Big Mouth — a Rear-Loaded Lowther Horn by James Melhuish The search for a Lowther horn that makes lots of bass is still on. Here is a brief review of a horn construction technique that I devised to build an efficient rear-loaded horn. A note of caution: I am not releasing the full dimension details because this horn appears to be problematic working with the Lowther driver. The large throat creates a high efficiency but also a lot of cone movement, with the potential to exceed the Lowther's X-max and cause damage to the driver. Be warned! You can use these construction techniques with a more conventional throat size (such as 70 cm2 for the Lowther) or whatever throat works for your chosen driver. You could also build a dedicated bass horn rather than a full-range design. Design I created a flare with an Fc of 40 Hz and a mouth size of 4500 cm2. The height of the horn is almost exactly 6 feet (1.8 meters). The width at the mouth is 26 inches (66 cm). The cross section is square throughout most of the horn. Materials For one horn: two sheets 8x4 3/8 inch plywood (cheap 3 ply from Home Depot) one sheet 8x4 3/8 inch bending plywood (whacky wood) one sheet 8x4 1/8 inch hardboard (masonite) about 15 feet of 1x1 inch pine square stock 3/4 inch MDF (for compression chamber) polyurethane glue (e.g. Gorilla glue) 2 inch deck screws (coarse thread) 1 or 1 1/4 inch drywall screws (coarse or fine thread) foam weatherstrip (compressable type) Construction Trace your side profile curve onto the sheet of masonite. Use a stick for a compass. Next, place the masonite template on top of two sheets of 3/8 inch ply, clamp together and saw out the profile using a jigsaw. You will end up with two pieces that look something like this: Next, cut out several pairs of "shaping sticks" as shown in the photo below. These sticks are a length to equal the width of the horn at the point you are going to fasten them. For example, the two long sticks are the same length as the horn mouth (the long side facing us). For the throat, I used a square of MDF rather than two sticks. Shape the ends of the sticks into pyramidal points, leaving a small flat for the screws to enter. This allows the plywood to take on an angle relative to the stick (not a right angle). Fasten the shaping sticks to one side of the horn with coarse thread deck screws (approximately 2 inch long). Start screwing the other side of ply onto the shaping sticks. Start with the mouth and work back to the throat. This is a fight and requires long clamps to help bend the ply into shape before screwing. Finished with the throat block attached, except for one more stick to go in the lower rear section. Another view. Lay the form down. Note the extra stick (black on one end) added at lower section of horn. Measure out the approximate shape of the mouth to top of horn on the bending ply and cut the bending ply to suit. Leave excess room on the sides (one or two inches), they can be trimmed later (no photo.) Soak the top few feet (narrow section) of the bending ply in a bucket of water (for the top curve). The joints are simple "butt" joints. Apply water to all contacting surfaces, then run a bead of polyurethane glue down the edges of the sides of the horn. Screw the bending ply with one screw to the center of the mouth shaping stick, then bend the ply down onto the glue towards the top of the horn. You need to work fast or the glue will start to set. Make sure the ply goes all the way to the top without "falling" off one side, then start screwing the bending ply onto the sides. This is difficult because of the extra width of the bending ply (it overlaps the sides of the horn by and inch or two). I used a metric ruler to measure the distance underneath the overlap from the edge to the side piece, then transferred that onto the top of the bending ply and placed the screw at the right place. Note that I have clamped the top. You can also use clamps on the middle section to help you. It is important to try and reduce any voids by clamping and screwing tightly. The glue provides the structural strength. You could possibly patch any holes later on with wood filler. Essentially repeat the above instructions for the inside curve. Note the excess bending ply overlapping the sides in the photo below. I used clamps (including band clamps: straps with a tightening ratchet). Note that the small inside curve was too tight for the bending ply so I stopped short. I used masonite for the tight curve. You can start removing some of the shaping sticks now. Cut masonite to fit the tight curve. When gluing, I had to clamp the masonite to both the side pieces and also the bending ply in order to make a smooth transition (the masonite is a different thickness to the bending ply). Cut bending ply or regular plywood to fit down to the throat. I used bending ply for the final section to the throat, but regular plywood might have been better as you want a smooth and essentially straight contour at this point, and the bending ply picked up the irregularities in my sawing of the side piece, making a slightly "wavy" contour. (Very slight.) I did not screw on either the masonite or the final plywood section. Cut bending ply to fit the final outer contour to the throat. Glue and clamp (I also used bricks for weights). I did not screw this piece on. Using 1 inch square stock, make a flange for the throat, glue and clamp the pieces on. Build a compression chamber (I used 3/4 MDF). I made it 10 liters in volume with removable side pieces so I could fill the chamber and reduce its volume. Pre-drill all holes. Fit foam weatherstrip onto flange. Screw the compression chamber to the flange. Use coarse thread deck screws (2 inch) and I used grease to lubricate the threads when screwing on the chamber. Fit driver and voila ! As mentioned before, this design allowed the Lowther cone to potentially exceed its travel and can cause damage to the driver. I have yet to investigate all possible avenues for addressing this problem. See some discussion in this thread. The sound was quite excellent in the bass even without tuning the compression chamber and not allowing for corner reinforcement. One option is to do away with the Lowther and put a dedicated bass driver in there. Then put the Lowther into a front horn such as the Oris and you're off ! The side walls did resonate with bass frequencies. Two different directions can address this problem: adding weight (damping and lowering the resonant frequency); or stiffening the sidewalls (raising the resonant frequency). I am intending to take the latter route. See some discussion about it in this thread and also in this thread on the Full Range Forum. Note: this design relies on completely equal flexing properties of the side 3/8 inch plywood to achieve good symmetry. Of course this is not really the case and the horn is slightly misshaped (if attempting to draw a centerline down the entire horn). To create perfect symmetry, a better solution would be to create a stiff frame in the center of the horn to which the shaping sticks are screwed. Now the plywood has to flex to take the exact shape of the sticks relative to a centerline, rather than relative to the plywood on the opposite side. You will need to find a way to disassemble the frame when the horn is fully glued together. Top