Sound Absorption and the Noise Reduction Coefficient (nrc)

When sound waves travel through air and encounterwith other factors constant. How do we know? By
another medium, the wall of a room, for instance, aperforming the standard ASTM C423 test,
portion of the sound will be absorbed by the wall whileindependent labs quantify the amount of sound a
the remainder will reflect from the wall surface. Add toparticular surface material absorbs and assigns to it a
the mix the other surfaces reflecting sound waves invalue referred to as the Noise Reduction Coefficient
various directions within the room, and the result is a(NRC). The NRC, calculated based on testing of a
jumble of sound reflections which interfere with thematerial at various frequencies within a concrete
clarity of the original, intended sound. The presence ofenclosure, can be interpreted as a percentage of noise
numerous hard, untreated surfaces is often to blamethat is captured and converted by a surface while the
for the heightened noise levels in busy restaurants asremaining portion is reflected. A NRC of 0.44, for
the voices of multiple patrons reflect and produceinstance, would be assigned to a surface which on
background noise. Acoustical improvement and soundaverage absorbs 44% of sound while reflecting the
reduction projects often involve the implementation ofremaining 54%.
treatments designed to absorb sound wave reflectionsLike materials used in construction, soundproofing
as a part of a comprehensive sound control plan.products are also tested for sound absorption at
Taking into consideration the tendency of soundvarious frequencies and assigned a Noise Reduction
waves to reflect from surfaces, it is apparent why theCoefficient. The absorptive value exhibited by a
absorption of sound reflections is an important elementparticular sound treatment can help determine the
of sound control and acoustical improvement.circumstances in which the treatment should be
Capturing sound reflections becomes particularlyapplied. A project aimed at controlling low bass tones,
important in environments such as auditoriums,for example, would call for more absorptive
restaurants and places of worship, where the qualitytreatments than one targeting the higher frequencies
of acoustics experienced within an area isproduced by the human voice. Since each acoustical
fundamental to its purpose.improvement solution must be tailored based on
The reflective tendencies of the particular surfacescharacteristics of the targeted area and frequencies
present within a room may contribute to the type andto be controlled, it is advised to consult with a
extent of sound absorption treatment necessary toknowledgeable soundproofing consultant prior to
achieve the desired result. Carpet, for instance, is moreimplementing sound reduction treatments in order to
absorptive than tile, so a carpeted floor would produceensure that critical variables are addressed.
less significant sound reflection than would a tile floor