I am going to attempt to build a new sub enclosure so I can fit my 400W sub in the boot of Nina's 244. It is about a 25cm sub, but there is only 30cm of vertical space under the parcel shelf, and 45cm of width (6x9s get in the way after that).
I am planning to use 12mm MDF to build a wedge-shaped box. The amp will be mounted to the back of it because I'm lazy and in 10 years I've never had an amp fail from being bolted to the sub enclosure so take your theories about component vibration and get out.
Here's my initial sketch, based on some quick measurements of the boot space:
I'm going to try and avoid buying any new tools for this.
What I have:
- Wood saw
- Plunge router
- Bench drill
- Non-bench drill
- Dremel
- Ryobi Jigsaw
- a chisel
- Large clamps
- About a thousand wood screws
To buy:
-
12mm MDF
-
Dust mask
-
Polyurethane wood glue
-
Countersink drill bit (I may get lazy and just use a giant drill bit to do this)
-
Speaker wire terminals
Also needed:
- I need some aluminium sheet offcuts to make a circle-cutting jig for the router, and I think I need a new straight cut bit for the router, but we'll see - MDF cuts pretty easy.
Design
Sub enclosures need to be a suitable size for the sub.
To work out the exact dimensions needed for your sub, you need some values supplied by the manufacturer, specifically Q(ts) and V(as) values. Vas is a volume. For my sub it is specified in litres, but for many subs it will be in cubic feet. Watch out for that, as the result of the below function will be in whatever unit the Vas is specified in. Google makes light work of the conversion though (type in "123 cubic feet in litres")
You then need to take your Q value and select the corresponding multiplier from the table below:
<table><caption>Q multiplication index</caption><thead><tr><th>Q</th><th>Multiplier</th></tr></thead><tbody><tr><td>.2</td><td>.1</td></tr><tr><td>.25</td><td>.25</td></tr><tr><td>.3</td><td>.4</td></tr><tr><td>.35</td><td>.75</td></tr><tr><td>.4</td><td>1.1</td></tr><tr><td>.45</td><td>1.5</td></tr><tr><td>.5</td><td>2.0</td></tr><tr><td>.55</td><td>2.6</td></tr><tr><td>.6</td><td>3.5</td></tr></tbody><tfoot><tr><td colspan="2">(
source)</td></tr></tfoot></table>
So for me it is ~1.1. If I multiply 1.1 by the Vas value, I get the optimum enclosure volume. So for me that is 1.1 × 19.38L = 31.31 litres.
The subwoofer itself will occupy about 1.42L of volume (
source), so I'm looking for a total of 32.73L internal volume.
This handy calculator makes light work of the process of determining the box dimensions. I simply select 12mm MDF, and start plugging in estimated dimensions:
http://www.rockfordfosgate.com/rftech/box_wizard.asp
And we have now arrived at the desired external box dimensions:
Because I want to wedge this against the rear seats, I still need to confirm the rear seat pitch from the boot side, and may have to adjust my measurements to ensure the angled face of the sub matches it.
To stop the sub from hitting the seats, I'm going to use some bars, recycled from my old sub enclosure. I'll post photos of this later.
Planning
Now I can determine how much MDF I need to buy. Using basic trig we can determine the sloped face of the enclosure will be 32.87cm tall.
So the following panels are needed:
Front: 32.87 x 41.4
Sides: 40.4 x 29.6
Back: 29.6 x 41.4
Top: 26.1 x 41.4
Bottom: 40.4 x 41.4
By using a guide with the jigsaw, I can cut both side pieces from a single rectangle of (~40.4x29.6), if I factor in the amount of wood removed by the cutting process.
Bunnings sells 12mm MDF in sheets measuring 1200mm x 450mm for $10.75 each, and I can lay out everything on two and have plenty of spare: