Room modes

The input of acoustic energy to the room at the modal frequencies and multiples thereof causes standing waves.

Absorbent material can be added to the room to damp such resonances which work by more quickly dissipating the stored acoustic energy.

Open apertures, dispersion cylinders (large diameter and usually wall height), carefully sized and placed panels, and irregular room shapes are another way of either absorbing energy or breaking up resonant modes.

For absorption, as with large foam wedges seen in anechoic chambers, the loss occurs ultimately through turbulence, as colliding air molecules convert some of their kinetic energy into heat.

This means the closely spaced harmonic resonances are likely to lie in the low frequency region and thus the response tends to be more uniform.