NRC and STC. What are they? What’s the difference?

NRC and STC. What are they? What’s the difference?

In the literature for many acoustical products you will find an NRC rating.  On some you may also find an STC rating.  What’s the difference?  These two ratings have completely different meanings.

NRC is short for Noise Reduction Coefficient.  In techno-speak, it is the average of the individual sound absorption coefficients of an item at 250, 500, 1000 and 2000 hertz.  In much plainer language it is basically a measurement of how well something absorbs sound, mostly in the range of normal speech frequencies.

 

STC is short for Sound Transmission Class.  It is a much 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 is a measure of how well an item blocks sound from going 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.

If 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.  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 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.

Unfortunately, the STC value is not as easily explained.  It’s best to discuss it in terms of real life situations.  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.  Cinderblock 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 theaters 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 pretty 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 like our APS-3 or HIAPS-3 panels.

On the other hand, if the problem is sound going through a wall, you need one of our products with a decent STC like our Decorative Tackboards as reflected below.

Our APS-1.5 product offers both NRC and STC.  It will help make a room quieter so there is less sound to escape, while at the same time making the wall it’s on more impenetrable to sound.

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One Comment

  1. Robert Vaughn April 7, 2018 at 4:20 pm - Reply

    Gary,

    Appreciate your explanation of NRC and STC. Very helpful as we are researching sound management considerations for a home studio.

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