Audio Engineering

862 Chapter 29

3. Helmholtz-tuned frequency resonator/absorber.


4. Quadratic residue/phase interference absorbers/diffusers.

Porous absorbers include many common materials, including such items as drapes,
fi berglass/mineral wool, foam, carpet, and acoustic tile.

Figure 29.10 illustrates the frequency range over which porous absorbers typically
operate. Figure 29.10 clearly shows the performance of these types of absorber to fall off
at medium to low frequencies—unless the material is very thick (comparable in fact to
1/4 wavelength of the sound frequency to be absorbed).

Panel or membrane absorbers ( Figure 29.11 ) therefore tend to be used for low-frequency
absorption. The frequency of absorption can be tuned by adjusting the mass of the panel
and the airspace behind it. By introducing some dissipative absorption into the cavity
to act as a damping element, the absorption curve is broadened. Membrane absorption
occurs naturally in many studios and control rooms where relatively lightweight
structures are used to form the basic shell, for example, plasterboard and stud partitions,
plasterboard or acoustic tile ceilings with airspace above, and so on.

63 125 250 500 1000 2000 4000 8000

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Absorption coefficient

25 mm

Octave band centre frequency (Hz)

100 mm 75 mm50 mm Overall thickness

Figure 29.10 : Typical absorption characteristics of porous material.