The absorption measurement in a reverberation chamber or in a small cabin is also called absorption measurement in diffuse fields or under diffuse incidence. It follows the ASTM C423 or ISO 354 standards.
An impedance tube, also called a Kundt tube, is a tube fitted at one end with a loudspeaker and at the other end with a sample holder in which the sample of acoustic material to be tested is installed. The measurement is done using two microphones located between the loudspeaker and the sample holder.
The efficiency of an acoustic material is determined by measuring its absorption coefficient, denoted α. The absorption coefficient is defined as the proportion of acoustic energy that has been absorbed in the material over the total energy.
An acoustic absorbent material is a material in which sound waves penetrate and then are dissipated. There are two main families of absorbent materials: fibrous materials and porous materials.