NRC - stands for noise reduction coefficient

NRC measures how well something absorbs sound, how well materials stop sound waves from reflecting). The NRC of different materials vary significantly. Understanding these, differences can go a long way in knowing what materials to use to achieve a specific result.

A Basic diagram that shows the difference between NRC & STC Ratings

STC - is short for sound transmission class

Sound Transmission Class is more complicated calculation than NRC, involving the graphing of ”contour curves”, transmission loss curves, and identifying a number on the contour where the two graphs are closest together. In regular terms, however, it measures how well an item blocks sound from passing through it.

NRC is a measurement of how well a product makes the room you are in quieter, while STC measures how well a product keeps sound from escaping the room.

When to use NRC

When you are in a noisy room like a gymnasium, you want a product with a higher NRC on the walls or ceiling to make the room less noisy.

When to use STC

If you are in a conference room where you are talking about sensitive topics that you don’t want people on the other side of the wall to hear. You need a product on the wall with a high STC. The image below features panels mounted to a gym wall to help reduce the echo.

NRC Units of Measurement

NRC usually ranges from zero to 1.0. A reasonable rule of thumb is that the NRC of a product is the percentage of sound a product will absorb. A painted drywall wall has NRC of about .05, so it absorbs only about 5% of the sound that hits it and reflects back 95% of the sound. A deep-pile carpeted floor may have an NRC as high as .35. 1” thick fabric covered acoustical panels designed specifically to absorb sound have an NRC closer to .80, making them very sound absorptive, reflecting back only 20% of the sound that hits them.

STC Measurement is a little More Complicated

Unfortunately, the STC value is not something one can explain simply. It may be easier to explain using a real life situation. A metal stud wall with ½” thick drywall on both sides has an STC of 34. While that is certainly better than nothing, it’s not very good. A metal stud wall with sound attenuating batt insulation in between the studs and 5/8” thick drywall on both sides has an STC of 47. That is much better and probably the most common construction in commercial buildings. breezeblock walls typically have an STC in the upper 40’s to low 50’s.

There are ways to increase the STC of a wall even further – adding another layer of drywall to the wall, staggering the studs, using resilient channels – but that can be discussed elsewhere. Unless you are building movie theatres or some other application where blocking every bit of sound penetration is critical, you should not have to go to those extremes. STC ratings in the upper 40’s are good. STC ratings in the 50’s are excellent. When selecting the right Working Walls Solutions product for your noise problem, you need to first identify what the problem is. If you have a space with too much echo, you need a product with a good NRC. On the other hand, if the problem is sound going through a wall, you need one of our products with a decent STC.

Live Acoustics provide a comprehensive service to help you refurbish or develop your auditorium, multipurpose room or performance space.


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