REALWORLDGEAR
Architectural
Loudspeakers
The Renkus-Heinz IC Live
loudspeaker
system.
Column or line source, the choices are limitless. by Mark Frink
>>>
Mention coluMnS to grumpy
old sound men and Shure’s Vocal Master
immediately springs to mind, as every
’70s band utilized them. line sources
have been used for a half-century. For
portable applications, audiences tend
to be small. installed systems are often
employed in reverberant public spaces
– houses of worship, auditoriums and
passenger terminals – predominately for
public address. the characteristics of line
source columns – wide horizontal coverage, minimal vertical coverage above and
below the enclosure and coherent sound
in the vocal range – are all attractive for
these kinds of venues.
Driver spacing determines the highest frequency at which a column of identical drivers acts as a line source, while
the height of the column determines
the lowest frequency with directivity. As
with modular line arrays, a short system
might efficiently throw the midrange,
but leave a puddle of low-frequency
mud around the enclosure.
inexpensive column speakers remain
popular because they offer a compromise
solution to installers who need efficiency
in the vocal range combined with even
coverage and a skinny profile that makes
them acceptable on the walls of public
assembly spaces.
investigations into line source coupling behavior and pattern control tell
us that loudspeaker cones exhibit behavior up to a frequency whose wavelength
is half the distance between adjacent
acoustic centers. Another old sound
guy image is the column of JBl 10-inch
midrange drivers in clair S4 cabinets.
With their frames squared off to provide
closer coupling, their acoustic centers
could be placed 9. 5 inches apart, providing coupling to 800 Hz.
in architectural columns 6.5-inch
diameter cones, when tightly-spaced,
will couple up to about 1,000 Hz. Four-inch cones couple to about 1,600 Hz
and 2-inch cones to 3,300 Hz. Above
these frequencies top and bottom lobes
appear in the polar response, however
restricting high frequency response of
some cones can reduce lobing. this can
be achieved with passive filters in the
cabinet, or with active filters used in
DSP-driven steerable columns.
A specialized version of the line
source column is called “digitally steerable” with individual amplification, delay
and equalization for each driver, allowing the column’s vertical coverage to be
tilted down (or up) and focused for short
or long throws, though its horizontal
coverage remains fixed. the advantage
is that a column speaker can be placed
flat on a wall, while its coverage can be
oriented downwards. Physically tilting
a passive array adjusts farthest coverage,
while leaving the coverage towards its
sides near the original height.
Demanding applications, such as
music, require both greater bandwidth
and dynamic range, leading to two-way bi-amped designs that incorporate
separate high-frequency transducers in
addition to the column of tightly spaced
speaker cones.
in modular line array enclosures,
compression drivers are mounted onto
“isophasic” manifolds that convert
sound from their round opening into
a tall, thin opening whose output is in
phase along its entire height. columns
lack both the budget or depth for this, so
high-frequency drivers in 2-way column
speakers are reproduced by a few closely
coupled HF drivers, since their height
only needs to be a few inches, or by the
use of magnetic planar or “ribbon” drivers, which provide the required in-phase
output in a tall, thin HF driver.
it’s common for line source columns
to be combined as multiple cabinets to
achieve better performance as taller systems for bigger rooms. longer columns
provide pattern control reaching to lower
frequencies. A nine foot column can provide control to 125 Hz, so combining
three 3-foot columns can increase low
frequency performance. As with miniature line arrays, some systems have companion lF columns which extend pattern
control to lower frequencies. Alternately,
traditional subwoofers can supplement a
column’s lF response.
Mark Frink is Associate Editor for live
Sound international. He can be reached
at mfrink@livesoundint.com.
