There are three main categories of materials that can be used to absorb sound: porous materials, membranes, and resonators. Porous materials, such as stone wool, can be used to absorb sound. The effectiveness of a material to absorb sound is measured by a weighted sound absorption coefficient, which is expressed as a percentage. A coefficient of 0.01 represents minimal absorption (1%), while a coefficient of 1.00 represents complete absorption (100%).