Sonosub Baseplate distance from sub?

[ramchip007]

I'm building two down firing 24 inch sonosubs with 15 inch subwoofers and my question is what's a good general distance The base plate should be mounted away from the sub? I was guessing around 6 inches, but I don't have much ceiling height, and I don't want to sacrifice any output , thank you

[Mike P.]

3 to 4 inches is common. I did 4 inches on twin Shiva sonotubes.

[SteveCallas]

Yeah, 4" should be fine with a 15".

[ramchip007]

thanks, time to get the brad nailer.

[JDRoberts]

I used 5" on my 24" sono,seems to work good.

[Hakka]

The surface area of the opening between the endcap and baseplate should be at least equal to the surface area of the driver.

Hakka.

[jr1414]

I'm considering building a sonosub now as well, but was wondering if there is more of a modeled solution to this dimension. I'm sure as you get closer to the driver there is more of a tendency for the air between to act as an "air spring" and somewhat dampen the driver.

When you say the surface area of the opening, are you refering to the cylindrical section between the endcap and baseplate? For a 15" driver this distance between the endcap and baseplate would be approximately 3.75". Yet people seem to be more commonly using 5" in this situation.

Anyone else have a more scientific solution to this question?

[Warpdrv]

I was also curious of this calculation...

If I was building my boxes 15" TC2K in a downfiring scenario, with my boxes being 26" W x 36" L, could I actually go with a shorter height... like 3" or even less?

[Warmon]

I asked a similar question in the AVS forum when building my down fired subs. I believe the formula is min opening = [Xmax*2] x SD. This represents the volume of air of the cone fully retracted to fully protracted. If it can't push this volume of air unrestricted, it will act like a bandpass enclosure. You should be safe using 15 for SD and just being sure you've got the min sq inches of volume underneath. Three inches ought to be plenty.

[collo]

I've been meaning to write this up on the sonosub.exe page

The short answer...

gap = nominal diameter / 4 for most drivers or
gap = nominal diameter / 3 for high excursion drivers

The drawing....

The principle....

Air movement generated by the driver shouldn't encounter any reduction in the CSA through which it has to flow

The proof....

The smallest area is represented by the green cylinder, and has a total area of:
gap * circumference = gap * pi * (nominal diameter)

This must equal the Sd of the driver, or in our example, pi * (nominal dia / 2)^2

So ...

gap * pi * (nominal diameter) = pi * (nominal dia / 2)^2

dividing both sides by pi...

gap * (nominal diameter) = (nominal diameter^2) / 4

dividing both sides by nominal diameter...

gap = (nominal diameter) / 4

You could go more accurate by calculating actual diameter from Sd, and factoring in the height of the surround vs driver rebate depth, but this method is going to be fairly close.

If you have a driver with a large Xmax, you would build a bit of leeway in by dividing by 3 instead of 4

This would give you.....

A 10inch driver wants between 2.5 and 3.3 inches gap depending on excursion
A 12inch driver wants between 3.0 and 4.0 inches gap
A 15inch driver wants between 3.8 and 5 inches gap
A 18inch driver wants between 4.5 and 6.0 inches gap

[Mike P.]

Thanks for the detailed information, Collo. This is a big help for future reference.

[jr1414]

Finally, an answer that seems to have some basis in reality. Is there any difference in a sealed configuration as opposed to a ported configuration. Common sense tells me no, but it's worth asking the question.

[Josuah]

I have 15" drivers. The Hitchhiker's Guide to the Galaxy ended up pushing the grilles off my top-facing drivers (I have sealed sonotubes with drivers at both ends). So this past weekend I ended up redoing the legs and replacing the top grille with the legs and base as well. So now it looks like you can flip the sub either way. I went with 4" legs because I couldn't find screws any longer than 6" at Home Depot. Otherwise I was going to go with 5" legs.

[Warpdrv]

Thanks Collo for that detailed bit of information...

And for us mathematically challenged, the picture is also a work of art...

So back to 4 to 4.5" for my 15" high excursion...

[Bent]

would there ever be any reason to run tighter - as in using the baseplate for additional driver loading to raise system Q? or would we be better off to do it electronically?

I ask this, because IIRC correctly, SVS used their (some, maybe not all) baseplates for assisting in loading the driver.

[Warpdrv]

Great question there Bent.... I was curious of that thought as well...
My box design already has a low Q, around .5 or so, so it'll be kinda dry to begin with, but what other effects would that have, I read SVS used a shorter height to increase response to a givin area of response...