The International Standards Organization (ISO) has divided the absorption behaviour 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 ratio of the absorbing to the reflecting energy of materials and can take on values between 1, i.e. materials that absorb all the energy, and 0, i.e. materials that reflect all the energy.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.