A couple of days ago, I started a thread entitled "My subwoofer design methods and considerations" in which we simulated a number of different implementations of one specific driver, and worked out which enclosure would best be suited to my requirements.
From that discussion, I theorized that a bass reflex enclosure of around 200 liters internal volume, tuned to 20Hz would be the way to go with this build.
Herewith the link to the thread:
http://www.avforums.co.za/index.php/topic,13284.0.htmlIn this thread, I will cover the construction of the subwoofer itself - most of the theory is behind us now. This is why subwoofer builds are favoured by so many people - they are quite easy, and will usually yield good results if you have taken time to do your homework well.
The driver I will be using is this - an 18 inch high efficiency unit by 18Sound of Italy, the 18LW800. Data sheet:
http://www.eighteensound.com/renderPdf.aspx?prj=true&pid=30. Out of interest, this driver is the smallest 18" driver these people make - it is the baby
Amplifier is a class D plate amplifier from Hypex, the AS2.100 It will produce 140W RMS into 8 ohm, bridged and has full DSP and crossover functions via USB port and software. More info and ordering from the Hypex online store:
https://www.hypexshop.com/
200 Liters nett internal volume makes for a really large cabinet, so the trick is to make it look slightly more interesting. If you just built a massive cube or such it wouldn't look too great, so some small tweaks are required from a purely aesthetic point of view. Nothing drastic, just something that breaks the monotomy of perfectly parallel surfaces. So, I started up my trusy copy of Google Sketchup, and proceeded to play around with designs until I found something I thought I could work with.
What I came up with is this - you are looking at a 2D cut away sectional view from the top perspective:
And the 3D view (without the top installed):
The enclosure is on the large side of large - it measures in at 1100mm wide x 600mm deep x 550mm high external. The internal volume will be a tad over 200 liters after I have subtracted the volume displacement of the driver, ports, bracing, and amplifier section.
I phoned up my favourite board supplier here in JHB, and ordered 2 sheets of AA grade marine ply, 25mm thick. These are 17 ply, and use phenolic resin type adhesive between layers. It is expensive, but it is a really good quality sheet material. This marine ply will be used for the cabinet itself. I also ordered a sheet of 12mm MDF to be used as internal bracing.
With the enclosure being so large, and housing an 18 inch driver there will be a significant amount of panel resonance issues, even with the marine ply. To counter that, I will use the ultimate form of bracing - matrix bracing. Matrix bracing couples all sides of the enclosure to all other sides, thus we achieve a far higher degree of structural integrity. To make matters even worse, I will make dado cuts in the marine ply for the matrix bracing to fit into. By doing this, I no longer rely on the wood glue alone to create a bond between the cabinet wall and the bracing - the bracing is integrated into the marine ply in a manner of speaking.
The glue used in just about all plywoods are really heavy on router bits. Seeing as I need to cut grooves (dados) through a couple of layers, I have purchased some serious bits to use in the router. I am fortunate in that my router has a 12mm collet as well as a half inch. The 12mm collet allows me to use a number of high quality milling cutters. I went out to get 2 of these spiral upcut bits. When used in milling machines, these cutters are called slot drills. These particular ones I have are made of HSSCo8. Price is around R 140 each ex VAT. You can also get the solid tungsten carbide ones, but they are around R1k each...
Here you can see my first cut with the bit - the depth of cut is 5mm. I clamp a straight edge on the wood to act as a fence, then run the router against this fence.
Close-up view - the cut is very clean because the bit is extremely sharp:
This pic shows a side panel, with all 3 slots cut. The spacing between the slots is 120mm.
After I cut both side panels, I drew the layout of the panels onto the bottom panel, then proceeded to cut slots for the matrix bracing into it:
Next, I cut some of the MDF braces, and did a test fit. This is without glue or any fastners, and is done to check that everything fits and lines up well.
View from above. You can see the two slots on the top of the back wall of the enclosure. These are rough cut-outs of the port mouths. After I glue the ports in, I will run a flush cut bearing bit in the router through these exits to clean up the edges perfectly.
That's it for 2 days work. I will hopefully get through some more tomorrow. Cutting the braces takes alot of time and is rather tedious, but it will be worth it in the end...
Cheers,
Ian.
