Single number ratings

While the sound absoprtion coefficient is usuallly a strong function of frequency human nature being what it is we often want a single number representation of a material. For instance an architectural specification may refer to a material's acoustic properties and require a rating greater than a certain minimum value. In North America a single number rating was developed in the 1950's, it was the arithmetic average of the absorption coefficient in the 4 octave bands (250Hz, 500 Hz, 1 kHz and 2 kHz) rounded to the nearest 0.05. This is referred to as the Noise Reduction Coefficient (NRC).

Europe was a lot later in adopting single number rating, but now uses ISO 11654-1997 to derive a single number rating. This is a more complex rating method, and uses a weighting method that compares the absorption curve as a function of frequency against a reference curve to produce a weighted sound absorption coefficient $\alpha$_W. Shape indicators are also used to indicate whether an absorber is a low frequency, mid frequency or high frequency absorber.

Zorba calculates the single number rating according to both methods and displays either NRC or $\alpha$_W . You can toggle between the two displays by double clicking on the number.