Sound walls are performance rated in two categories: Sound Transmission Class (STC) and Noise Reduction Coefficient (NRC). The STC determines the amount of noise energy transmitted through the wall material. The NRC determines the amount of energy absorbed by the wall material and the amount of energy reflected back towards the source.

Walls having STC ratings of 30 or more means that less than 0.1 percent of the noise energy is transmitted through the barrier material. Many State Department of Transportation specifications require minimum STC ratings of 24.

NRC measures the amount of sound energy absorbed and measures the amount of sound energy reflected back towards the source. NRC ratings will range between 0 (100% reflective) to 1 (100% absorptive). A wall with an NRC rating of .85 means the wall absorbs 85% of the noise and reflects 15% of the noise back towards the source. NRC ratings equal to or greater than .85 are considered to be good sound absorbers and are often used as the minimum requirement when considering absorptive walls.

A good sound wall is a sound-absorbing wall with a STC rating of 30 or more and a minimum NRC rating of .8.

Is a sound wall the best mitigation option for every application? Certainly not. There are effective mitigation tools for any situation – from earplugs to ceiling tiles. But for many outdoor applications such as traffic, manufacturing, compressor and commercial retail noise, nothing outperforms a well-engineered and efficient absorptive sound wall.

* * *

¹ Infoplease, Copyright © 2000–2006 Pearson Education, publishing as Infoplease.

² H.I.N.S., Inc