Audio Engineering

Room Acoustics 863

The frequency at which maximum absorption occurs can be calculated from the following
formula:

f

md



60

,

where m is mass of panel in kg/m^2 and d is airspace in meters.

The action of porous and membrane type absorbers is often combined to form a single
wide frequency band absorber or to extend the low-frequency characteristics of porous
materials.

Bass traps are generally formed by creating an absorbing cavity or layer of porous
absorption equivalent to a quarter wavelength of the sound frequency or bass note in
question. Table 29.5 presents the dimensions required for the range 30–120 Hz.

Figure 29.12 shows basic principles and typical absorption characteristics of a Helmholtz,
or cavity, absorber.

The frequency of maximum absorption occurs at the natural resonant frequency of the
cavity, which is given by the following formula:

F

S

res lV

 55 Hz,

where S is the cross-sectional area of the neck (m^2 ), l is the length of the neck (m), and V
is the volume of air in the main cavity (m^3 ).

Sound absorbent

Wall

Membrane (plywood, hardboard)

Sound absorbent

Wall

Perforated membrane

d

Figure 29.11 : Typical panel or membrane absorber construction.