Avalon Acoustics Isis - Manual (Page 28)

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Conversely, improper placement of the loudspeakers can result in uneven
frequency response. This results in diminished bass quality. Please refer to
Figure 8.6.
As frequency increases and wavelength becomes more similar to the
distance to the boundary, the phase difference between original and
reflected waves increases, and the air coupling effect is diminished. In
particular, when the wavelength equals about four times the distance to the
boundary, the reflected wave is antiphase to the original wave, resulting in a
cancellation (dip) in the output. At frequencies above this level, the effect
becomes less significant and creates similar but smaller variations in output.
Figures 8.7 and 8.8 illustrate these concepts.
Figure 8.6 - Uneven frequency response caused by improper
placement of the speakers. In this case, the speaker is 2.0 feet from
both the side and rear walls.
Figure 8.7 - In-room response when the speaker is placed 2.3 feet from
the side wall, and 3.9 feet from the rear wall.