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The sound absorption coefficient α of sound‐absorbing materials (curtains, drapes, carpets, clothes, fiberglass, acoustical foams, etc.), is defined as

(3.73)

Note that α also depends on the angle of incidence. The absorption coefficient of materials depends on frequency as well. Thicker materials absorb more sound energy (particularly important at low frequency). See Figure 3.19.

Figure 3.19 Sound absorption coefficient α of typical absorbing materials as a function of frequency.

If all the sound energy is absorbed, α = 1 (none reflected). If no sound energy is absorbed, α = 0:

If α = 1, the sound absorption is perfect (e.g. an open window).

The behavior of sound‐absorbing materials is described in more detail in Chapter 9 of this book.