The acoustically most problematic frequency range is below 200 Hz, because of the spatially and frequency wise irregular distribution of room resonances. Many computer programs have been written that calculate the resonant modes of a given room and recommend optimum loudspeaker and listener placements. Usually, real rooms are much more complex than the calculated models. Walls are not infinitely stiff, rooms have windows, doors, openings, suspended floors or ceilings, etc. In addition, it is the polar pattern and the acoustic source impedance of the given loudspeaker that determines which of the potential room modes are actually excited and to which degree. The usefulness of such programs is marginal at best. Likewise, recommended proportions for room length, width and height should not be taken more seriously than other proportions that may be based on visual aesthetics.
The conventional closed or vented box design, that is used for the majority of loudspeakers on the market, contributes significantly to the room problems below 200 Hz. These designs are omni-directional radiators and they tend to excite a maximum number of room resonances, particularly when located in room corners. While this adds to the perceived bass output at certain frequencies, it can lead to a falsification of the recorded material, namely when the room resonance decays more slowly than the original sound. In general, the low frequency response of omni-directional speakers in small rooms is quite non-uniform. Attempts to treat the room with absorbers will make only marginal differences unless very many absorbers or large absorbing surfaces are used. It is best to attenuate peaks in the bass response with parametric equalization. Holes in the response cannot be filled in.
By far the perceptually most uniform response in the range below 200 Hz is obtained with an open-baffle, dipole or figure-of-eight radiating source. Because of its directionality, the dipole excites far fewer room resonances than an omni-directional source. The measured room response is not necessarily any smoother than that for an omni-directional source. But the perceived difference in bass reproduction is startling at first, because we are so used to hearing the irregular and booming bass of the typical box speaker in acoustically small rooms. Quickly one learns to recognize the distortion of this combination and it becomes intolerable.
