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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 9.4 and 9.5 illustrate these concepts.
6
3
0
-3
-6
dB
10
Hz
20 50 100 200 500
Figure 9.4 - 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 9.5 - Same conditions as above, except speaker is 6.6 feet from the
reflecting surface. Note how the cancellation and reinforcement occur at
lower frequencies.