OK, as promised, here it begins....
Well, we need to get from a hornresp model we like, and turn it into something we can build. I am going to use The Shiva X2 driver from Exodus Audio, and do a 20Hz horn that is under 300 liters.
Here's the model:
Now the question is: How big is this thing going to be? Hornresp says it will be about 264 liters, under 10 cubic feet.....but hornresp doesn't factor in the fact that you will be using actual building materials....
So, rule of thumb I use is to add 20-30% to the hornresp volume figure. That takes us to ~345 liters or ~12 cu ft, which means around 20,000 cu in. I chose 20" as a good width figure, and ended up with ~32" for the other two dimensions. I don't have necessarily a definite form factor I want to obtain with this exercise, so I am just using a flat rectangle, that when up against a wall won't jut too much into a room.
OK, so the 20" dimension has been chosen. Converting to centimeters, that 20" becomes 50.8cm. Now you need to decide what building material you are going to use. 1/2" plywood is usually 12mm thick. So, for the large 32"x32" sides, we have to take away 2.4cm from the 50.8cm dimension, leaving us with 48.4cm. We will also need bracing, which I think can be accomplished with 1/4" plywood (6mm). For a 20" span, we'll use 2 sets of braces, taking us down another 1.2cm, to 47.2cm.
20" x 2.54 = 50.8cm
50.8cm - 2.4cm for sides = 48.4cm
48.4cm - 1.2cm for bracing = 47.2cm
So now we get back into hornresp, and use that 47.2cm figure as our horn 'height'. When looking at the schematic drawing, Go to File, Export, Horn Data:
You get a dialogue box like this:
Use the 'UNI' flare, and choose whatever you want for the increments. I use large increments because this horn has a single, conical flare. After the first two segments (which I keep the same size so I can fit the driver), the horn doesn't change.
Name the file, and open it in Notepad:
Highlighted are the important numbers you will need to turn this from a bunch of numbers into a drawing:
Now, get into Sketchup and draw out the basic horn shape. As the numbers above show, it should be ~3.7cm 'tall' at the throat, and ~19.07cm 'tall' at the mouth. The throat stays at ~3.7cm for 34cm, then begins expanding until it hits the ~19.07cm mark 425cm later.
Here's a drawing:
Now, we need to 'line' it with 12mm plywood. Using the 'offset' tool in Sketchup, you can specify a 12mm offset, and color it like wood:
Now all you need to do is fold it up into a box...and get the rear chamber volume correct. To find out how much area needs to be in the rear chamber, we use the fact that we need a 50 liter chamber from the model, and work backwards:
50 liters = 50,000 cc
50,000cc + space the driver takes up (.13 cu ft = 3,681cc) = 53,681cc
53,681cc divided by 48.4cm (assuming no bracing in driver chamber) = 1109 sqcm.
So we need the driver chamber area to be ~1109 sq cm after we fold up the horn....
There are 2 basic folds, 90 and 180deg. Here are some illustrations:
Basic Simple OD horn shape:
90 deg fold:
180 deg fold:
Using those two folds, and folding either clockwise or anti-clockwise, you have a decent set of tools in your arsenal to come up with many different unique folding patterns...
One (the one I will use for the tutorial) is a spiral:
If you wish one dimension to be longer than another, you can use a spiral, or a few 180deg folds prior to a spiral (you have seen this on CinemaHorn designs):
Next we will go over how to do the folds in Sketchup.