DIY subwoofer project - low budget - critique please

[98_1LE_2]

My dad and I are going to build a sub for my bedroom. He already has several sheets of 3/4" MDF, and a sheet a finished stainable plywood (I believe it is oak). I have not designed or built a sub since '94 or so, so please be my second set of eyes. Trying to keep this under $300 including driver, amp, feet, hardware and port.

The plan is a
to use a Dayton 12" Titanic Mk III sub, paired with a
Dayton SA240 240W amp. The internal enclosure volume would be 3.666 ft^3, with a 3" flared port 12" in length. Goals are a sub that will play down to (or close to) 20Hz, at lower volume levels.

Planned box design is a tad unusual, and where I would like feedback. It will basically be a box within a box. The innermost box will be made from the 3/4" MDF, all mating surfaces glued, panels screwed together, all seams siliconed and clamped to dry. At that point we plan to use liquid nails to attach a finished plywood outer box, then use screws from the inside to attach it. The only planned internal bracing is a piece of angle iron screwed and glued down the center of the longest side (the side behind the driver. A larger hole will be cut in the outer box such that the driver is recessed.

Finally the corners will be rounded off, the whole thing sanded, stained, and assembled.

This is more about a father and son project than a sub, but will be cherished for many years to come.

[michael hurd]

It sounds like a bit of overkill to use a double box ( complete ) rather than single 3/4 ply and a simple brace. ( no real need for angle iron )

Go with singled 3/4" construction, the 12" Titanic and use the increased internal volume to get a larger port in there. I would go with no less than a 4" port. Double up on the baffle to make your recess. The amp is fine, just make sure you take the volume of it and the port into consideration when planning and modelling.

Did you have a size requirement? If not... consider the Dayton Reference HF 15, it is the same price as the 12" Titanic... and 1.4 db more sensitive ( meaning more output for the same wattage ) as the extra surface area of the cone is an advantage here.

Also, you step up to a more robust voice coil and cone... the aluminum cone helps to dissipate heat. This will help keep thermal compression down, and the motor has shorting rings in it.

[unclemeat]

The oak plywood is a huge mistake. You want veneer. Also, you may find something better than
oak. Unless it's quarter or rift-sawn it's about as attractive as CDX plywood. Make a cut list,
what size your finished MDF panels will be. This will give you a total
square footage of veneer needed. Allow an inch extra in both width & length per panel for cutting the veneer flush. If you have a local
hardwood supplier (not a lumberyard) they probably carry veneers too. Definitely the first
place to look. Though you will probably have to buy a full sheet. Or
search for veneers in Google. You may be able to buy less than a
full sheet of veneer which can be expensive.

You
can build your MDF box, make sure all surfaces and joints are flush and
smooth. Get rid of the Liquid Nails, buy good carpenter's glue from
Home Depot or Lowe's or even your local hardware store. When gluing MDF
edges it's a good idea to use a very thin coat of glue first and allow
that to dry just to seal them. They soak glue up pretty quickly and you definitely want a good bond. Pre-drill the screwholes. MDF edges do not hold screws very well at all and you may end up splitting them. Countersink your screw heads and fill with Bondo. Yes auto
body filler, carpenter's secret. You can find a small tube of it at an automotive store which is all you'll need. Fill any other gaps as well & sand
it flat & smooth. At this point you're ready to apply the veneer.

Just keep in mind how you'll be looking at the finished box. Unless you veneer the MDF panels first and then miter all meeting edges (possible but a nightmare which is why you only see it on the highest-end speakers) you'll have exposed edges on the veneers. Of course that edge is only about 1/64"-1/32" thick. So you want to veneer the bottom first. Then the back (though I would suggest just painting it flat black), then the sides, then the top and finally the face. This will overlap exposed edges as best you can.

Use contact adhesive. Water based definitely has less VOC's but if you and Pop want a cheap high go with the solvent based. The face of MDF is not very absorbent but the back of the veneer may be. Two thin coats won't hurt, just allow the first to dry then do your second coat and the MDF. Placing the veneer can be tricky, you only get one shot. This is why you cut the veneers larger, you'll never fit them perfectly. And orient the grain with the longest edge. It'll look much better than across the shorter dimension. The excess can be trimmed off with a router or laminate trimmer if you have one and a bottom bearing flush trim bit. Make sure to lightly sand the trimmed edges flush afterwards

Once the veneering is done, you're ready to make all your cutouts. You won't be able to round off the corners without going through the veneers of course. But then again, you couldn't round off the ply either. Exposed plywood edges are almost as ugly as flatsawn oak. And remember, you can't polish a turd. You're already spending over $300 on the innards so you might as well make it look good.

Roman

An example of my work, click on it.

[TheEAR]

Like Mike pointed the Dayton Reference HF15 is a better choice,with 14mm of Xmax and a larger piston it surpasses the Titanic III 12" in displacement,plus these drivers are known for very high sound quality.

As was posted use a quality carpenter glue, it is ideal to glue wood products like MDF and ply or any wood.