Front vs down firing subs

[Phil Olson]

Any thougts on what's best? I thought I read somewhere that downfiring couples better for louder output but don't know if it sounds better.

I'm currently building two sealed SDX-15 cubes per their suggested .577 layout, (25x25x25).

If downfiring is best, how close can it be. Again, I read that 4" was optimal but is that enough space to move the air? I would think so. It would be open on three sides.

Finally, if downfiring, would their be any problem with putting them on casters? They will weigh a ton so the ability to roll them around to get the best response would be very adventageous.

Thanks,
Phil

[Saturnus]

Downfiring subs should generally be tuned with a slightly lower Qt than front firing ones. I'd suggest 0.5 to start with especially if you intend it to be relatively near a side wall.

Clearance should be enough that it is above SD of the driver but not more than twice SD or you'll loose much of the room coupling effect.

I personally don't think there's much difference between casters and proper spikes, and certainly not enough to outweigh the spouse friendly effect of being able to roll them out of the way when not used or vacuuming.

[pkm]

why would there be more of a coupling effect?the wavelengths involved are 3-30metres long.

What you will change are floor vibrations,and room modes,by changing the subwoofer position.

Also you might lose 1% of excursion to sag.

Yes you dont want it too closed up - or you will be making an acoustic low pass filter.You already have an electrical one.

[Phil Olson]

Saturnus,

I assume you mean that the opening should be the same area as SD which is 48 sq in, or double that for max benefit.

PKM,

I don't recall specifics but the coupling had something to do with Pi space or what have you.

It's so sad, it seems like anything I read nowadays tends to have a half life or a day or two.

[Mikey p]

For down firing height see post #10 of this thread:

http://www.hometheatershack.com/foru...html#post65272

[Phil Olson]

Mikey,

Thanks, That was just what I was looking for!

From that, considering I have a 15" long throw driver, it looks more like I'll need 5" clearance.

I've cut all the panels and routed out the speaker holes so I now only have to route out the bracing to let it breath then start assembly.

[Andersonix]

Originally posted by pkm
Also you might lose 1% of excursion to sag.

Is that what the calculation gives for this woofer?

Sag is a real concern, and there is a formula for determining if a woofer is suitable (although I've forgotten where it is).

[Svante]

Originally posted by Tosh

Is that what the calculation gives for this woofer?

Sag is a real concern, and there is a formula for determining if a woofer is suitable (although I've forgotten where it is).

Easy...

x = F*Cms = g* Mms*Cms = g / (2*pi*fs)²

g=9,81m/s²

So for a drver with fs=20 Hz the sag becomes

x= 9,81/(2*pi*20)²=0.62 mm

...then again, there are other mechanisms than Cms that get involved when a driver is subjected to a constant force; the sag becomes larger. How much larger depends on the material of the driver suspension. There are no formulas for this that are based on T/S parameters.

[Andersonix]

Hej Svante,

That's it. The first time I saw this in the Adire Audio paper, I hadn't made the connection that sag is entirely dependent on Fs. This seems too simple. But who am I to argue?

I wonder if it would be worthwhile to introduce some DC offset to get back the full excursion...

[Phil Olson]

I think that makes intuitive sense. Lower Fs would mean more compliance which would mean more sag.