Whilst it is possible to buy a variety of mid and high frequency horns of varying sizes, it is not that difficult to make your own. If you are making a design where the sides of the horn don't have compound curves it is possible to build it using sheet material. Where compound curves are required the best method is to use fibreglass. This requires a mould to be made which forms the shape of the horn.

I have found that the easiest method to make the mould is to shape it out off wood. For the traditional round, axis-symmetrical horns I cut a number of circular discs and glue them together in a stepped cone. This can then be turned and smoothed on a lathe. Any imperfections can be filled with car body filler and sanded smooth while still on the lathe. To ensure that the profile is accurate use the calculated radii at regular intervals along the horn in a CAD software program. By using the spline tool and entering the co-ordinates you can draw and print out a template to check your mould.

For small rectangular horns blocks of wood can be glued together and shaped on a band saw.  Examples of both a round and rectangular mould can be seen in the pictures. The round horn is the one used for the Crescent Moon PA. Once satisfied with the shape of the mould give it a couple of coats of resin and sand down after each coat with some fine wet & dry; 600 grit or finer. Giving the mould a couple of coats of different coloured paint helps highlight any high or low spots when sanding. The aim is to get an extremely smooth finish. Coat the mould with a couple of layers of release agent and you are ready to fabricate the horn.

You will notice in the pictures that the base of the mould extends out further than necessary. I do this so that I can laminate a flange larger than needed which can be cut back once the horn has been removed from the mould. You should also notice that there is no flange for the compression driver to fix to. To make this I laminate a flat piece of fibreglass on a sheet of glass using about three layers of mat. This is then cut to the appropriate diameter for the compression driver and a hole for the throat drilled in the centre. With the smooth side towards the compression driver the flange is then glued to the horn with epoxy resin and extra layers of mat built up to strengthen the flange and the neck of the horn.

If you add extra layers of fibreglass to already cured fibreglass make sure that it is sanded down well to remove a film of wax that is added to the resin to enable it to cure properly. Gel coat doesn't have this wax and remains tacky when exposed to air. I don't usually bother with a gel coat, and as long as the mat is fully wetted the finish will be smooth. Also optional is the addition of a colour to the resin. Because I have the spray equipment and enough left-overs from larger projects I just use the natural resin colour and paint the finished horn.

An alternative to fibreglass is carved wood. This technique works well with radial horns where the plan view of the horn is an arc of a circle and the height of the horn expands as increasing concentric circles. Unless you are experienced in wood work and are confident in your construction techniques I advise keeping clear of this method of horn construction.

Once you have decided on the horn profile, cut a circle out of a piece of plywood that has a radius equal to the length of the finished horn. Blocks of wood are then glued to the ply base deep enough to accommodate the horn profile. The plywood is then screwed to a faceplate and attached to the outboard spindle of the lathe. Even for a small horn the wooden disc can be 60cm in diameter so select the slowest speed on the lathe. When you start carving the horn it is unlikely to be balanced and

could come apart if not of sound construction. This is why I advise this method to only be attempted by the competent. Don't blame me if the whole assembly flies apart. If it does and a block of wood hits someone it could do more than hurt. Only attempt this if you are 100% confident.

To carve the profile I used a square metal tube clamped in the vices of two sturdy benches as a tool rest. As with the construction of the moulds for the fibreglass horn it is best to cut out a template to ensure an accurate profile. Once the profile has been carved the disc can be removed from the faceplate and then cut in segments. Two of the segments are then fixed together as shown in the diagram with rectangular plywood side plates. Another piece of ply was fixed to the throat end of the horn to allow fixture of the driver. The easiest way to do this is fix four captive screws facing outwards a second plate (I used aluminium) is cut the same size and holes drilled so that it can be fixed to the first plate Two or three additional holes are drilled and countersunk to allow the alloy plate to be fixed to the drive unit. The combined plate driver is then coupled to the horn using the captive screws.