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/for...sonosub-baseplate-distance-sub.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]

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]

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.

 

[Andersonix]

Does it seem intuitive that for example a 15in paper cone woofer and a 6inch poly cone woofer have the same exact amount of sag just because they have the same Fs? It's not intuitive to me, but I'll go along with it.

 

[Phil Olson]

In order for the 6" woofer to have a low Fs I would think it would need a very weak compliance so it would sag more.

On the other hand, if the 15" has a tight compliance, meaning less sag, it would lead to higher Fs.

I may be wrong but from my physics background it would seem to make sense. If I got this wrong feel free to set me straight.

 

[EWorkshop1708]

IMO, A good sub should not sag much. I like those kind of sub that seem really stiff and hard to move when you push on them, but when you play them, they move a lot and play hard down low.

I'll be building a down-firing 12" sub for home use. I went with down firing because I want to use a sealed box, and want the bass to be somewhat even around the room. The plan was to mount the sub and box on a flat piece of wood with wood or rubber feet suspending the box about 3 inches above it with the sub firing down towards it.

I'm hoping this approach makes the sub not so dependent on position or wall reflection, and plays good bass wherever placed. I already have the box and sub, but it's front firing for now. It works good, but it's picky about where you aim the subwoofer.

 

[Andersonix]

EW: Low frequencies shouldn't depend on the direction the sub is aimed, so maybe your sub is not crossed over low enough. Maybe you are altering the relation between it and a boundary (wall) by turning it around?

Also (if I'm reading you correctly) if the sub sits on its feet on top of a piece of board (as opposed to sitting on its feet directly on the carpet), you allow more high frequencies to reflect out (and may cause the board to vibrate). Plus you don't get the opportunity to use cone or spike feet to couple the sub to the building structure.

In general I prefer my speakers to have a smaller footprint, so I'm not likely to build big downfiring subs. But if I built a sub, I would design it so it would be flexible about the direction (side/downward) it was placed.

I also think casters are a good idea if it's going to get moved around. Otherwise, feet that prevent it from sliding around are better (rubber feet on hard floors, spikes on carpet).

 

[EWorkshop1708]

Well, with a front firing sealed sub, the placement makes it sound different, and it does matter because of standing waves in a room. ALL subwoofers do this. I'm just trying to make placement more flexible. If the down-firing doesn't sound better, then I'd change it back to front firing.

Yes, low frequencies are omnidirectional, but the loudness and depth of the bass is still affected by moving a sub around a room.

 

[marchel]

Yes, low frequencies are omnidirectional, but the loudness and depth of the bass is still affected by moving a sub around a room. [/B][/QUOTE]



I agree, And I think there is a difference in SQ between front and down firing subs. It's about the difference in standing wave in the room and not about the directionality of the sub's driver.

Personally I place the sub's driver as close to the corner as possible, no farther than the diameter of the driver, this setting have the best coupling to my room.

 

[Svante]

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

Yes, Lower fs could also mean larger mass, ie more sag.

And if the suspension is made stiffer, mass has to be added to keep fs. If fs is kept, the sag is the same.

Then again, keep in mind that the long term sag is controlled by other mechanisms.