L-Acoustics V-DOSC, Operator'S Manual

Page: 146 / 158

L-ACOUSTICS V-DOSC Manual Version 4 6/29/2005 Page 146 of 158
We can now compare the number of sound sources inside the constructive rings to the number
of sources inside the destructive rings. When these numbers are almost equal, sound sources
cancel each other and produce an incoherent wavefront at M. When there are significantly more
sound sources within the constructive rings, a coherent wavefront is produced at M.
It should be noted that this method is due to Fresnel – he used this type of analysis to describe
light interference at the beginning of the century!
If we repeat this analysis for different M locations, we can draw a map that shows where the
sound field is coherent or incoherent. When it appears that there is no constructive wavefront
over a given area, the wavefront is chaotic. If we can define an area where the wavefront is
highly constructive, the sound pressure level will be much higher than in a destructive area. We
can thus define the effective coverage of the array for this given frequency.
The goal for the sound designer is to clearly identify an area where the wavefront is coherent -
not just for a given frequency, but for the whole operating bandwidth of the sound source.
How can we achieve this goal?
The first step is to minimize the distance between sound sources. By doing this for a given
frequency, we have a better chance of maximizing the number of sources within the first
constructive ring. This situation is more likely to be achieved on the main axis (perpendicular to
the line array), as shown in Figure 106. Note that in Figure 106, the source separation is smaller
than in Figure 105 so that more sources lie within the first constructive ring. Moving away from
the main axis as in Figure 107, the number of sources within the first constructive ring decreases
progessively, until there are equal numbers of constructive and destructive sources and they
cancel each other outside of the coverage region. Clear separation between coherent and
incoherent wave-fields defines the consistency of the main coverage region.
Figure 106: Constructive interference rings for a condensed point source line array at observation point
M.