• 3/4 thick high quality birch plywood,
• box is only 4" deep inside (21" x 21" x 5.5" outside dimensions) to allow for keeping the spare tire,
• box is sprayed inside with Dynamat Dynashield which is no longer available,
• internal corners are sealed (do not use silicone based sealant - see notes below),
• four braces support top wall of the box,
• tee nuts are used inside the box for subwoofer flange attachment,
• subwoofer flange is sealed, and
• Deflex  pads are used  inside the box to reduce standing waves and enclosure resonance.
  

At this time I am still working on attachment method to the floor but it will be based on use of rivetnuts, and on the top bezel construction.  Amplifier rack bezel will butt end with subwoofer box bezel.   Both will be made out of plywood, smooth finished and painted flat black.   

• Phase Linear 12" Aliante , also discussed by Stephen here, unfortunately discontinued for a while as all other Phase Linear products, came in wooden box with it's own "birth certificate"!
  

Some questions I have asked and very helpful advise I have received when deciding on what subwoofer I want:

...Referring to Kevin O'Byrne [SMTP:kobweb@carsound.net]) design, one needs 19.5 litters or 0.7 SQ F.   In My 928, I plan to elevate hatchback floor by about 4".  This would limit my enclosure to 18" x 18" x 4" size.   WOULD IT WORK?

You can increase box dimensions, just be sure to perhaps place "corner" braces if you can to help straighten the box at corners. Just make braces that go from the bottom to the top of the enclosure and are perhaps 1-2" wide.......you'll be fine.
Kirk
Ampmanaudio Owner

I used to handle technical support for Phase Linear Europe but they don't employ me anymore for that (and they don't really have anyone else doing that job now), so I can't really help too much (sorry but I don't have the time to do the work for free).  But I worked out that your box (18" x 18" x 4") would give 0.675 cubic feet, which is close enough and should work fine. The depth of just 4 inches is not ideal, but it should be OK. 

Regards,

Kevin
www.carsound.net

Sealed vs. Ported Box Design

I don't know what style of music/bass you prefer, but maybe you should try to have it as a closed box first. That should give a more natural bass response and will be able to handle the power safely up to about 200W at low frequencies. Then if you still want more power and "punch", you could add a vent later (but adding a vent will cause the bass to roll-off sooner, so it won't sound so deep).   A vented box is usually better for people who like dance music or very heavy bass around 50Hz.

Regards,

Kevin
www.carsound.net

Subwoofer Box Material
 

I have checked out a 3/4 MDF (smooth and dense but appears that it may brittle on the edges), a 3/4 excellent plywood with thick venire   layer of the birch, and the same style plywood but finished with oak.   Which is the best.  I want to take "the best" approach regarding box materials.  How would You rate these 3 in order of  decreasing box quality.

MDF is always best unless the plywood is of very good quality and does not have air gaps in its lamination.
Kirk
Ampmanaudio Owner

Is plywood acceptable material for the box, or only particle board must be used?

Plywood is a very good material - maybe even better than particle board because plywood is less porous. But it is more likely to flex than particle board, so for larger enclosures it is usually necessary to internally brace the panels.  In your case (not a large box) it probably won't be necessary to brace the panels, but try to use a heavyweight thickness of plywood - say 12-15mm thick.  Most guys use particle board because it is much cheaper, easier to find and a little easier to cut and work with, but personally I think plywood is better.

best regards,

Kevin
www.carsound.net

Subwoofer Size

Is 12" preferred over 10" sub for any reason other than max. handling power?

I've listened to the Phase Linear subs, and thought they sounded great.  It was the 12" in a ported box.  It went nice and low, was very quick. I'd recommend the 12" sub over the 10" sub, I'm a big fan of cone area  (decreased distortion). 
 

Mike Kim
TeamAMP Technical Director

Number of Subwoofers

Is there real advantage to two 12" Alienates vs. one?

The only advantage is more output - the frequency response should be the same (assuming the box size is correct - i.e.. about twice the size, or two boxes).

Regards,

Kevin
www.carsound.net

What amp should be used (wattage) to drive 12" subs? 

I suggest at least 300 watts to drive it to full excursion.  It's quite a robust sub.
Good luck!
Mike Kim
Team AMP Technical Director

There are many controversies over plywood vs.. MDF vs. particleboard,  etc...as I made a living designing, building and prototyping enclosures for very high-end speakers.  I've probably built about 7000-10,000 enclosures of very high quality....here are my thoughts: Overall I prefer quality plywood (lumber core) and HDF (high density fiberboard of over 50# weight).  The reasons are uniformity,   workability, and material stability relative to its fabrication. The heart of an enclosure is not so much a function of its make-up, rather the mechanical constitution of how it is made. In your case, being a very small enclosure, you face few problems, the main being a large back panel relative to the sides. This large back panel (opposite the front where the speaker is placed) can, and will act as a "drum" face and will resonate. It us therefore prudent to apply low frequency reduction to lower the resonant frequency of that panel and if at all possible, apply a "link" between the rear of the driver and the rear panel (as we discussed with a sorbothane puck). this is what I term, "a resonance sink", similar to what a heatsink on an amplifier accomplishes....it drains heat away from the output devices, and so on. Many materials will accomplish this, requirements being mainly mass, high viscosity (closer to a liquid state than solid) and in a pinch sand or lead shot in a bag works nicely as well although will settle and therefore cant be used  between the driver and the rear panel as a "link", but it works just the same.  Overall, we want to reduce the tendency of the rear panel to resonate. Build the enclosure robust with wall thickness of 3/4" and you will be fine. Using "constrained layer dampening" buy doubling up panel thickness and is highly advisable if possible using perhaps Dynamat in between the panels. You can mechanically attached the 3 payers with screws, cabinetmaker staples (best) and/or contact adhesive. this is the surest way to reduce panel resonance and at the same time greatly increase enclosure rigidity.  Pay close attention to how you attach the driver to the enclosure. I use t-nuts and bolts, usually #10 gauge, wrap the bolts in plumbers Teflon tape and use lock washers between the bolt and the driver surface. Using "Mortite" (a common window sealing tape-caulk is a cheap and highly effective way to seal the driver as a gasket as opposed to closed cell foam, which looses all useful properties over time. Lastly, coat all internal seams with "phenoseal" caulking...try to avoid silicone as it releases gases that can erode driver adhesives.....Mounting the enclosure is very important as well to couple it to the car, do this well and you may want to use a layer or two of dampening materials in the area you are mounting to is adjacent to any cavity (again to avoid releasing low frequency energy into that cavity).  CDT crossovers use a wire attachment system that is self-contained, there is no need for other terminations, nor recommended.  As a rule, it is advisable to mount xovers nearest the drivers and run (bi-wire if you can, 1 wire to mids, 1 wire to highs on xover) wires from the amp to the crossovers. Having the shortest distance of wire between the speakers and xovers is the best.  This reduces strain on the amp and creates the biggest "pipe" from amp to xover..... 
All the best,
Kirk 
Ampmanaudio Owner