# How can we compare the quality of acoustic materials?

The quality of acoustic materials is determined by their sound absorbing capacity, which means the amount of sound energy per square meter that is being absorbed. The ideal value of this parameter equals 1, which represents absorption of 100%.

In acoustic terms, this situation is called an “open window“ through which all sound disappears and doesn’t come back inside. Although this is physically impossible, we are able to obtain this value by adding more area per square meter to a room (e.g. a pyramid-shaped air duct or acoustic panels with a so-called “sandwich construction“).

To indicate the absorption capacity of a room, we use the measuring values NRC, Alpha w and Alpha, while measurement unit is always sabins.

The respective sound absorption curves can look as follows:
green: 2.5 cm material strength, yellow: 5 cm strength, blue: 7.5 cm strength.

Besides its strength, the density of the absorption package which is measured in kg/m3 plays an important role.

D20, for example, represents 20 kg/m3, D40 40 kg/m3

What is NRC?

NRC is the abbreviation for Noise Reduction Coefficient corresponding to the ASTM C 423 standards. It represents the arithmetic mean of the sound absorption at 250, 500 1.000 und 2.000 Hz and is measured in thirds or one-third octaves. The NRC is not weighted with the sensitivity of the human ear, which is displayed in the reference curve Alpha p. The value usually lies between 0 and 1 and is not used for acoustic calculations.

What is Alpha w?

Alpha w stands for the weighted sound absorption coefficient corresponding to the EN-ISO 11654 standards. It represents the arithmetic mean of the sound absorption at 250, 500 and 1000 Hz and is measured in thirds or one-third octaves. The Alpha w is weighted with the sensitivity of the human ear, which is displayed in the reference curve Alpha p. The value usually lies between 0 and 1 and is not used for acoustic calculations