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I am starting a new enclosure build, similar to what VO has done with the replacement floor setup in the hatch. I have begun building the box and have come across a roadblock. When I started building the box, I was basing my airspace requirement on what I thought was the official airspace needed for my sub according to the Eclipse website. The website lists the VAS as 3.2 cubic feet. Thinking that was the airspace recommended for the sub (since no other volumetric data was listed anywhere) I began building the box with that in mind.
The box I have built is about 2.77 ft^3 before displacement of the sub basket, but loses some airspace due to rises and waves in the hatch floor (which is the bottom of the box). So I think my final airspace before sub displacement is about 2.5 ct^3. Seemed good...
Now, reading some other forums where people are using this same subwoofer, I'm seeing audio dealers and install pros saying they always install these Eclipse 12s in 0.88 cubic foot boxes! Others suggested a 3.5 cubic foot box for a pair of them (so 1.75 per sub). Either way, the advice out there is all over the place. Can anyone here sort this out for me?
Should I expect crap sound if I drop a single Eclipse 12 into a sealed box that's 2.5 cu. ft? (people say a too-large box produces flat response). Or should I section off part of the inside of the box and reduce the volume? If so, what airspace should I go for? Should I consider porting the enclosure? Some pics are attached of what it looks like so far. I'm waiting to solve this problem before I glue the box to the floor of the hatch...
The top of the box is 6.75" from the floor of the hatch, so much like VO's setup, once you factor in the thickness of the rear carpet and sub gasket, the sub will be sitting about a half-inch above the floor.
To combat the airspace issue, and to avoid having to tear it all apart if I ever need to change it, I think that instead of sectioning it off to reduce the internal volume, I'm just going to add some pillows or styrofoam to reduce it to 1.25 or less cubic feet. If it sounds bad, I'll pull some of it out until it sounds right.
I would shoot for around 1 cu ft. Using polyfill of foam will not work as a filler to reduce volume. You need to use wood. Glue and/or screw wood pieces to the existing wood walls of the enclosure to reduce the volume. Of course this is going to get heavy if you need to reduce the volume by 1.5 cu ft.
Using polyfill typically does not work at reducing volume because it is porous and does not have the effect of reducing box volume. "But when you 'overstuff' (using about half to three quarters pound per cubic foot) you change from isothermic to adiabatic loading. Basically what is happening is that the additional stuffing converts the pressure waves to heat. Some sealed box designers use this function to reduce box volume by ten to twenty-five percent."
Also, any solid object such as styrofoam, bricks, 2x4s, expanding foam or thick pillows will reduce internal volume, obviously. You can find this method recommended on many audio boards, for reducing the volume of a box with fixed size.
At this point, adding a wood divider isn't an option, because getting it to line up smoothly and airtight with the top baffle will be impossible because the floor of the hatch is not flat.
Obviously, I'll not be using the brick option...I will probably look for some nice dense stryofoam, or build a box out of MDF and fill it with Great Stuff, then remove the wood once the foam is hardened. And because I am sealing the top of the box with weatherstripping instead of silicone, it will be removable, so I will be able to quickly listen and test the audio output using every method above until the sound is just right.
Airspace/enclosure question
