L-Acoustics V-DOSC, Operator'S Manual

Page: 145 / 158

L-ACOUSTICS V-DOSC Manual Version 4 6/29/2005 Page 145 of 158
Figure 104: The effect of varying frequency and listener position M on Fresnel rings
As seen in Figure 104, when decreasing the frequency while maintaining the same listener
position, wavelengths get larger and the ring spacing gets bigger so that more sources fall within
the ring and couple constructively at M. Conversely, as the frequency increases, fewer sources
fall within the ring and add constructively at M. Maintaining the same frequency (and therefore
the same ring spacing) and moving the position M closer to the array means that the radius of
curvature of the rings is decreased, therefore fewer sources fall within the ring. Moving the
listener position further away, the radius of curvature increases and more sources fall within the
constructive ring.
Now let’s draw all circles with radii defined by:
δ
d = (2n + 1)
λ
/2. The destructive areas where
cancellation occurs are shown in gray in Figure 105. White areas are constructive interference
zones.
Figure 105: Destructive and constructive interference rings for a line array at observation point M.