absorption class
The International Standards Organization(ISO) has divided the absorption behavior into five sound absorption classes in ISO 11654. The absorption classes have the designations A to E and reflect the absorption coefficients of the various sound-absorbing materials.
The alpha value represents the quotient of the absorbing to the reflecting energy of materials. It can take values between 1, which are materials that absorb all the energy, and 0, which are materials that reflect all the energy.
Sound absorption classes are concerned with the frequency range between 250 Hz and 4 kHz, which is crucial in acoustics. ISO has defined reference curves for the sound absorption classes, which drop by about 30% at frequencies below 500 Hz and above 2 kHz. In between, the frequency response curve is constant.