Avalon Acoustics Isis - Manual (Page 29)

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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 three and seven feet from one of the walls
(side or rear), and between five and fifteen feet from the other wall. The
measurements are made from the wall to the center of the 13" 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 ISIS 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.
Figure 8.8 - In-room response when the speaker is placed 3.6 feet from
the side wall, and 6.6 feet from the rear wall.