| Any single loudspeaker may measure to have
exceptional response, and the sound quality of a
single loudspeaker may be nothing short of
hi-fidelity, but that will not be the case once
arrayed. With these direct-radiator packaged
speaker systems grouped in a cluster, the
individual drivers can not be positioned to
minimize time delays between devices and the
resulting destructive interference, cancellation
and response variations. These variations are
important because the coverage uniformity in the
1,000Hz to 4,000Hz range is critical for good
intelligibility and uniform frequency response
from 250Hz to 1,000Hz is important for natural
sounding speech. The problem is related to the
physical distance between individual loudspeaker
components. The special signal
processor/crossover that comes with the
loudspeaker can only correct for time offset of
the signal between the low and high frequency
drivers along a single speaker system axis. The
signal delay in the processor cannot adjust for
every possible off-axis listening angle,
especially in the vertical plane, where the
physical offset between high and low frequency
devices changes the signal delay for various
listening positions by fractions of a
millisecond. The processor certainly can't fix
the additional multiple signal delay variables
thrown in by having additional boxes added to the
mix.
In the four box cluster we will show here,
there was approximately 30" between the box
centres, both horizontally and vertically, and
this was the manufacturer's minimum interference
configuration (That's a half wavelength at
225Hz, one wavelength at 450Hz, two wavelengths
at 900Hz). At the crossover point, where the
horn and the 12" bass speaker in each box
are operating at equal levels, there are a total
of eight devices
operating at the same level with
multiple wavelength spacing between all eight
sound sources. This is the offset distance
vertically, horizontally and diagonally between
all devices. Where the coverage of the boxes
overlap, there will be significant lobing of the
coverage and comb filtering caused by these
physical-offset-induced signal delays.
Below the crossover point, the 12" bass
drivers gradually broaden their coverage angle
until the boxes each become omnidirectional. In
the frequency range between 250Hz and 1000Hz,
very large variations in level will be found,
with the highest level exactly on the centre axis
of the cluster, and spurious lobes all around
that centre hot spot. This produces plainly
audible sound quality variations with seating
position, and may produce feedback prone
positions under the cluster.
Second one is related and that is uneven
spl/power. Lines (even those made of single long
elements) exhibit a much different
"acoustical density" over distance,
essentially falling off at half the rate of
attenuation of a point source. (And the line
itself becomes a point source at a frequency far
above your subwoofer when you build a short
line.) This means that your placement is going to
be iffy in your room, and the mesh between
driver(s) and driver banks will need a great deal
of adjustment. The theory is understood but the
setup still requires trial and error when you try
to integrate lines and points.
Lastly, a little known phenomenon of lines is
their corruption of transient information. Lines
are 2-dimensional radiators (having no vertial
dispersion when they are used standing upright)
and thus cannot disperse sound in all directions
like points. The transients are produced with
artifacts, or "wakes". Kinergetics
published a white paper some years ago detailing
this if you can find it. In fact, I think it was
Venderkoy who also published a paper discounting
lines entirely for this and other reasons.
Somebody please correct me if I misremember...
Having said all this, obviously, some lines
are very entertaining and make big waves in some
audiophile circles. While the jury may be out as
to the absolute "accuracy" of these
devices, listeners report an enormous image and
big rock and roll factor.
It is also important to understand the effects
of physically mis-aligned loudspeakers, and to be
able to measure and adjust the time-offset of
devices. The small packaged boxes described above
may relieve the buyer of the need to think about
the selection of devices, but that doesn't alter
the final results as we've shown here. Cluster
design can't be left to a manufacturer's
marketing department!
What follows is a brief review of the
behaviors of point, line, and plane sound
sources.
jlm
There are a few notes here. The classic
Series/Paralell connections will result in the
Back EMF of the various drivers interfering to
some degree with each other, it's unavoidable.
The other issue is the "Bessel"
Effect which creates a comb-filter effect with
multiple Driver arrays. A 1 X 2 or 2 X 2 Array is
(IMHO) the largest that could be used succesfully
without actually angling in the Drivers and
applying other "extreme" measures.
If you want to play with larger groups, there
are two options. Philips have a Patent for
multiple driver Linesources that are on a flat
Baffle, but use specific gain and phase
coeficcients for each driver, combating the
combfilter effect. It does require a lot of slip
stick (or PC) work though.
The other option is mechanical and works (at
least in practice) very well. As long as all
drivers are placed on a baffle that offers a
close approximation of a section of a sphere,
having the same radius as the listening Distance.
This quite succesfully eliminates the problems
with the "Bessel" Effect and presents a
very coherent sound for vertical array without
messing with overly complex crossover.
Measurements should prove much smoother
responce from curved baffle linesource.
The more serious problem is with the frequency
characteristics of multiple dirivers: as the
frequency increases the wavelength decreases and
when you get to the point where the wavelength is
1/2 the distance between driver centers you will
get wave destructive interferance causing deep
nulls in the response. Especially the off-axis
response will suffer from this earlier than the
on axis but generaly what you will get is a very
sharp narrowing of the on axis response, i.e
lobbing. Thus a wide frequency reproduction and
even off-axis response from multiple drivers is
not possible.
With an extended-range driver used in a line
array, there is a significant chance that the
beaming problem is really lobing - that is,
multiple lobes at different angles, but of nearly
equal loudness. For instance, a large array of
sources spaced uniformly 6.75" apart (close
to the str8-8 spacing) at 4kHz will have
far-field peaks at 0 (on-axis), 30 degrees, and
90 degrees. There will be nulls between, at 14
degrees and 49 degrees. At higher frequencies,
there will be even more beams, more closely
spaced.
The best way around this is to move the
drivers more closely together. Tweeters spaced
3,5 cm apart will work up to 20kHz, or
approximately 7 cm apart will work to 10kHz,
before becoming excessively lobed.
If multiple woofers and a single tweeter is
used, than there is a problem with different
pressure decays, woofers act like infinite line
source, tweeter acts like a point source. With
this kind of behaveur tweeter must be padded more
when listening closer to the speakers, and less
when listening far away. It's an interesting
effect to walk up very close to the speaker. The
highs get louder and louder, much more quickly
than the mids.
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