Architectural Boundaries
A potential source of destructive interference can exist outside of the loudspeakers or clusters themselves, within the physical boundaries of a given space.
If loudspeakers are placed close to room boundaries (walls, floor or ceiling), there is interaction that results in interference patterns.
This varies across the frequency spectrum.
At low frequencies, there is reinforcement (boost) caused by boundaries, causing an increase (sometimes desired) in low-frequency levels.
At mid frequencies, some points in the spectrum also are increased due to reflection, but many other points are impacted by cancellation caused by reflected sound combining with the loudspeaker’s direct output (See Illustration 6A).
At high frequencies, there is very little (or no) effect, because these frequencies are well controlled and projected forward, away from nearby boundaries.
Often we’re forced to place a cluster or fill loudspeaker in front of the proscenium walls surrounding a stage, with the resulting frequency response altered, primarily at mid frequencies.
Lows are largely unaffected (because they are long enough to wrap around the proscenium) and highs, much more controlled and directed outwards, are hardly affected at all.
But at lower-middle frequencies, where the wavelengths are long enough, stray energy will wrap around (diffract) the enclosure, reflect off the proscenium wall and combine with direct energy coming from the loudspeaker (See Illustration 6B).
Again, the primary culprit is the reflected energy lagging behind the direct energy, and again, this cannot be corrected other than by removing the source of reflection, usually moving the loudspeakers away from the boundaries.
In recording studios it has long been common practice to soffit-mount large-format monitor loudspeakers into the front walls of the control room. In doing so, diffraction induced anomalies are minimized, and low-frequency boundary reinforcement is maximized.
