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# How Noise Reduction Coefficient (NRC) Numbers Work

Regarding acoustic absorption, the quantity of absorption is numbered in "sabins". Sabins are derived by the "NRC" (noise reduction coefficient). NRC numbers are available on all acoustically rated products, usually on the data sheet. The higher the NRC, the better the absorber. The NRC may be thought of as a percentage of absorption per square foot. Sometimes an NRC number is higher than 1.00 (i.e. NRC 1.05). This cannot mean that it absorbs 105%.

Long ago, a man named Sabin found that some mysterious material was the best absorber known to man (maybe it was lambs wool) and they called this NRC 1.00 as this was the best known absorber at the time. As time went on, other materials were discovered to have a better NRC so the numbers had to grow.

Hence we find NRC numbers greater than 1.00.

## How many sabins of absorption do you need?

The best way to ballpark this, is to take the cubic area of the room to be treated. Multiply this number x .049. The answer is the number of "Sabins" you need to get a 1 second reverb time (this would be library quality). BUT! You don't need that much for a social hall, gymnasium, industrial shop or swimming pool area, etc., so halve that number to get a 2 second reverb time and that's about where you want to be.

Example: room size 24' x 50' x 15' high
Cubic area (length x width x height) = 18,000 cubic ft.
18,000 cubic ft x .049 is 882 sabins of absorption for a 1 second reverb time.
To get the 2 second reverb time, divide the 882 sabins by 2.

This means if you have an 18,000 cubic ft. room, you need 441 sabins of absorption to make it sound like a high school auditorium.
So, divide the 441 by the NRC number from the data sheet and that's how many square feet of that material you will need.
*NOTE: This is only a guideline. You can start with as little as one-half this amount of absorber but don't expect the same result as if you were to use the entire 441 as in the example above.

*ALL ACOUSTIC ABSORBERS ARE RATED WITH AN NRC AND IT SHOULD HAVE A TEST NUMBER FROM A RECOGNIZED NATIONAL LABORATORY SUCH AS:
ASTM, FACTORY MUTUAL, MVSS, or RIVERBANKS LABS