Shape of subwoofer - important? (1 Viewer)

[Brian Bunge]

Greg,
What do you think of North Creek Music's theory behind cabinet design? In case you aren't familiar they recommend 3/4" MDF with 3/4" Baltic Birch braces spaced at unequal distances from the baffle. Then they recommend a double thickness baffle and rear panel consisting of the Baltic Birch and MDF. I think they also recommend 1" dowels connecting the braces to each other and the front baffle and the rear panel.
Also, as an FYI, a recent article in Speaker Builder Magazine showed the affects of different shapes of enclosures for midrange drivers. The tests concluded that a perfect cube wasn't nearly as bad as everyone states and that the difference between the cube and a golden ratio enclosure were very minimal. Of course the sphere and egg shaped enclosures worked the best. It was determined that enclosure size and amount of stuffing had much more of an impact than the shape did.
Brian
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[Greg Monfort]

Hey Brian, what can I say, there's more than one way to 'skin a cat', as the old saying goes.

I've built cabs this way, and I found it a huge waste of time/$$ Vs braced 3/4" no-void plywood. Like them, I also recommend tying the bracing together so that all six sides are coupled together, regardless of the wall material/thickness used.
I've had some folks (one today on another list as a matter of fact) claim a double layer of 3/4" no-void ply, with no bracing, is superior sonically. Having tried it long ago, and not being able to tell any difference, I stuck with a single sheet/bracing since it's easier/cheaper for me.
WRT the cube test, I can't comment on mids, but in testing larger cabs/midbass that was more my norm, there's a BIG difference. I assume it has to do with the relative energy being coupled. To get a better idea, get in an empty room that is ~cubical, clap your hands, and listen to the crack-boom they make. Then do it in a ~acoustic ratio room, and all you hear is the crack (and harmonic echo, if it's very reverberant).
GM
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Loud is beautiful, if it's clean

 

[Brian Bunge]

Greg,
Do you also prefer no-void ply for midrange enclosures like bookshelf speakers, etc. as well or are we strictly talking subwoofers here? I'm just wondering since most of your responses have been concerning questions about sub building.
As always, thanks for your input!
Brian

 

[Jack Gilvey]

Brian,
Re-reading Greg's posts, I think his preference for no-void ply is based on the fact the its resonance is much higher than that of MDF, and is unlikely to be excited by the frequencies encountered in a subwoofer application. The very virtue that makes it good for subs might make it less suitable for midrange cabinets, where its higher resonance would be more often exposed.
I hate to paraphrase Greg, I usually get it wrong.
Jack
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[Greg Monfort]

Right on, JG!
The higher you push the cab's Fs, whether no-void ply, composites, or metal, the higher the useable BW can be used without worry. Actually, since the higher the cab's Fs is, the lower in amplitude it is, so when excited it's easily damped with fiberglass, self-stick vinyl floor tile, etc., so you can build fullrange speakers with it. The bracing/damping construction that I've discussed has been used in fullrange speakers, midbass/mid horns to mine (any many others) satisfaction, though at least one pro horn designer claims that double 3/4" plus bracing is the best tradeoff of cost/efficiency based on his measurements. For getting max efficiency at high power, he's probably right, though for home HIFI/HT, it's overkill IMO.
FYI, since rigidity goes up at the cube of thickness, MDF needs to be >1.13" thick to match the no-void ply's stiffness, hence my remark that it will need to be larger and heavier. So if Kevin had suggested laminating some combination of MDF thicknesses that summed to >1.13", making it stiffer than no-void ply, his assertion about the MDF cab Vs no-void ply cab would have been correct.
According to Tom Danley, probably the best cost effective construction is constrained layer damping (CLD) using 3/4" Baltic Birch with aluminum sheet bonded to it with a non-hardening flexible glue. While I've used several different versions of CLD to my satisfaction, I haven't tried this one, though it makes sense to me.
GM
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Loud is beautiful, if it's clean