Resonant room modes
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Resonant room modes affect the low frequency response of a sound system at the listening position. They are actually one of the biggest obstacles to high fidelity reproduction with modern equipment as they exist to varying degrees in all rooms and can only be reduced by the use of very big and bulky absorbent materials — like the foam wedges used in anechoic chambers. Most rooms have several main resonant modes in the 20 Hz to 200 Hz region, each related to a room dimension like length, breadth and height. Some relate to corner to corner reflection, and are stimulated more if loudspeakers are placed in the corners. The effect on music reproduction is heard as 'muddy' bass, with some notes, especially on bass guitar standing out and persisting longer than they should and some notes 'disappearing' (at antinodes). Most people are surprised when they first walk around a room while listening to low frequency test tones, the changes in level from almost nothing to loud often being quite startling. Attempting to 'equalise' the bass response can only work on steady state signals for one listening position, and even then, it fails on music because of sluggishness of the modes to build and die away.
Recently, attempts have been made to cancel the reverberant field, using digital techniques that create inverse signals corresponding to the room reverberation, but they only work well for one listening position. Very large rooms like concert halls, or outdoor stadia do not suffer this problem, and hence tend to sound much more solid on bass notes.
