Horn folding in AutoCAD

Some time back, on one of the speaker / DIY internet forums, someone described their method of folding bass horns as "drawing the straight profile of the horn on a piece of paper,cutting it out and cutting it into a number of segments, and gluing a piece of string along the centre line". The string keeps the centre of each segment aligned whilst folding the horn around. I cannot remember who suggested this, nor what forum it was, so if it was you then email me with details and I can credit you with the idea.

This method makes sense, although in reality is not entirely accurate as the accoustic centre of a bend is towards the outside of the bend. However, using the physical centre line results in only a slightly longer accoustic horn length, causing no degradation in performance.

The following describes the way in which I implemented the above idea using the power of AutoCAD. Undoubtedly, other Cad software may be used provided it can run a command script file. Whilst manipulating 3D segments in AutoCAD is straightforward, generating the initial profile is a difficult task made easier with the use of a Microsoft Excel spreadsheet downloadable here. Please note that this spreadsheet only calculates single hypex type flares in metric millimetres. The first page of this spreadsheet contains instructions on its use.

As noted in the spreadsheet instructions, AutoCAD should be set up for millimetres before running the script. After the spreadsheet output has been pasted to file horn.scr and saved, the script can be run from within AutoCAD by selecting Tools --> Run Script and selecting horn.scr as shown below.

Note that when doing a bend, ALL segments from the bend point to the end of the flare must be selected to keep them all together.

At this point, it is advantageous to construct a temporary box to the desired maximum cabinet size.

Using the same technique, the horn can be bent around bit by bit until the required shape is achieved, as shown partially below.

Hmmmm.... here it seems that another fold is still neccessary, and possible speaker placements could result in inefficiency of internal space.

Personally, I think "S" folds are probably more suited to horns where the path length is short relative to the mouth size.

AutoCAD's undo feature is very handy here, as we can just keep hitting undo until we get back to a position where we can retry the fold. I will try a spiral type fold as used in the Labsub.....

Here it can be seen that the length is not a problem and the degree of folding is minimised, however there is a lot of wasted space, indicating this type of folding scheme is quite efficient with long horn paths.

Again using the undo feature, we can go right back to the first fold and retry with a reduced cabinet depth, as shown below.

Here it can be seen that although there is a extra bend, the folding is still quite mild and still plenty of room.

Also shown here is another handy feature of AutoCAD. The green area is a closed polyline representing the rear chamber area of the speaker. The properties of this polyline gives us the actual area, and from that we can estimate the rear chamber volume. In this case, the volume is pretty much exactly what is required. However, the area below the baffle, either side of the throat section, is still wasted space. By not allowing the baffle plate to seal off this area, it can be included in the rear chamber volume.

Again, using the undo, I will reduce the cabinet depth a fraction more at the front, resulting in the fold below.

Here the total of green areas is the desired rear chamber volume, meaning the only wasted space in the whole cabinet is the three corners (although these can be utilised for electronics such as internal power amplifiers, processors etc).

In the case of this example, the speaker is a 10" model, cabinet height is 900mm, depth is 770mm and width is 400mm. This will form the basis for a future compact pro sound sub capable of extension to 40Hz.

All that is left to do now is to surround this path with pieces of wood, using standard AutoCAD techniques, and then removing the path (sounds simple, but is still quite involved - but highly accurate).