Page 242 - Master Handbook of Acoustics

downward as it enters the much denser concrete blocks. There is further heat loss G within the
concrete blocks. As the ray travels on, getting weaker all the time, it strikes the concrete-air boundary
and undergoes another reflection C and emerges with refraction D with additional heat lost (I, J, and
K) in all three media.

The sound ray S experiences many complex events during its travel through this barrier, and every
reflection and passage through air or acoustical material dissipates some of its original energy. The
refractions bend the ray but do not necessarily dissipate heat. Fortunately, this minutia is not involved
in practical absorption problems. We usually consider only the aggregate of these individual actions.

Absorption Coefficients

Absorption coefficients are used to rate a material’s effectiveness in absorbing sound. Absorption
coefficients vary with the angle at which sound impinges upon the material. In an established diffuse
sound field in a room, sound is traveling in every imaginable direction. In many calculations, we need

sound absorption coefficients that are averaged over all possible angles of incidence. The random
incidence absorption coefficient is a coefficient that is averaged over all incidence angles. This is
usually referred to as the absorption coefficient of a material, designated as α. The absorption
coefficient is a measure of the efficiency of a surface or material in absorbing sound. For example, if
55% of the incident sound energy is absorbed at some frequency, the absorption coefficient α is said
to be 0.55 at that frequency. A perfect sound absorber will absorb 100% of incident sound; thus α is
1.0. A perfectly reflecting surface would have α of 0.0.

Different references may use different symbols for the absorption coefficient; for example, a is
sometimes used instead of α. Partly, this is because there are several different absorption coefficients.
As noted, absorption varies according to the incident angle of sound striking the surface (absorption
also varies according to frequency). One type of absorption coefficient measures absorption at a
specific angle of incidence. Another type of absorption coefficient measures absorption from a

diffuse sound field, that is, sound from a random distribution of angles. In this book, α refers to the
absorption coefficient from a diffuse sound field (averaging all angles of incidence) at a given
frequency. When an absorption coefficient at a particular angle is cited, it will be referred to as α θ
where θ is the angle of incidence.

The sound absorption A provided by a particular area of material is obtained by multiplying its
absorption coefficient by the surface area of the material exposed to sound. Therefore:

where A =
absorption units, sabins or metric sabins
S =
2
surface area, ft or m 2
α =
absorption coefficient

Sound absorption A is measured in sabins, in honor of Wallace Sabine. An open window is
considered a perfect absorber because sound passing through it never returns to the room. An open