC 35 curve, that window has an STC rating of 35.

Techniques to Improve STC Rating

You can use several techniques to improve a room’s STC (Sound Transmission Class) rating.

Increasing the mass of the assembly

Adding more mass to a wall or floor, such as thicker drywall or concrete, can help block more noise.

This is because a sound is a form of energy, and thicker, heavier materials can absorb and dissipate more of that energy, resulting in less sound transmission.

Additionally, having a greater mass can help block sound from entering or leaving a space by creating a more solid barrier between the two sides.

Installing soundproofing materials

There are a variety of soundproofing materials, such as acoustic insulation, sound-deadening paint, sprays, and sealants.

You can use them to improve the STC rating of a room without making complicated modifications.


Decoupling in soundproofing
Decoupling in soundproofing (Source: Everkem)

Decoupling is an effective technique for reducing the transmission of sound between walls or floors in a building.

In decoupling, the walls are isolated from the structure of the building to prevent sound waves from transmitting through the contact point of the structure.

You can do this using materials or methods that isolate one surface from the other.

Some techniques include using sound isolation clips and resilient channels to decouple drywall from studs in a wall or using rubber isolation mounts to decouple a floor from the joists beneath it.

Adding an air space

Adding an air space between two layers of drywall, also known as a “double drywall,” helps in soundproofing by creating a decoupling layer.

This means that the two layers of drywall are not directly attached, allowing them to move independently.

A double drywall installation
A double drywall installation (Source: MicronXD via StackExchange)

When sound waves hit the first layer of drywall, they cause it to vibrate. Without the air space, these vibrations would be transmitted directly to the second layer of drywall and ultimately to the other side of the wall.

But with the air space, the vibrations are absorbed and dissipated before they can reach the second layer of drywall.

This can significantly reduce the amount of noise transmitted through the wall.

Adding a resilient channel

Adding a resilient channel between two layers of your drywall can help to soundproof as it creates a decoupling layer.

A resilient channel is a type of metal framing designed explicitly for soundproofing.

You can install it between the two layers of drywall. It works similarly to how an air space would work.

For the resilient channel, use a flexible and “resilient” material for movement, meaning it can move independently from the drywall layers.

Installing a resilient channel between the floor
Installing a resilient channel between the floor (Source: Simpson Properties – YouTube)

When sound waves hit the first layer of drywall, they cause it to vibrate. Without the resilient channel, these vibrations would be transmitted directly to the second layer of drywall and ultimately to the other room.

But with the resilient channel, the vibrations are absorbed by the channel and dissipated before they can reach the second layer of drywall.

The resilient channel also has the added advantage of providing a more secure and stable mount for the drywall. It’s more resistant to impact noise compared to an air space alone.

Using acoustic doors and windows

Special doors and windows designed for sound control can block sound from entering or leaving a room.

Acoustic doors and windows effectively reduce the transmission of sound through them, but they may not eliminate all noise.

They are designed to reduce sound transmission, not absorb/dampen.

If you are interested, I have written an article on the best soundproofing windows.

To fully soundproof a room, you will need to use a combination of methods, such as adding insulation, sealing gaps and cracks, and using sound-absorbing materials on walls, ceilings, windows, and floors.

If you work with a professional acoustical engineer, they can help you better evaluate the specific sound transmission issues and design a solution to your situation.

STC vs SRI vs STI ratings

STC, SRI, and STI are all internationally recognized standards for measuring the sound insulation performance of building materials and products.

However, the specific requirements and testing methods for these ratings can vary depending on the country or region.

For example, ASTM International (American Society for Testing and Materials) sets standards for STC, SRI, and STI testing and ratings in the United States.

Additionally, some countries or regions may have specific regulations that differ from international standards.

It is essential to check the local regulations and standards when specifying sound insulation products for a specific project and to ensure that the products used meet the required ratings.

STC (Sound Transmission Class) is a rating system primarily used in the United States. It is also used in Canada and other countries that have adopted similar building codes.

However, it’s worth noting that these countries can have different measurement systems and standards for sound transmission in buildings.

So your STC rating in the US may not be directly comparable to Canada’s.

SRI (Sound Reduction Index) rating is also similar to STC. SRI is primarily used in Europe and other countries that have adopted the European standard (EN ISO 717-1) for measuring sound insulation in buildings.

Some countries using SRI include the UK, Germany, France, Spain, Italy, etc.

Speech Transmission Index (STI) is a rating system similar to STC but uses different quantitative measurements.

It is specifically designed to measure the reduction of speech transmission rather than overall sound transmission.

STI rating is primarily used in countries like Australia, New Zealand, the United Kingdom, etc., adopting similar building codes.

Final Thoughts – Is STC effective?

STC only measures sound transmission through walls, floors, windows, and other similar installations.

It doesn’t measure sound transmission through airborne paths, so it is not a complete measurement of the sound insulation of a building.

Overall, STC is an essential tool that can be used to ensure that buildings are constructed in a way that promotes acoustic comfort and privacy.

Understanding STC ratings can help you make informed decisions about the materials and methods to construct soundproof walls, floors, doors, and window installations.

Frequently Asked Questions

What’s Drywall’s STC Rating?

The drywall’s STC rating is typically between 35 – 40. The rating of drywall can be influenced by various factors, such as its manufacturing process, specific raw materials used in production, the thickness of the drywall, etc.

What is a good STC rating for walls and floors?

A good STC rating for walls and floors varies depending on the intended use of the space. For example, a higher STC rating of 55 and above is typically desired in a recording studio or home theater. In contrast, a lower rating (under 45) may be acceptable in a room with less critical acoustics, like a laundry room.

What does STC 40 mean?

An STC rating of 40 means that the tested material or assembly can reduce sound transmission by 40 dB. This can be considered an average value for sound insulation buildings or spaces with less critical acoustics, such as a garage or storage room.

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