Audio Engineering

94 Chapter 3

Change in media

hence change

in velocity

Reflected

sound

Transm

itted

sound

Reflectedsound (1 –

a)

Flank

ing path

Absorption (a)

Sound

source

Mass Law

T.L. 20 log [fw]− 47 dB

f frequency in Hz

w weight of barrier in kg/m^2

a1 – 10(dB/10)

dB 10 log (1 – a)

Figure 3.6 : Absorption, refl ection, and transmission of boundary surface areas.

path. This illustrates why feedback modes change as air conditioners, heating, or crowds
dramatically change the temperature of a room ( Figure 3.7 ).

3.11 Absorption

Absorption is the inverse of refl ection. When sound strikes a large surface, part of it is
refl ected and part of it is absorbed. For a given material, the absorption coeffi cient ( a ) is

a

E

E

A

I

 (3.14)

where EA is the absorbed acoustic energy, EI is the total incident acoustic energy (i.e., the
total sound), and (1 –a ) is the refl ected sound.

This theoretically makes the absorption coeffi cient some value between 0 and 1. For
a  0, no sound is absorbed; it is all refl ected. If a material has an a of 0.25, it will absorb
25% of all sound energy having the same frequency as the absorption coeffi cient rating,
and it will refl ect 75% of the sound energy having that frequency.