NRC – What Is It?
While the STC rating measures how much noise is held inside an area, the NRC rating has nothing to do with a material's
barrier qualities. Instead, NRC measures how much noise is absorbed into a material itself, rather than being reflected by
it. This difference is important depending on the type of noise that will be in the room. If your priority is blocking human
voice from reaching other rooms, you would want your room to have a high STC rating. On the other hand, if your priority is
improving the acoustics in a room, increasing speech clarity, and reducing echo, you would want a room with a high NRC
Let's look at a home theatre system as an example. Audio systems produce 80–90 decibels (dB) of continuous sound
power while the human voice produces around 50 dB. A room with a high STC rating will hold this high level of sound in the
room, rather than release it into other rooms. With the high level of continuous sound produced by a home theatre system, the
sounds waves in a room with a high STC can begin to interfere with each other, and may lose clarity and become garbled. This
is where NRC is important. NRC measures the level of sound that is absorbed into the room rather than reflected around it.
More absorbent rooms have a higher NRC rating, thus absorbing sound waves rather than allowing them to bounce back and
interfere with other sounds.
How NRC Is Determined
The material in questions is tested in a concrete enclosure using various frequencies that are usually 250, 500, 1000, and
2000 Hz. More modern laboratories may increase this to frequencies between 100–5000Hz. The amount of sound absorption
is measured using the sound absorption coefficient rating of .00–1.00. A rating of .00 is very reflective, .50 absorbs
50% of sound and a rating of 1.00 would absorb 100% of sound. The average of the sound absorption coefficient ratings is what
gives us the NRC rating.
Other Helping NRC Info
Carpet, vinyl, cork, rubber, and other types of resilient flooring have higher NRC ratings than hard, unforgiving
surfaces, such as tile, stone, hardwood, and bamboo flooring. Carpet, having an NRC rating of .40–.50, has the highest
NRC rating of any type of commonly used flooring. Most other types of resilient flooring only have an NRC rating of
.15–.25 with hard surfaces (tile, hardwood, etc.) having an even lower NRC rating.
Even the highest rated NRC flooring is only moderately effective in absorbing sounds, which is why the NRC rating is not
that commonly used in the world of flooring. On the other hand, acoustical ceilings can have typical NRC ratings in excess of
.50. In addition, NRC ratings in a room can be increased with the use of sound absorbing drapes, furniture, and other