Down Fire Clearance?

[jct404 10+ year member]

I'm looking at building a single 12 down fire box for gmc crew cab. My question is how much room should I allow between the floor add the sub?

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[Slammer 10+ year member]

Most importantly, leave adequate room for excursion of the woofer, so that you are NEVER having to worry about the surround coming in contact with the floor. If the sub that you are using has a peak to peak excursion of 1.5", allow for the height of the surround (above the mounting surface), and then add .75" for movement. I would do extra to be on the safe side.

Secondly, how much room "can" you allow? I ran down-firing 12s in my Sierra in a prefab box, and there was an opening in the front and on the sides. This was only about an inch off the carpet. This was with an internal mounting depth maximum of about 6", if I'm not mistaken. I found that I got significantly more volume and "effect" from the base, by propping the box up slightly. I got a couple of round plugs from Lowes, which were about an inch tall, put one under each corner, to hold the box up. This worked nicely until I pulled it and gave up on the whole under seat box idea.

With all of that said, I would leave as much open room as possible, without killing all of your mounting depth. If you move the bottom board up, creating more room, your mounting depth will be reduced by the same amount. Know the subs you are going to use first, then build the box for the subs.

Hope this helps.

 

[subzero 5,000+ posts]

Deflection is the key. you may have to just build the box. then experiment with diferent deflection distances and see what sounds best to you. Once you find it, measure it and finish your box. I have seen boxes with deflection shields made of plexy glass . dont remember the distances..

 

[audioholic 5,000+ posts]

Distance from the floor (or reflecting surface) depends on the amount of area the sound waves will have to disperse. The main thing is to avoid a situation where the area becomes a quasi-port and alters tuning. So this means height from the reflecting surface is not the only factor.

 

[subzero 5,000+ posts]

yea, but he didnt say if or where the port was.

 

[audioholic 5,000+ posts]

yea, but he didnt say if or where the port was.

He didnt say the enclosure was ported at all actually. Nor does that effect my response beyond avoiding a ported effect in a sealed system, or a quasi-bandpass (6th order) if the enclosure is ported. You simply dont want the area the sound disperses out through, between the enclosure and radiating surface, to restrict airflow, as that would create a porting effect and alter tuning.

If the setup is down-firing, and ported... the port AND sub should fire down.

Down-fire can be a bit tricky because you want the reflecting surface to be close enough to create a front-loading situation, but far enough away (again dependent on dispersion area) to avoid an unwanted porting effect by restricting airflow.

Sorry if my explanation is not clear enough, Ive been drinking. //content.invisioncic.com/y282845/emoticons/tongue.gif.6130eb82179565f6db8d26d6001dcd24.gif

 

[subzero 5,000+ posts]

I C. I been at work. bored out my mind so I been killing time on the forum.

 

[jct404 10+ year member]

I stared on the box last nite I didn't get very far, but I sure knocked the s#!t out of a 12 pack. Anyway the box is going to be sealed, the sub is in my sig. I don't think the P2's extrude very much. I was thinking around an 1" to an 1-1/2" off the floor. I don't think it is possible to do a ported box in the truck with out removing some steel and a seat, second I would know how. The reason I'm doing this is I traded truck for an 07 classic, my 03 had the seat relocated and the duct work removed behind the seat which would allow for the two 12's and the box. I don't want to do this to the 07...three kids and a wife.....

 

[audioholic 5,000+ posts]

If all else fails, use temporary blocks to experiment with enclosure height until you get the desired result, and then reinstall the seat to accommodate.

 

[jct404 10+ year member]

yeah I ready that on the previuos post thanks for the idea.