Avalon Acoustics Sentinel - Manual (Page 51)

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There are typically three reflective surfaces near each speaker: the floor, the
rear wall, and the side wall. Each of these surfaces produces its own
reflection, and hence its own cancellation and reinforcement. By properly
selecting the distances to each surface, we can provide a uniform and
extended bass response. Conversely, improper placement of the
loudspeakers can result in uneven frequency response, resulting in diminished
bass quality.
In order to take full advantage of bass reinforcement and to provide the most
uniform and extended bass response, it is recommended that the
loudspeaker be placed between four and seven feet from the side wall, and
between eight and eighteen feet from the back wall. The measurements are
made from the wall to the center of the 9" woofer cone. The exact distances
are not overly critical, although the two distances should not be within about
33% of each other.
A good rule of thumb in establishing the lateral position of the Sentinel Active
Reference System is to apply the ratio 4 : 10 : 4. In other words, the distance
to the side wall is 4/18 (or about 22.2%) of the room width, and the distance
from speaker to speaker is 10/18 (or about 55.5%) of room width.