Do You Hear What I Hear?

An Overview of Sound Transmission Class (STC) and Cover Boards 

Both building owners and industry regulators continue to have growing concerns about managing sound in residential and business environments to ensure comfort for building occupants.

In the past, a building’s sound issues were tackled from the interior looking at walls and ceilings. Now, there is a continued focus on the exterior of a building with a desire to create systems that can better handle sound generated outside the building, such as air traffic or highway traffic, and within sensitive buildings, such as hospitals and schools, in a more effective way. In this post, we will provide an overview of Sound Transmission Class (STC), how it is measured and tested, several ways to increase STC and finally how cover boards help create better environments for STC.


STC is the most common sound isolation metric used today and is for airborne noise, or unwanted sound. Typically measured in decibels (dB), the higher the STC value the better that system is for blocking airborne noise. It is important to keep in mind that dB’s are a logarithmic unit and displayed on a logarithmic scale, therefore, going from 40 dB to 50 dB isn’t a 125% increase in perceived sound. Rather, it means the perceived sound is twice as loud. This is because the measurement is taken over a frequency range of 125-4000 Hertz (Hz) and uses transmission loss to calculate the STC. The STC value of a system (wall, ceiling, roof, etc.) indicates how much sound is being stopped by that system. For example, if a sound of 100 dB is generated in a room and only 60 dB is heard in the room next door, that means the wall has a 40 STC rating. Since STC does not focus on low frequencies, it also has limitations in real world applications for sounds that are generated above a building. An example is air traffic, as it has a great deal of low frequency noise that is missed by STC but outside inside transmission classification (OITC) indicates how a roof system blocks these lower frequencies. Despite its limitations, STC is an easy number to use and allows agencies and companies to gather data on systems in order to help solve concerns around sound and a building. 


Calculating the STC

Third party laboratories with trained acousticians on staff typically conduct the STC testing. There are several standards for measuring transmission loss, but the most popular ones are ASTM E90 (Standard Test Method for Laboratory Measurement of Airborne Sound Transmission Loss of Building Partitions and Elements) and E413 (Classification for Rating Sound Insulation). These ASTM standards create a method for testing airborne sound transmission loss in partitions and obtaining a single number rating called STC. There is a lot that goes into the actual testing of the partitions, but here’s an example of how it works: 

  • A partition is constructed between two rooms in a laboratory setting.
  • While noise is generated on one side of the wall, the sound levels over the frequency range of 125 to 4000 Hz are measured on both sides of that partition.
  • The transmission loss of the system is calculated based on the difference between the sound levels in the two rooms. The sound level is then combined with factors regarding the area and room characteristics while in the laboratory environment, such as wall reflectance, temperature and humidity. 
  • STC is calculated for that partition based on the transmission loss data collected.

Improving STC

There are many applications that might be able to help improve airborne sound transmission, and there are several ways to increase the STC rating of a roof and ceiling assembly specifically.

First, adding mass to a roof and ceiling assembly can have a significant effect on blocking sound. Adding layers of material, such as a cover board, accomplishes this. Increasing air space between the ceiling tile and a roof deck increases the STC more than having ceiling tile directly under the roof deck. Additionally, using an absorptive material such as a specialized ceiling tile or increasing the amount of insulation will help improve the STC. Finally, decoupling is also an option. It is a mechanical solution that separates layers of the roof and ceiling assembly, making it difficult for sound to pass through. This typically requires the use of sound isolation clips.



There are many ways to increase the STC rating of a roof and ceiling assembly. 

Recent testing shows that cover boards help with the complex issues surrounding building exteriors and STC. Some of the testing results include: 

  • Adding roof boards increased the system STC incrementally.
  • Glass-mat and gypsum-fiber cover boards provided similar results for STC.
  • The type of insulation used impacts STC.
  • Mechanically attached and fully adhered systems provide similar results for STC testing.

To ensure the best system is installed and the highest STC rating for the project is secured, three elements must be understood and considered. Those elements are an understanding of STC and its impact on a space, how STC is measured and lastly, how cover boards can be used to create a better STC.