Sound Absorption and the Noise Reduction Coefficient (nrc)

When sound waves travel through air and encounter another medium, the wall of a room, for instance, a portion of the sound will be absorbed by the wall while the remainder will reflect from the wall surface. Add to the mix the other surfaces reflecting sound waves in various directions within the room, and the result is a jumble of sound reflections which interfere with the clarity of the original, intended sound. The presence of numerous hard, untreated surfaces is often to blame for the heightened noise levels in busy restaurants as the voices of multiple patrons reflect and produce background noise. Acoustical improvement and sound reduction projects often involve the implementation of treatments designed to absorb sound wave reflections as a part of a comprehensive sound control plan.

Taking into consideration the tendency of sound waves to reflect from surfaces, it is apparent why the absorption of sound reflections is an important element of sound control and acoustical improvement. Capturing sound reflections becomes particularly important in environments such as auditoriums, restaurants and places of worship, where the quality of acoustics experienced within an area is fundamental to its purpose.

The reflective tendencies of the particular surfaces present within a room may contribute to the type and extent of sound absorption treatment necessary to achieve the desired result. Carpet, for instance, is more absorptive than tile, so a carpeted floor would produce less significant sound reflection than would a tile floor with other factors constant. How do we know? By performing the standard ASTM C423 test, independent labs quantify the amount of sound a particular surface material absorbs and assigns to it a value referred to as the Noise Reduction Coefficient (NRC). The NRC, calculated based on testing of a material at various frequencies within a concrete enclosure, can be interpreted as a percentage of noise that is captured and converted by a surface while the remaining portion is reflected. A NRC of 0.44, for instance, would be assigned to a surface which on average absorbs 44% of sound while reflecting the remaining 54%.

Like materials used in construction, soundproofing products are also tested for sound absorption at various frequencies and assigned a Noise Reduction Coefficient. The absorptive value exhibited by a particular sound treatment can help determine the circumstances in which the treatment should be applied. A project aimed at controlling low bass tones, for example, would call for more absorptive treatments than one targeting the higher frequencies produced by the human voice. Since each acoustical improvement solution must be tailored based on characteristics of the targeted area and frequencies to be controlled, it is advised to consult with a knowledgeable soundproofing consultant prior to implementing sound reduction treatments in order to ensure that critical variables are addressed.

About the Author: Mark Rustad is President of NetWell Noise Control, based in Minneapolis, MN. Founded in 1991, NetWell is a leading supplier of acoustic products, soundproofing treatments, sound reduction products and online acoustical consulting services. NetWell?s sound management skills are packaged into the industry?s premier website. Discover first hand why so much of NetWell?s business stems from the referrals and repeat orders they receive from satisfied clients around the world.