All building materials have some acoustical properties in that they will all absorb, reflect or transmit sound striking them. Conventionally speaking, acoustical materials are those materials designed and used for the purpose of absorbing sound that might otherwise be reflected.
Noise Reduction Coefficient is a method for providing a single number rating of sound absorption. It is defined in ASTM C423 as the arithmetical average of the measured sound absorption coefficients for the four one-third octave band frequencies centred at 250, 500, 1000 & 2000 Hz, rounded to the nearest 0.05. This US based system has been used extensively in Europe for many years but is now becoming less popular with the advent of the Weighted Sound Absorption Coefficient αw.
Alpha w (weighted sound absorption coefficient) is a method for converting a wide frequency based range of sound absorption coefficient values into a single number but this is done using a curve fitting process. Although more complex to derive, Alpha w is considered to be more representative of how the human ear interprets sound. The method is fully described in EN ISO 11654 and has now become the preferred European unit for making a simple and rapid comparison of sound absorption performance.
Absorption Class is a system for the classification of Alpha w values which is defined in EN ISO 11654. Because it groups consecutive Alpha w values together into six broad-bands (each successively identified as A to E & 'Not classified'), it is not so precise and therefore provides less useful detail than when individual Alpha w values are specified or selected.
This absorption diagrams below shows the practical sound absorption coefficient, αp in accordance with the international standard EN ISO 11654. The same standard defines the sound absorption single value, αw, and the sound absorption classes.
The absorption classes are designated A-E, where absorption class A has the highest sound absorption. A specified overall depth of system (o.d.s) must always be stated for a given absorption class. The ASTM standard C 423 specifies two different single values, the NRC (Noise Reduction Coefficient) and the SAA (Sound Absorption Average). Both values are calculated as an average over the frequency ranges (250-2000 Hz and 200-2500).
Independent tests were carried out in the UK on the Armourcoat Acoustic® Plaster System to determine the Sound Absorption Coefficient (αs), conducted in accordance with European Standard EN ISO 354. Single figure ratings of sound absorption performance, known as the Weighted Sound Absorption Coefficient (αw) and Sound Absorption Class, are derived from these measurements in accordance with European Standard EN ISO 11654. Tests were carried out by the AIRO Acoustics Laboratory, a UKAS accredited testing laboratory No. 0483, on 18th December 2019.
BS EN ISO 11654
Sound Absorption Class A
αw = 0.95
NRC = 0.95
Sound Absorption Class B
αw = 0.80
NRC = 0.85