Line arrays have actually been around…
lor a lot longer than most people realise; the column loudspeaker
f ound in many traditional E nglish churches is actually a line array, with
many of the designs used for these loudspeakers dating back to the
1950s and 60s. I n fact the concept of line arrays is mentioned as far
back as 1896.
When a group of individual loudspeaker drivers are arranged in a
vertical line, we get two very useful benefits. Firstly, the vertical
dispersion of the array is more tightly focused than for a single
loudspeaker, meaning more sound energy is directed to the audience,
which is desirable, and much less to the ceiling and floor, which is
not so desirable. Secondly, as the output from all the drivers in a line
array should sum coherently, they wi l l give us a much greater acoustic
output than the single device.
The problems start however as soon as you put one loudspeaker
driver in close proximity to another. Sound is a pressure wave and
the wavelength varies between around 12m at 30Hz (one of the
lowest frequencies found in most musical material) to just over 2cm
at 16kHz (one of the highest most adults can hear). To produce a
uniform sound field, the drivers need to be spaced no more than half
a wavelength apart. Further apart than this and the array will suff er
from lobing, which can be heard as changes to the tonal character
of the system as you move around the listening space. For the low
frequencies, bass drivers are typically 18″ and 15″ speakers, so spacing
them less than half wavelength is not a problem. E ven the mid-range
speakers can be spaced close enough to maintain the rule. T his ^
Most sound, including our voices, naturally…
emanates from a single point. T he only sound I can thi nk of which
emanates from a line, is a stringed instrument. The theoretical
propagation pattern of a point is spherical, and an example is the
ripples generated by a stone dropped into the pond, although this
is only in two dimensions. The pattern from line arrays is more
complex. On a sliding scale, lower frequencies and longer wavelengths
will couple and tend towards a flat wavcfront. At higher frequencies,
where the wavelength dimension is smaller than the spacing of the
components in the array the loudspeakers wi l l start behaving as
a multitude of small point sources with narrowing dispersion as
the frequency rises. T his has been the problem with line arrays.
T his multitude of point sources would be quite acceptable if the
components were positioned on the surface of a notional sphere. In a
line array they are positioned almost on a Hat plane; wherever a listener
is positioned in front of a line array most of the loudspeakers can be
heard, which sounds like a positive thing except that every loudspeaker
is in a different position and therefore a different distance from the
listener which spells much trouble. Consider the crack from a snare
drum. In a split second it goes from no sound to f ul l on sound. W hat
makes it exciting is its attack or fast transient. W hen this is heard from
a line array the sound from all the differently positioned loudspeakers
arrives not all at once but in a closely spaced series of hits which has
the effect of smearing or blunting the attack. It is like appl ying soft
focus to a picture. The audible effect of this is a loss of crispness, and
the sound retreats into the enclosure. As point source clusters
