Absorption (chapter 9, Everest)
1. Dissipation of Sound Energy
a. “Conservation of Energy”
i. Energy Can’t be created or destroyed by can be changed into different
forms
ii. Sound energy is in the form of motion of Air Molecules (Kinetic Energy) This
can be dissipated by converting it into “heat” or thermal energy (i.e.
uncorrelated motion of air or other particles, and/a electromagnetic
radiation infrared).
2. Absorption of Air
a. Frequency A(Sabins per 1000ft3)
b. 1Khz 0.9
c. 2KHz 2.3
d. 4Khz 7.2
3. Types of Absorbers
a. Porous and Resonant
i. Porous
1. Rely on sound penetrating “interstices” of material and causing
material to move. Frictional losses in this motion result in
dissipation of energy as heat. (i.e. sound does work on something
that something dissipates energy as heat.)
2. Porous absorbers are often “fuzzy” or “foamy”
ii. Density and thickness effect the amount of absorption
1. Thickness has more of an effect on low frequencies than in high
2. Density also affects which and how many frequencies are absorbed.
a. “Too dense” - Sound may just reflect off resulting in poor
absorption
3. Not dense enough, sound will be “transmitted and not be affected
by the absorber.
b. Say you have a piece of porous absorber and a sound of intensity. Ii is incident on it.
Some of the sound will be reflected (with intensity, TR), Some transmitted (IT) and
some absorbed
i. Ii = IR + IT + IA
ii. I = R (reflection) + T (transmitted coefficient) + A (energy absorption
coefficient)
iii. IR/Ii IT / Ii
iv. The energy absorption coefficient α
v. α = 1-R
vi. The energy absorption coefficient α is
vii. α = 1-R
