The back-loaded double horn
In the late 1980s early 1990s I built a few stage bass cabinets from the Eminence high power enclosure cookbook. One of these, the ‘W bin’ cabinet, lodged in my memory and I decided to try and adopt the style for a hifi cabinet some 20 years later. The result of this endeavour is illustrated in the following set of photos.
It should be noted that the original ‘W bin’ was front loaded, but this started as an experiment, trying a rear loaded enclosure using the well loved Fostex FE167.
Design
The first stage was to draw out the shape on some wallpaper(lining paper) as this could accommodate both height and depth. The basic outline was a double horn but instead of straight sides as in the ‘W bin’ it should have a curved shape.
‘W bin’ layout. Somewhat resembling a bat, hence pipi from pipistrelle.
The drawing was refined and transferred to a sheet of 18mm Baltic birch ply, with a mirror image on another piece of ply. The dimensions of the drawing were super scientific and technical, i.e tailored to the size of a sheet of ply – 4’ high. This was done twice for a pair of speakers. Speaker builders should not be alarmed here. I had copious experience of reading the background technology of back loaded horns but this was an exercise in intuitive building, no science involved.
The only recourse to actual scientific theory was the volume of the coupling chamber for the Fostex 167 and the vent size for exit to the horn.
Construction
Looking at the plan for more than a couple of seconds gave realisation to the fact that bending the horn sides was going to be a challenge. It was elected from the start that the ply for the horn sides would be constructed from scratch using a lay-up method in presses.The presses were constructed from 18mm carcass ply, stacked in layers until the correct width was achieved. To ensure alignment as the press layers were added a dowel pin was inserted.
Ply press taking shape. Successive layers of carcass ply until reaching the width of the speaker cabinet.
After some experimentation the material chosen for the horn shape was 10 layers consisting of alternate 2mm ply and 1.5mm carcass veneer. The individual sheets were laid and glued in the press.
A number of shooting boxes were constructed to plane the ends of the curved ply to exact angles.
The baffle and bottom front panel were fitted to accommodate the mahogany trim that would run on the front of the side panels. The void left in the cabinet at the top and the bottom was filled with builders expanding foam, which had been found to be very effective in other builds.
The overall result for this wreckless endeavour was truly rewarding. The presentation gave a razor sharp image, presumably because of the narrow baffle, but more than this, the bottom end was enhanced compared to other FE167 cabinets. It, the bottom end, was also found to be very controlled, and again, this may be a result of the constrained loading. It has been successfully demonstrated with many valve amplifiers, both SE and PP and seems well suited to the minimally damped mosfet amps I have used. Considering the original idea was based on intuition then this is an outstanding triumph. Having said that the driver chosen has been championed by many and it has been said that it’s difficult to produce a bad cabinet for the FE167.
