L-Acoustics V-DOSC, Operator'S Manual

Page: 99 / 158

L-ACOUSTICS V-DOSC Manual Version 4 6/29/2005 Page 99 of 158
3.5.2 CENTRAL LINE ARRAY WITH ELECTRONIC DELAY PROCESSING
A central ground stacked line array optimizes low frequency SPL output since all subwoofers couple
acoustically and the mirror image ground plane effect doubles the number of subs. Given the same
number of subwoofers, the overall SPL obtained by ground stacking in this manner is higher than any
other configuration. An additional advantage of the centre line array is that the vertical directivity is
omnidirectional - this is useful for venues where there is an audience tribune. Since the subwoofers
are physically separate from the flown V-DOSC arrays they are normally driven via aux send in either
INFRA, X AUX or 4W mode (although the system can also be operated in 4-way mode).
Limitations of the central ground stacked line array configuration are as follows:
Tonal balance has an exaggerated low frequency contour for the closest members of the audience
(integrating distributed front fill loudspeakers or a stereo infill system with the central line array of
subwoofers can help offset this).
Increased amount of low frequency energy behind the central line array which can cause feedback
problems on stage
Unless electronic delay processing is applied, the horizontal directivity will narrow (the extent of
narrowing depends on the length of the central line array)
Potential for reduced throw due to audience absorption (this problem is likely to occur with any
ground stacked configuration).
A good technique for controlling the directivity of a central ground-stacked horizontal line array is to
use delay processing to electronically arc the array. Electronic delay processing in this manner will
decouple sections of the array so that tonal balance is improved up close while at the same time low
frequency coverage is smoother throughout the audience in the horizontal plane. The optimum arc
radius is typically equal to half the length of the central line array - delay taps are calculated
geometrically based on this arc radius and the physical distance to the centre of a given subwoofer
group off the center line reference.
(a) Horizontal line array
(b) Delay processed horizontal line array
Figure 76: Centre subwoofer line array (a) without and (b) with electronic delay processing
To implement electronic delay processing, a 4- or 6-channel DSP is dedicated to subwoofer drive and
the SB218 DELAY ARC 80 Hz preset (subs with negative polarity) used in conjunction with the 3WX
preset for V-DOSC. Alternatively, the SB218 DELAY ARC 60 Hz preset can be used with the 3W
INFRA preset (subs with positive polarity). SB218s are circuited in blocks of four symmetrically about
the centre axis and powered by half of an RK124 amplifier rack. The appropriate delay tap can be
selected using the DSUBTK comb connector kit (set of 6 COMB connectors for T1-T6).
An EXCEL spreadsheet tool to perform delay and offset calculations is available in ARRAY2004 (SUB
ARC). Several examples are given in Figure 77 and further details on the SUBARC sheet are provided
in Section 3.5.4.