# Down firing sub clearance



## McKinneyMike (Jul 24, 2014)

I want to put a small sub into my truck and I want it to down fire. What is the minimum clearance that I should allow between bottom of sub box and floor? Is 2" enough?


----------



## SHAGGS (Apr 24, 2011)

Some say 1/2 the diameter of the sub, but I've seen as little as an inch, with no noticeable affect. 
In fact, most of JL's Stealthbox's are between 1 and 1.5".
I've done a couple, and usually shoot for 2-3, unless it's really tight.
Just make sure there's enough room for full suspension travel and then some. 
Depending on where the box is located, steel mesh is a good way to block off the opening, to prevent anything from rolling under it and damaging the sub.


----------



## diy.phil (May 23, 2011)

I think I saw this similar question the other day. Somebody here pointed out some home subs are like 2" off the floor. 
And the we look under there and the sub sticks out half an inch or slightly more... so... the 1.5" or less should be ok depending on where we measure from.

(edit: my sub at home is on a shelf 65 inch off the floor (steel bar reinforced shelf lol, and add another 2 inch)... another sub is 77 inch off the floor in the wall measured at the center. crazy custom install and then I don't use them ha ha)


----------



## McKinneyMike (Jul 24, 2014)

Thank you both for your input.


----------



## osamu (Jan 24, 2006)

mine have like an inch and sound alright, but they are really low powered subs (~200w) with not much suspension travel at all.


----------



## LBaudio (Jan 9, 2009)

I left 2-3" from sub to the floor, if It is ported box with port positioned on the same side as driver, then I left clearence the same as port crossection or slightly bigger. Sometimes I slant the lower side with driver installed so pressure created by driver between sub and car's floor can escape quicker....


----------



## macro (Jun 30, 2014)

I remember reading somewhere that you don't want the driver to be compressing air into the floor. I think it's because once it starts compressing air then that will affect your box tuning (more force would be needed to move the cone the same distance). So it depends on the size and excursion of your sub. If you imagine a cylinder around the edge of your sub form the driver to the floor, you want the surface area of that cylinder excluding the top and bottom circles to be larger than the effective piston area of your sub. So if you want to get super technical, you can do the math.

For example, for an Ultimo 12 the Sd is about 70 in2. I'm going to guess that the diameter of the sub measured from peak to peak of the surround would be about 10.5". A cylinder with a surface area of 70 in2 (excluding the top and bottom) and a diameter of 10.5" would be about 2.12" tall. So based on that, the Ultimo 12 should be mounted with at least 2.12" ground clearance measured from the lowest point on the sub at max outward excursion. That would probably be around 3 inches between the baffle and the floor if you surface mount the sub.

It gets more complicated when you add a second sub or place the box against a wall or a seat that blocks air travel on one side. I think the same principal applies though; bottlenecking air travel will affect your box tuning. In theory I think the box would sound a little smaller, since more force is needed to move the cone. But that's not always a bad thing. Just something to think about.


----------



## LaserSVT (Feb 1, 2009)

Downfire. I have only the feet as the space between the sub and floor. Maybe 1.5"? I flipped the box around, threw it on the seat and even tried rear fire and oddly enough the downfire produced the most pleasing sound.


----------

