how big does a sealed sub really have to be to be IB

[indygreg]

if i do the math right according to the formulas a 15" soundsplinter only needs about 5 cubic feet behind it to qualify as an IB (10x the Vas). is that correct?

could i do something like this behind my screen with a pair of 15"?


i don't want to dump the backwave into the rest of the house but i have some room backstage. i thought that perhaps i could get some of the mechanical benefit from opposing drivers.

greg

[Jack Gilvey]

What Vas are you using?

[indygreg]

their website says 163L.

greg

[cjd]

Well, 10x is the point at which there are diminishing returns - you can model it easily enough. Here's the thing. The larger you go, the closer you get. Life is full of compromises, and this is one you can probably make and be quite happy, you know?

This is one of the reasons people find they can stuff ports and they get great sound quality - they're generally using a "large box sealed" system at that point.

C

[ThomasW]

If one runs the numbers, it actually takes a space 25 times Vas for Qts =Qtc. That's not really practical for anyone bulding a box.

So for those building structural based IB subs, we recommended the 10 times Vas Chris noted.

When box size is = the driver's Vas, the Qts is increased by a factor of 1.4. So the standard recommendation for those having to build a 'box' for their IB is go no smaller than 4 times Vas, and bigger is of course better.

[Jack Gilvey]

their website says 163L.

greg

And 10x that is 1630L, or ~58ft^3. Did you mean 50 in your first post?

[cjd]

Heh... I think someone made a floating point error. Musta been using an old P60 computer maybe.

Though a 4x8x2 foot box would be over 60cu/ft so not *too* unreasonable to build, realistically.

C

[ThomasW]

As big and heavy as this box will be just aim the drivers forward.

[cjd]

As big and heavy as this box will be just aim the drivers forward.

Where are the cats to sit if you do that?!

[indygreg]

oops, i guess i didn't move the decimal correctly. i thought that seemed small.

so i can build this in place and never move it. compared to the stage it is not all that big and heavy

so i need a 4 x 8 x 1.5 for each driver. if i point the driver forward. something i have never understood is the issue with cabinet depth vs volume. obviously a cabinet 1/3 as deep and 3 times as tall is not the same but i don't really understand how that works. if i build something with this odd a shape is it ok to put the drivers in front with only 18" behind them?

g

[Jack Gilvey]

Where are the cats to sit if you do that?!

I've found paw prints in my manifold. The only thing saving him is his timing (or my lack thereof), I suppose.

[cjd]

Dunno, one of my cats likes to sit in there during movies and stuff. Our theory is that the vibrations are a comfort, much like a purr is for them.

C

[indygreg]

ok i think i have a plan. after searching and reading on the forum i found adam hennings double tc2000 ported sub and decided to go with a proven design. i need it to fit behind my screen so here is what i want to do:

- modify it to be 97 x 37 5/8 x 18 with the same volume (800L before bracing).
- run it on its side so that it is 37 5/8 tall and 18 deep so that i can put it behind my screen.
- i would really like to just do a square port so i don't have to hunt 10" round material. 10" round = 78.54 sq inches = 8 7/8" square. i can still flare the end with a router. can i do that?
- drive it with a crown K2 amplifier.

will this plan work?

g

[cjd]

So, why not the sealed setup again? It's somehow unproven?

Sure, it will work, assuming those numbers are accurate and stuff. You can do a full-width slot port (may take some bracing at 18" wide and technically be 3 slot ports or something) perhaps more easily than a square...

C

[indygreg]

i am probably just not experienced with sub building enough and i have never built a big sealed sub. i thought that the curve would flatten out with a lot of volume but in unibox it looked like this.



is it normal for it to roll off that much? no matter how big i modeled the cabinet it did this. i thought that an IB would extend a bit flatter.

greg

[Dennis H]

That looks just right. Room gain will boost the low end so it's flat.

[indygreg]

so what does a big sealed box sub sound like compared to a ported sub? is it tighter and the ported sub is more boomy? i hear a lot of people saying they would do sealed if they had the space or money for drivers so it must be a characteristic sound that some people prefer.

greg