Large 3-ways design / build

[MJL21193]

It's that time of the year when I get the itch to build more speakers. Even though I have TOO many projects already under way, none have the appeal of a new major speaker project.

These are to be large speakers, room dominator's - rough initial estimates put the internal volume at around 300 litres, approximately 50" tall, 26" deep and 16" wide. They will have two 12" woofers each, a 7" mid (actually a midwoofer) and a 1" silk dome, horn loaded tweeter.
They will be actively driven using a DCX2496 as the crossover.
The drivers:
Custom 12" woofers from Eminence. Bought from Davygrvy on this forum, measured specs HERE.

The mid is the PL18 from Vifa. I have used this driver a few times before and love the way it sounds. I think it is very hard to beat in the midrange.

The tweeter is the Morel cat378. I haven't heard this driver in person but reviews are very good and it would be perfect for this. I wanted a horn loading on the tweeter and originally I planed to shape the front baffle this way. This tweeter seems to have what I need.

[MJL21193]

The box will be vented to get the most out of those woofers. Using Unibox and the measured T/S parameters I have come up with what looks to be a pretty good compromise of response, low end extension and size:

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I have the box volume at 260 litres, leaving some space for the mid enclosure, ports, etc. There will be 2 ports - 12cm diameter, 27cm long. Box is tuned to 27Hz and F3 is ~28Hz.

VB Response EMINENCE 12.gif

It will easily handle 200 watts without running reaching its excursion limit:

VB Excursion EMINENCE 12.gif

So far so good.

[MJL21193]

The mid will be housed in its own sealed enclosure and will have fairly good low end response. It will most likely be driven low pass only through the DCX since it is more than capable down there. Box volume will 15 or 20 litres and deep to reduce the chance of reflection back through the cone.

Now for some construction details. I've been wanting to try out an idea I've had for a while - a different way to build the panels of the box. Basically here's the idea:

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It is a sandwich but with a twist: the inner and outer skins are joined at the edge and bowed apart in the middle. It is my belief that this will make one hell of a stiff panel, especially when it has been filled with pourable urethane foam. The skins of the panels will be 1/4" hardboard and the idea is to use these panels on 5 sides with a traditional solid front baffle. It will be tricky to put together, especially where the sides meet the bottom and top (compound curves to deal with) but I'm up for the challenge.

Here's a view of the box from above looking down:

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And a view of the overall construction with cut-away view:

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I'm not sure yet about what material I'll use for the baffle. I'm considering solid hardwood but this is a risky choice - it would need to be ultra dry to use here to avoid movement.

[MJL21193]

Hi Andrew,
Hardboard is high density fiberboard and probably has high tensile / compressive strength. Don't forget the curvature and how that will stiffen the panel vertically and transfer the load to the edge. Once filled with 4lb foam, it will be hard to match for stiffness.

I did a bit of a mock-up of the side and back panel sections today:

NPX_057.JPG

And a closeup of the vertical corner:

NPX_056.JPG

I used 10 degree fillets of softwood at the vertex of the panel to increase gluing surface. The vertical corners will have solid wood cleats (as seen in the pic above) that run from top to bottom to reinforce this joint.

All in all it should be a whole lot of fun to build and I'll post pics along the way showing as much detail as possible.

EDIT: Oops, I guess Andrew deleted his post.

[crazyhub]

Sorry for my bad english.

Such a big enclosure needs a very stiff and dampened structure; you will never achieve this goal by using tight hardboard panels; bending them will largely not enhance their stiffness enough; and pourable urethane foam will only damp tiny part of the fr bandwidth.
You will get a dancing enclosure.

[MJL21193]

You have no faith my friend.

Curving the panel massively increases strength - the way these panels are designed and joined together leaves only one "weak" point: the vertical centre axis of the tallest panels may flex slightly. To help combat this I will use steel flat bar for the divider shown circled here:

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Up the side panels and the bottom and top panels will have 1/4" x 1 1/2" flat bar and the back will have 1/4" x 1".

[Andrew Eckhardt]

Sorry I deleted my post. I had thought I didn't want to get into this one, but I think you will want to see how little air and how much sand or limestone chip you can wind up casting in-between your panels. Without added mass those panels will become a huge dome midrange.

[MJL21193]

Originally Posted by Andrew Eckhardt

Without added mass those panels will become a huge dome midrange.

I expect we will see.
I see the structure of these panels and how they tie together as a more effective way to restrict movement. A midrange has a surround that allows the cone (or dome) to move in and out. The "surround" for this is actually the strongest point.
A curve (or arch) will transfer any applied load to its edges - it will by this very nature resist movement. The edges are rigidly tied to the adjacent panel at 90 degrees - how can it move by any significant amount?
Adding mass is not where I want to go with this.

[Cal Weldon]

Those will be nice little speakers John.

[MJL21193]

Originally Posted by Cal Weldon

Those will be nice little speakers John.

Tiny compared to some of your gear Cal but big for us common folk...