Specifications

TI 323 (6.0E)
18 - 36
5.2.2. Finite line arrays
The length of an array has a large impact on its
properties, and due to the finite length of an array
the initially cylindrical wave transforms into a
spherical wave. This near field/far field transition
radius is dependant on the length of the array and
the frequency being produced. This radius can be
calculated using the following rough formula:
r
near
=
l
2
f
2
c
.
.
Where:
r
near
= near field/far field transition radius (m (ft))
l = length of the array (m (ft))
f = frequency (1/s)
c = speed of sound (m/s)
The table below shows the transition distance for a
5 metre (16.4 ft) long array at various frequencies.
fr in m (ft)
100 Hz 3.7 m (12 ft)
1 kHz 37 m (121.4 ft)
10 kHz 370 m (1214 ft)
The result at high frequencies is a level drop of 3 dB
per doubling the distance. In other words this
behaviour shows that a line array works optimally in
the far field where all sources of the array arrive with
minimal path length differences, and where the
maximum coherent addition of sound energy occurs.
Close to the array the path length differences are
greater, leading to an increased incoherent addition
of the sound energy radiated by the array.
Comparing the line source with a spherical source
radiating the same energy shows that the 3 dB level
drop comes from a lack of energy close to the array.
At very low frequencies a line array behaves in the
same way as a normal spherical source.
5.2.3. Curved line source
The discussion so far has only covered situations
where arrays are set up in one straight line with no
vertical angle set between the loudspeaker cabinets.
In a typical real life situation a straight line set up will
not produce sufficient coverage in the vertical plane
and this requires improvement. The easiest way to
achieve this is to arrange the boxes in an arc, all with
the same vertical splay.
The first thing to recognize is the usual 6 dB level
drop per doubling of distance. As described in the
spherical sources section, the wave front now diverges
horizontally and vertically, exhibiting the behaviour of
a spherical source. The energy for the far field can
now be adjusted by changing the overall vertical
angle, halving the angle between cabinets increases
the energy in the far field (and only there) by 3 dB.
5.2.4. J-shaped line source
A J-shaped array allows adjustment to the amount of
energy according to the distance that has to be
covered. This means no more than the number of
cabinets targeting the listener area is proportional to
the distance.