Budget Subwoofer Project
Well, this thread/project is a spin-off from my guitar/bass-guitar cabinet project in "Full Range" section.
As some may know,
when rummaging around in my garage,
I found a car subwoofer box left by a previous tenant.
Initially I pulled the 12" drivers,
and they were kinda beat up, surround had come off,
and looks like the dust-caps were deliberately pushed in.
The surrounds had been repaired with a glue-gun unsuccessfully,
so I scraped all that off and re-glued with a kind of 'super-glue'.
I picked up new gaskets at the Speaker Shop,
and I was on my way to restoring two 12" subwoofers!
Here was my post from there:
although I don't have any budget for full re-coning.
Now comes Part 2:
PW 1258-US (12" 8 ohm version)
I repaired the speakers (I'll post some pics in a sec).
I started by sawing the original cabinet (it had a divider) in half,
and making a subwoofer cab out of it.
This needed serious reinforcing, and stuffing,
but it seemed to work, although after plugging the port,
the cab is in fact too small for a 12" I think.
(I'll calculate the cubic feet in that cab in a moment).
Anyway, I wasn't satisfied with either the look or size/sound.
Now comes the fun part.
I'm driving along looking for garbage as usual,
and low and behold,
someone has thrown out an end-table.
This is a modern type, with Ikea-type locking bolts and glued parts,
and pegholes for shelves.
But most important of all, to make the wood look big and thick,
they've of course cheated: Its hollow!.
Its constructed the way they make modern doors, and airplane wings:
With secret struts and cross-braces to give very thin plywood the same stiffness as solid board.
...Actually BETTER. For sound, and other purposes,
these panels are actually stiffer than plywood twice the weight and thickness, i.e., 3/8" or 1/2" ply!
But they are half as heavy too! because they are mostly air!
If you think this wouldn't make a good speaker-cabinet wall,
think again, because modern speaker-makers are using the same techniques!
Even for bass cabinets!
So, Since I can't re-size or refinish the thing if I were to break it apart,
I resorted to measuring the inside volume: 2.5 cubic feet!
Just about perfect for one 12" woof or two 10" woofers!
But it had no back or front, so I had to cobble my own.
While doing that, I used the same reinforcement strategy as in my own modifed Marshall Cabinet:
and to give something to screw to through the back plate.
I only used 5/8" ply for the front, but 1" ply for the back, because hey,
the front is mostly speaker anyway, and all the edges are going to sit on an inner rim/shelf,
which I will screw the front-plate down to.
Not bad for a half-day's monkeying around!
I'll also stuff it and maybe dampen the side panels,
and add a front screen for looks and protection.
How does it sound?
Have you tried playing your bass through it?
FWIW your bracing scheme is never used in speakers I've seen because
it has no effect on the top / bottom / side panels. Rotated 45 degrees
its far better, adding offsets to avoid coincident panels even better.
It will be a subwoofer for my near-field Focals.
For info, this cab has the following internal / external dimensions:
Internal: 17.5" wide x 17.75" tall x 14.25" (effective) deep. = 2.5 cu ft.
back panel - 1" ply
front panel - 5/8" ply
external: 20.25" wide x 20.5" tall x 15.74" deep.
Waiting for glue to dry.
Also, here's an amazingly convenient thing.
Whereas with solid walls you have to brace from the inside (for looks),
taking up precious cabinet volume,
here you can drill into the walls from the inside,
and fill the cavities with foam insulation.
The very effectively stiffens all the sides even further,
without adding any significant weight to the enclosure,
OR decreasing the effective cabinet volume at all!
With commercially manufactured offerings,
it comes down to cost.
Since sides have to be braced,
its cheaper for cabinet makers to brace using the St.George Cross,
rather than use the St.Andrews X, and still have to brace sides, top and bottom.
But DIYers are not limited by cost in the same way,
because they are only doing one-offs, and only spending the money/time/effort ONCE.
Cheaper does not equal Better.
With these new-style fabricated walls,
you can stiffen sides very effectively even more than they are,
without struts, and without losing any cabinet volume.
Ergo, The St.Andrews X is now the best option.
keep in mind that the one thing all rectangular cabs face
is Standing Waves.
Triangular compartments kill them dead.
There are no standing waves in a subwoofer box (unless it's huge, think refrigerator sized). Even with the internal bracing you still have four panels that have the same open panel dimension, meaning they will all have the same resonance. Stiffness isn't everything.
Not the one about no standing waves.
I always run a relatively small hardwood diagonal brace across rectangular walls inside a cab;
this brace doesn't have to be much in size or volume to kill a rectangular plate vibration.
In my living-room scenario, the bottom plate will be strongly muted by the floor,
and the top weighed down by the mids/tweeters.
Don't forget that all 2-dimensional harmonic vibration patterns are non-integrals, that is, not whole-number frequency multiples,
and so like a drum head quickly die.
The key is to mute the fundamental and the 1st few harmonics
of the standing waves themselves,
which are more like a 1-dimensional air-column type of vibration, with musical harmonics.
Finally, the X-cross shape is the whole point.
It kills all standing waves between parallel sides.
The only issue left is the front/back standing wave.
This is broken up effectively by the central core-block (whatever orientation),
and of course the stuffing.
Again you can't have it both ways.
The wavelengths on the panels (vibrating plates) are
as small as the distances between parallel panels.
So the frequency-range of both is about the same here!
If the panels can vibrate, then so can standing waves in the chamber, and vise versa.
You seem to be claiming that the panels can vibrate at frequencies lower than the cavity-waves.
That is impossible.
That might be true for heavy steel plates or thick dense but springy hardwood,
but these panels have no weight, and so their resonances will be quite high relatively.
You've done it so it must be better. It isn't, and quite frankly i don't
care for your nonsense arguments. Triangular compartments or
rooms don't kill resonances at all. Certainly in your case front to
back, and by simply adding a mirror as the side wall all your
compartments are effectively square and will resonate badly.
I can't remember offhand what the transform is, but anything
resembling a repeating tiling pattern will have resonances.
If that box is 34cm deep the front to back primary standing wave would be around 1000Hz. That's a pretty high XO you're running. Even if you consider the first quarter wave it's still at 250Hz...
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