Practical Considerations - Electro-Voice Xi-A Series Manual

the uniform appearance of both the ground stack and the flown array. When designing an Xi-A
loudspeaker array, three primary acoustic goals should be kept in mind. First, the array should be
designed so that the acoustic energy is directed at the audience and away from reflecting
surfaces outside the listening area. This maximizes the intelligibility and clarity throughout the
listening area. Second, the array should be designed to achieve even coverage throughout the
audience. This ensures consistent levels and sound quality throughout the listening area. Third,
the array should be designed to achieve the required sound pressure levels throughout the
audience. This also maximizes the intelligibility and clarity throughout the listening area. The key
to achieving these three goals is to select the Xi-A loudspeaker system with appropriate coverage
patterns and acoustic-output capabilities, orient the enclosures and coverage patterns for optimal
array performance, and accurately aim the systems in the array. The reader is referred to
Appendix B.1 for a list of references covering the topics of acoustics and loudspeaker array
design. Because sound coming from numerous loudspeakers should arrive at any seat at the
same time, the speakers in an array should be curved as if mounted on the outside of an
imaginary sphere. The loudspeakers should be close-spaced to minimize lobing and maximize
coupling and each speaker must be accurately aimed. The apparent source then becomes the
single point at the center of the sphere. The horizontal array curve is simply achieved by hanging
the cabinets closely spaced, aimed outward at diverging angles. Achieving the vertical array curve
is a bit more complex to obtain. The Xi-A loudspeaker systems utilize a unique two-point
suspension system (shown in Figure 2.1) that can achieve all of the design criteria for a true point-
source array. To tilt an Xi-A loudspeaker system downward, the rigging straps at the top of that
cabinet are shifted along the rigging track towards the rear of the enclosure. To align the top-rear
corner of an Xi-A cabinet with the bottom-rear corner of an Xi-A cabinet above it, the rigging straps
at the bottom of the upper cabinet are shifted along the rigging track until the rear corners of the
adjacent cabinets are in alignment. A number of examples are presented in the following section
that will demonstrate both the mechanical and acoustical aspects of an array design with the Xi-A
loudspeaker systems.

2.3.2 Practical Considerations

The vertical angle of the flying Xi-A loudspeaker system may be adjusted by choosing different
positions of attachment along the rigging track on the cabinet. There are a number of attachment
points allowing for a wide variety of angles. Although the center of gravity is slightly different for
each of the systems, the balancing concept is the same. The further the top attachment point is
towards the back of the cabinet, the greater the downward angle, as shown in Figure 2.5. As the
attachment point is moved forward, the cabinet will have less downward angle and, at the furthest
point forward, will have an upward tilt. This principle holds true when hanging any of the Xi-A
systems in either rigging orientation (rigging track on the top and bottom, or on the sides). Vertical
arrays are constructed by hanging Xi-A systems from one another in succession. Both the
curvature of the array and the angles of the individual cabinets are controlled by the rigging
attachment positions. The shape of the array curve is determined by the position on the lower
track of an already hanging cabinet from which the next cabinet is hung, the further back the
attachment point is on the upper box, the further back the lower cabinet is shifted and, hence, the
greater the curvature of the array. The goal is to have the back, top and bottom edges to adjacent
cabinets in alignment. Hanging one cabinet from another affects the angle of the first. This is best
demonstrated in an example. In Figure 2.6a, one Xi-A enclosure is hung so that it points straight
ahead and a second is added below. The addition of the second enclosure causes the top cabinet
to point upward. Shifting the upper attachment point of the first cabinet back, as shown in Figure
2.6b, counteracts the additional load and results in the upper cabinet pointing straight ahead
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ELECTRO-VOICE Xi-A Rigging Manual 8