Offset Driver Horn Folding Tutorial Using Sketchup

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:



The schematic:



Response:



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.

Let's Review:

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.

JSS

Before you jump in, please take the time to become familiar with Sketchup, so that I don't lose you when doing the vids...

Here are some GREAT video tutorials:

http://sketchup.google.com/intl/en/training/videos.html

Become familiar with the following tools:

Line
Paint Bucket
Move
Rotate
Tape Measure
Offset
Dimension
Navigation
Position Camera/Look around

Also look at the more advanced videos for:

Inferencing
Copy and Arrays
Accuracy

This is basically all I use to get a 2D model/fold of a horn.

Once you've made yourself familiar with Sketchup, you can start drawing and folding horns.

The first video attached shows how to quickly generate a horn shape prior to folding:

To make a 90 deg fold, use the line tool to generate a split in the horn, select the segment you need, and use the move tool with Ctrl+C to copy the section, rotate it, and reattach:

For a 180 deg fold, basically the same thing:

So far, this is the barest of bare bones...making these vids is easy, but they are very big, and the resolution is OK...I thought I would be able to fold up the Shiva Horn with the vids, but that would make for a file that is too big....

Basically, I can answer specific Sketchup questions w/ vids, but I may not be able to show the whole process....

Any questions so far?

JSS

I'll get to posting more vids in the coming days...the hardest part is when your folds start to interact with the structure already drawn, that's where the 'group' function reigns supreme....

JSS

In a 180, I do the same thing, I pull back the divider by the amount of horn width at the beginning of the 180....I also add reflectors later to avoid the 'expansion' of the horn at the 180 that you talked about...

See below:

JSS

A vented box would be a close comparo...the things the horn has going for it compared to the vented box is sensitivity advantage above cutoff, and excursion control below cutoff...Also, the horn takes advantage of corner loading a little better than the vented box, giving a slight sensitivity advantage at the low corner, only when corner loaded.....

Basically, the design is hamstrung by the size. 300L built is just too small for a 20Hz FLH for the Shiva. Grow to 400L and you get a much more capable horn...see below for 300L vs 400L 20Hz Shiva OD FLH horns, 2V in, 2Pi space:



JSS