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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 8.3 and 8.4 illustrate
these concepts.
6
3
0
-3
-6
dB
10
Hz
20 50 100 200 500
Figure 8.3 - Change in frequency response resulting from placement of speaker 3.3
feet from a reflective surface (relative to an anechoic environment).
6
3
0
-3
-6
dB
10
Hz
20 50 100 200 500
Figure 8.4 - Same conditions as above, except speaker is 6.6 feet from the reflecting
surface. Note how the cancellation and reinforcement occur at lower frequencies.