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1. Introduction
Figure 1.2: Vertical Array coverage, 16 modules @ 2kHz: (a) perfectly straight array, (b) progressive curvilinear
array.
extend the frequency range at which the interference between the sources can be controlled, even when
their frequencies reach the highest levels.
Those assumptions raise great doubts, first of all because, beyond a certain frequency, the emis-
sion of a driver membrane is no longer flat
acoustic assumptions which are valid only when the wave length is very short if compared to that orig-
inating from the geometry of the reflecting surfaces, that is at frequencies for which the source (the
driver membrane) is already above the frequency break-up point, which means out of control, trying to
re-phase a wave that was originally already out of phase.
Furthermore, a perfectly cylindrical emission from each module of the array is to be desired only
when the array is mounted perfectly straight. If we were to analyse the emission according to the vertical
axis of a perfectly straight array, we would immediately remark that its practical applications would be
rather limited, as in most cases the array needs to cover a deep audience area which is below the centre
of the array itself, rather than project a sound beam the furthest away possible (figure 1.2). In 99.9% of
cases vertical arrays are mounted in a curve in order to obtain a wider vertical dispersion.
The wave front curvature cannot and should not be useless: in practical applications that require a
curved array it is logical to have curved wave fronts.
Obviously, cylindrical wave fronts, or flat wave fronts if seen in section, are the best solution when
the array has to be mounted straight. In most practical applications with a curved array, if the emission
of each single element is flat, it creates energy vacuums in the polar response, resulting in an unstable
vertical coverage (figure 1.3).
1.1.2 AXIOM Vertical Array Systems
During the Axiom Systems planning phase we ran a statistical analysis on curvatures that are actually
employed for vertical arrays and we found out that, on average, the angle between the elements varies
◦
◦
from 5
to 8
.
The brilliant performance of vertical array systems as compared to a traditional array solution comes
from the correct control of interference between several sound sources and not from the supposed cre-
ation of a cylindrical wave front. For the higher portion of the spectrum, the AXIOM Systems directivity
control is due to unusual conic waveguides emitting on diffraction and horn slots which minimize dis-
tortion and losses, keep energy at the highest range and obtain the desired directivity pattern with a
minimal vertical dispersion and very wide horizontal dispersion. In order to increase horizontal angu-
lar dispersion in the medium-lower range, every model features a purpose-designed diffraction device–
A.C.I.D. (Acoustical Coverage Improvement Device) – which guarantees the correct distribution of en-
ergy in space.
1.1.3 Typical applications of AXIOM Vertical Arrays
Thanks to their scalability and ease of mounting, vertical array systems are the ideal solution for a
great deal of live events and installations, both outdoors and indoors. Nonetheless, we need to critically
analyse the downward scalability of such systems, when a small number of elements complicates set up
7
Above a certain frequency the membrane of the driver no longer behaves as a piston.
(a)
7
; secondly because those devices are based on geometrical
1.1. THE SOUND OUT FRONT!
3
(b)
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