Audio Engineering

Room Acoustics 865

Often membrane absorbers are combined with cavity absorbers to extend their range.
Some commercial modular absorbers also make use of such techniques and, by using a
range of materials/membranes/cavities, can provide a wide frequency range of operation
within a standard size format. Figure 29.13 illustrates general characteristics
of membrane, cavity, and dissipative/porous absorbers.

A newer type of “ absorber ” is the quadratic residue diffuser. This device uniformly
scatters or diffuses sound striking it so that, although individual refl ections are attenuated
and controlled, the incident sound energy is essentially returned to the room. This process
can therefore be used to provide controlled low-level refl ections or reverberation enabling
studios or control rooms to be designed without excessive absorption or subjective
oppressiveness that often occurs when trying to control room refl ections and resonances.
When designing a studio or control room, the various absorptive mechanisms described
earlier are taken into account and combined to produce a uniform absorption/frequency
characteristic, that is, reverberation time.

29.3.4 Reverberation and Refl ection

Reverberation times can be calculated using the following simple Sabine formula:

RT

lV

A



016.

,

0.2 63

0.4

0.6

Absorption coefficient

0.8

Cavity absorber

Membrane absorber

Dissipative absorber

1.0

1.2

125 250 500 1 K 2 K 4 K 8 K

Frequency (Hz)

Figure 29.13 : Summary of typical absorber characteristics.