Speaker Placement
The interaction of any Speaker System with its environment is complex and is largely out of the speaker designer's control.
Most listening rooms present a more or less reflective environment, depending on size, shape, volume, and furnishings. This means the sound reaching a listener's ears consists not only of sound directly from the speaker system, but also the same sound delayed by traveling to and from, and being modified by, one or more surfaces.
These reflected sound waves, when added to the direct sound, cause cancellation and addition at assorted frequencies, thus changing the character of the sound at the listener's ears.
The human brain is very sensitive to small variations, including some of these, and this is part of the reason why a speaker system sounds different at different listening positions or in different rooms.
A significant factor in the sound of any speaker system is the amount of absorption and diffusion present in the environment. Clapping one's hands in a typical empty room, without draperies or carpet, will produce a zippy, fluttery echo which is due both to a lack of absorption and to reverberation, or repeated echoes, from flat reflective walls, floor, and ceiling.
The addition of hard surfaced furniture, wall hangings, shelving and even baroque plaster ceiling decoration, will change the echoes, due primarily to the diffusion caused by differently reflective objects with shapes and surfaces having sizes on the order of the sound wavelengths being diffused.
To some extent, this breaks up the simple reflections otherwise caused by bare flat surfaces, and spreads the reflected energy of an incident wave over a larger angle on reflection.
In a typical rectangular listening room, the hard, parallel surfaces of the walls, floor and ceiling cause primary acoustic resonance modes in each of the three dimensions, left-right, up-down and forward-backward.
Low frequencies excite these modes the most, since long wavelengths are not much affected by furniture compositions or placement.
The mode spacing is critical, especially in small and medium size rooms like recording studios, home theaters and broadcast studios. The proximity of the speakers to room boundaries affects how strongly the resonances are excited as well as affecting the relative strength at each frequency.
The location of the listener is critical, too, as a position near a boundary can have a great effect on the perceived balance of frequencies.