Accidental Line Array
Introduction.
At the start of the twenty-first century the "in" design for sound reinforcement
systems is the line array. After a slow start, all the major manufacturers have
got their own variation on the theme which of course confers some advantage over
their rival's systems with no disadvantage The main selling point for all these
systems is their ability to approximate a true theoretical line source that
radiates cylindrically rather than spherically. This reduces the rate of level
reduction for each doubling of distance, as one moves away from the array, from
6dB to 3dB, greatly improving the "throw" of a system in large venues. Not to be
outdone, I designed my own line array loudspeaker back in 1995 albeit for very
different reasons. At the time I was unaware of any major manufacturer producing
similar designs and it wasn't until I became internet enabled a few years ago
that I became aware of the success of the Vdosc system and the subsequent
development of similar systems by the other manufacturers.
The idea for the design originated from when I was working as one of the house
sound engineers at a local venue. The installed system was beginning to show
signs of its age and the owner asked me to think about what might be a suitable
replacement. I thus thought about it and also the possibility that I might be
able to design and install one of my own systems. Not long afterwards, the venue
closed because the owners of the building sold it and the new owners wanted to
use it for their own purpose rather than lease it. So apart from a couple of
prototypes my design got no further.
The design.
I had never been a fan of the contemporary full range loudspeaker cabinets and
their use when arrayed. The overlapping sound waves from the individual drive
units created as many destructive interference patterns as constructive.
Although it is possible to reduce these problems, in doing so the manufacturers
would run into ergonomic, visual and manufacturing problems. With the available
budget that the club had, and thus the type of equipment that they would be able
to purchase these problems would have probably been ignored by the manufacturer.
The club was quite small with a capacity of approximately 400. It had a low
ceiling and was wider than long (given the stage/PA position). Members of the
audience were also free to move anywhere they wished and as such there were no
dead spots. Given that everyone was on the same level vertically but at
different positions horizontally, I wondered how I could reduce the loudspeaker
system to a point source looking from a horizontal perspective. Column
loudspeakers had been used historically but the traditional design would not
provide a sufficient frequency range even if it could be built to produce the
required sound levels.
As I was also involved with the design and construction of both hi-fi
loudspeakers and the occasional studio monitor system, I was aware of the use of
the mid-top-mid lay out used horizontally in a number of studio monitor designs
(Quested, Dynaudio) and vertically in hi-fi speakers as introduced by Dr J
D'apollito. Using the former layout but scaled up and with cabinets placed
vertically on top of one another might produce the super column that I was
looking for. The next stage was to draw a few sketches and do some rough
calculations to see if the technique was worth pursuing.
As a starting point a non-trapezoidal shaped box 1m by 0.6m by 0.35m was
assumed. The dimensions limited the maximum size of bass drive unit to 12". If
two such units were fixed at either end of a 1m front baffle there was only room
for an 8" drive unit to fit between them. Whilst there are a number of coaxial
8" drive units on the market it was felt that the cost and sensitivity of such
units was restricted for this design. That left three choices. Make the cabinet
larger, abandon the design concept or move the bass drive units off the front
baffle; this could be done by using a band pass type enclosure.
The next stage of the design was to calculate the response for various drive
units. The conflicting parameters to consider are: frequency response, enclosure
size, and limitations on the maximum SPL (sound pressure level) due to
electrical and mechanical restraints of the drive unit. Calculations were done
for drive units manufactured by Beymer, Eminence, Celestion, Fane, ATC, JBL and
RCF. The best compromise was the Fane Studio12b, two of which would fit in the
cabinet size specified, and achieve an output of 123dB over a range of 50Hz to
274Hz with no excursion power limitations and a ripple of less than 3dB. The
diagram shows the lobing effects of two point sources as a function of the
wavelength between the sources. If any control over the directivity of the
cabinet is to be maintained, which is a major feature of the enclosure, the
upper crossover frequency must be at some frequency where the spacing between
the drive units or ports in the case of a band pass enclosure is about 1/2l ,
where l = 1 wavelength of the sound. Although the responses shown in the diagram
are obviously different the transition in response is gradual as the frequency
increases. The actual cross over point can, therefore, be chosen to best
compliment the response and directivity of the midrange section at some point
between 200Hz and 250Hz (0.4l and 0.6l ).
As a matter of interest calculations were done for a band pass enclosure where
the rear enclosure is connected to the front enclosure with a port. As a "what
if" exercise a simulation was done with the port for the front cabinet set at
the area of the front portion of the cabinet and the tuning frequency set so
that the port length would be close to zero. The response was similar to that of
an ordinary bass reflex cabinet, which opened up the possibility of using a
greater range of drive units, also given the omni-directional directivity of low
frequency sound it should be possible to leave off the outer walls of the
cabinet completely and have the speakers fire sideways. This could lead to
problems of preventing damage to the drive units or grille during transit, but
would be worth experimenting with. In the smaller of the two cabinets (assuming
the rear volume remained as it is) the cabinet could be reduced in width by
240mm. Where cabinet size is less of an issue 15" drive units could be used
which would give an extended bass output.
The band pass enclosure type means that there are now approximately 28 inches of
baffle space left for the mid and high frequency units. This would allow either
two 8" or 10" drive units to be fitted either side of a central horn. If the
same principles are applied as used for the bass units, regarding directivity,
the mid range units would be only operable up to about 500Hz and a small horn
would most certainly not operate down to such a low frequency. The other
alternative would be to horn load the mid range drivers using either a single
10" unit or two 8" units. In the enclosure considered the area of baffle
available for the mouth of the horn would give an effective diameter of 0.34l at
the lower cross over frequency. This gives a reasonable loading to the drive
units with little variation in the acoustic impedance.
When the cabinets are used in multiples the effective mouth area becomes even
larger improving the loading of the drive units even further. After calculating
the acoustic impedance of both exponential and conical horns and their
theoretical response it was decided to use a conical design which allowed easier
construction of a constant directivity type horn. Ease of construction coupled
with better directional control was considered to outweigh the increased
acoustic loading of an exponential horn. For the drive units I finally chose two
8" units that would face each other, a technique that I was familiar with.
The upper frequency limit of the horn is principally limited by the combined
mass of the diaphragm and voice coil. The volume of air between the diaphragm
and throat, however, acts as a compliance and although if chosen correctly can
extend the upper response slightly it more often than not reduces the upper
frequency limit of the horn. The lower individual mass of 8" drive units should
give a wider operating range than 10" units. By careful design the response of
the mid range horn should extend from 200Hz to 3500Hz.
Initially it was envisaged that the high frequency horn(s) would be mounted
coaxially in the mouth of the mid range horn. With the possibility of a cross
over frequency of 3500Hz it would be possible to use a number of cheap small
horn driver combinations if cost was a major consideration. Where performance
was more of an issue more expensive units could be used. While sketching
possible layouts for the cabinet it became clear that fitting a slot horn in the
throat of the mid range horn would be possible. This technique offers a number
of advantages. Firstly the path difference between the mid and high drive units
is less giving better time alignment and no lobing problems. Secondly the
directional characteristics of both the mid and high sections are identical at
the crossover point ensuring a smooth power response and thirdly the high
frequency horn is fabricated into the cabinet saving the cost of a commercial
unit. The idea for this technique came from the idea of making a vertical slot
coaxial unit. I had always liked the sound of Tannoy coaxial drive units and had
designed several successful cabinets using them. I had also been intrigued by an
old Vitavox slot horn design (shown right), so combining the two techniques
seemed like a good idea. The high frequency horn was made simply by cutting some
inch thick board (diameter of the compression driver throats) to the required
shape and sand witching it between two sheets of thinner board. The whole thing
then fitted between the two midrange drive units.
The design was now looking feasible. Slightly more detailed drawings were
created and the response of the mid horns predicted. These were modelled for
some eminence 8" drive units that I already had and would use in the prototype,
and also for 8" RCF drive units that I had considered for the "production
models". The hi-frequency horn inside the mid-range throat would work down to
2000Hz in free air so I foresaw no problems with this part of the design and
just went ahead with it. The prototype was loaded with some old JBL2420 drive
units that I owned.
Whilst this is by no means a simple cabinet to construct, for anyone
experienced in building loudspeaker cabinets should have no problem building
such a cabinet. As well as considering it a good design from an acoustical point
of view, the cabinet works well as a package. The design is simply a mid/high
horn with the low-frequency enclosures fitted in the space that is normally
wasted air in the horn box.
The prototype cabinet was powered by some of the Mosfet power amplifiers
described here. A 600W mono amplifier drove the bass and either side of a
350W/ch amplifier drove the mid and top. The crossover was also a home made one
that was lying about in my workshop. The crossover frequencies were slightly
wrong (250Hz and 2.5KHz) but close enough for me to dismiss the idea of building
another.
Note the sophisticated computer measuring system. This was a Toshiba portable
computer with a 386 processor and 4 meg of memory. An analogue - digital
converter connected to the parallel port and the software ran in DOS.
The speaker was later re-measured using windows based software Spectralab.
Initial measurements showed that the bass response was pretty much as expected,
the mid horn worked better at low frequencies than I had expected, but the
response was a little irregular at higher frequencies. Despite this the midrange
sound was clear and coherent. The results for the high frequency response just
revealed that in their old age the JBLs had lost a little bit of their top end.
Despite throwing in the drive units that I had on hand, and using a crossover
that had been designed for another system the cabinet sounded well. I was
impressed by the detail in the mid range and the fact that even close up to the
cabinet you could here the entire frequency range. Others seemed to be more
impressed by the depth and clarity of the bass. The response seemed to go lower
than measured. Despite the favourable comments no one was interested in the
design because of what "other people" might think if they used it. It was just a
little bit too different from all the sort of vaguely Turbo TMS4 copies that
were being used.
For a couple of years the cabinet stood in my workshop as a pretty good system
for playing tapes through. In 1997 someone contacted me with regard to building
some, but by that time Fane had stopped making the studio 12b drive units and
the order seemed dubious anyway. I recently moved house at which time the drive
units were removed and the cabinet was disposed of.
If you are interested in this cabinet design and want to know more details about
it please email me and I'll get back in touch.
Spurred on by a group of enthusiasts designing their own Line array on Harmony
Central here is my idea of a compact loudspeaker that
fits their brief.