My first experience of PA systems was while still at school back in the 1970s. At that time most systems consisted of a stack of JBL 4560 cabinets with an equally large pile of radial horns stacked on top. My first design sort of mimicked this type of system. The bass cabinet looked more like an Altec Voice of the Theatre horn and was essentially a horn/reflex loaded cabinet. The cabinet was loaded with an ATC 30cm drive unit and the port determined using the formula in Beranek's book Acoustics. Out of interest I have recently calculated the response (shown below) using Thiel Small parameters. This does not take into consideration the horn loading but wasn't too bad.

More interesting was the midrange horn that I built to go with the bass horn. The actual horn was nothing new, a four by two multicell. Each cell was made from thin ply and assembled individually they were then all glued together and reinforced with glass fibre. The scan below is of an original early sketch of the whole system. What was interesting was the use of a small cone loudspeaker as a

drive unit. Although it is quite common now at the time I didn't know of anyone using this method for PA work. The reason behind using a cone was simple. I couldn't afford compression drivers so why not stick a four-inch drive unit behind a horn whose throat area was smaller than the drive unit cone. That, after all was what a compression driver was. The efficiency wasn't great but they sounded smooth, until someone plugged them into a bass amp and blew up the drive units.

In the next phase of their life, which was while I was working full time for a PA hire company the drive units were changed to RCF TW101 compression drivers. These were coupled to the horn using adapters that I cast and machined from aluminium. Anyone who was around in the late seventies and saw bands like The Police, Orchestral Manoeuvres, Joy division, a young Mick Hucknall and many more well known artists, will probably have heard these multicellular horns.

On to the final phase in the life of the multicell and it had turned full circle once more using a cone speaker. This time it was 6½ inch Audax unit the HD17HR37 which Audax described as  a high efficiency, high power handling unit. Today's equivalent is not uncommon on large high quality PA systems. So was I far ahead of the rest of the world in the use of small cone units? If so, then it was accidental. Once more it came down to using what I could afford. It is worth noting though, that the main complaint

from people using the system was that it sounded too much like a hi-fi and not like a PA. Before moving on, the reason for using a multicell was to achieve an even dispersion. The original crossover frequency was 800Hz which I reckoned to be the frequency where the bass cabinet (the little Altec copy) and the horn would have similar dispersion characteristics.

The first cabinet I designed as a professional was a folded bass horn. The scan from my original sketch shows a cross section. I didn't consider it anything special. The flare was exponential, the cut off frequency would have been 60Hz if I remember correctly and the box size was 48"w x 20"h x 30"  d. This gave a horn

length of about 4 ft.  (All the calculations would have been in metric but the wood sizes were imperial) The drive units were 15" Gauss and the cabinets were intended for use in multiples of two. The horns must have produced as much bass as I remember because they generated much interest with the reggae bands we worked with. They couldn't understand how such a small speaker cabinet (small to them) produced so much bass.

The next design is a bit more interesting. It was again a horn but this time it used two 12" drive units facing one another with a gap of 1.5" between them. The scan from my sketch shows this clearly. I don't know if anyone else had done this before me but if so, at the time I wasn't aware of it. No doubt someone had already done it back in the 1950s anyway.  The thinking behind the design was fairly straightforward.

If you want a horn to operate down to a given frequency, that frequency fixes the mouth size needed and also the flare rate. To improve the efficiency the throat area has to be smaller than the drive unit. This effectively fixes the length of the horn. It seemed that you could either have a long efficient horn or a short inefficient one. The other alternative was to increase the drive unit size but a bigger cone would need to be thicker to achieve the same stiffness increasing the moving mass. The magnet size would not be appreciably different from that of a smaller drive unit. Why not use, then, two smaller drive units coupled to the same horn so that they act as a single drive unit. Using two smaller drive units, whilst increasing the moving mass also has the added benefit of two magnets Sticking the units side by side would not accomplish what I wanted because the throat area would have gone up as well creating what in effect would be two low efficiency horns side by side. The way I did it meant that the total throat area was 12" x 4" for both the drive units, which were made by Fane.

I have tried a number of variations on this theme that I will come to in a minute. On the initial design I carved a couple of pieces of polystyrene to stick to the cones to reduce the volume of the throat cavity. I couldn't tell any difference with or without them so didn't bother for subsequent versions. Back then I didn't have any measuring equipment except for a cheap Eagle sound meter so everything was based on listening tests. I also remember having built the first cabinet wiring the drive units through a switching box to convince the rest of the company that the drivers needed to be wired the same way so that the cones moved towards and away from each other and not following one another.

This cabinet got nicknamed the mushroom because although the prototype was boxed shaped about 21"w x 21"d x 14"h I later stuck an additional flare to the front, to increase the mouth area, so that when they were stood on their back they looked like a cross section of a mushroom.

A basic four way system which we operated back then consisted of two of the bass cabinets mentioned above, two of the "mushroom" cabinets the multicell (sometimes in conjunction with a smaller radial horn) and a couple of tweeters per side. My contribution to the tweeter design was the box they were mounted in and, just to prove that you can get paid to mix the sound for bands and still pick what are considered the cheap and nasty option I preferred the sound of the little piezos over that of some more prestigious makes. I believe using a cross over instead of running full range improves their performance.

Coming back to the design using horn loaded, facing drive units. I tried a number of variations using drive units  form 6" diameter to 15". One variation used a hyperbolic flare rate with the now familiar trapezoidal shaped plan view (see sketches) This together with the curved mouth was an attempt to make a large radial horn. It was at this point that I was starting to try and create some form of continuity to the directivity patterns through the crossover regions by using similar size horn mouths for the various frequency ranges.

At this point in time (1980-84) I was using Richard Allan 8" -15" drive units, Audax 4.5"and 6.5" drive units and either fane or RCF compression drive units.

During the late 1980s I didn't do much audio work but by the start of the 90s I was back designing loudspeakers. Despite a number of commissions for small PAs, none were spectacular, simply a speaker and a horn in a box. In 1995 I was asked about the possibility of designing a PA for a small club. I came up with a design that I thought was a little bit different from the norm at the time. If you haven't already done so you can read about my line array cabinet here.