Room Treatment: A Primer

When a speaker emits sound waves, they travel to your listening position but also to the ceiling, sides walls and floor, where they are reflected toward the listening position. While these first reflections arrive later and are measurably distinct from the initial signal, they reach our ears too soon for our psychoacoustic system to distinguish them from the initial signal and they become one in our head. What can be lost or overwhelmed by this wave interference are the subtler sounds that can help depict hall size, performer placement, soundstage width and depth, and the music’s overall tonal signature. This is more the "psycho" part of psychoacoustics and is distinct from the later reflections that bounce around the room and result in frequency peaks and nulls that accrue from room modes and the like. These later reflections might also interfere with the initial speaker signal much as first reflections do, but they further screw up the sound as they contribute to a less than smooth room frequency curve and/or cause “ringing”. Treatment at first reflection points helps a lot, but whole room treatment is necessary to deal with these later reflections.

Absorption simply captures these interfering wave reflections and converts them to heat. Diffusion breaks the waves up and reflects them in different directions instead of directly back at you. To better understand diffusion’s role, imagine being in a room and shooting a shotgun directly at a flat wall (no diffusion) versus against a rock wall (diffusion). The return path of the former is predictable and most are flying directly back at you. The latter will have much of the shot going elsewhere and taking a more circuitous route, bouncing off the surfaces of the room and even bumping into other shot before they might return to you. Because they are fewer, have been delayed, and have a lower energy, their return will not bother you as much as if they came flying straight back at you. Similarly, diffusion provides a time delay along with a weaker signal, allowing your psychoacoustic system to ignore or at least differentiate the reflections from the initial signal. Scattering panels work similarly and often are combined with absorption.

At a minimum, you want to avoid "first reflections" and instead have either "first absorption" or diffusion. Opinions are mixed as to which is better. It is probably easier to do absorption with standard home goods/furnishings such as rugs, blankets, curtains, overstuffed couches, etc. Some suggest that books and records have significant absorption properties, but I would argue they are primarily reflective and if there is absorption, it is over an extremely limited frequency range.  Bookshelves, racks, and the like do not give a predictable diffusion pattern. You may be getting different reflections from various media or equipment but they will not have the significant and ordered differences in depth to achieve meaningful diffusion. Neither will a popcorn ceiling nor textured wallpaper. On the other hand, if you wanted to arrange your equipment or books to follow a QRD pattern, you might see some benefit. Also, while too much absorption can be a bad thing as it sucks the reverberant life out of the room, I am not sure there is such a thing as too much diffusion.

If you cannot go full Monty because  of WAF, expense, room layout, etc., at the least put a rug down in front of you, avoid any hard surfaces such as a table between you and the speakers, and hang draperies on your windows and tapestries on your wall at the first reflection points.