Centre speaker design

[paulspencer]

Here is a design for a centre speaker. It is a 3 way using Vifa XT tweeter vertically mounted above the Vifa P13 open back for the midrange and 6.5" Peerless CSX on either side for the bass.

My mains speakers have the P17 and Vifa D25AG metal dome in MTM arrangement and they are TLs.

Rendered image:

Crossover idea:

3rd order electrical filter to the tweeter at 3kHz or 3.5 kHz
P13 crosses to the woofers at say 300 Hz
My idea is to give one of the woofers a 1st order slope only so it acts as BSC, so the xo is in effect more like 3.5 way

The open back should easily have large enough effective "wings" so that dipole eq won't be needed. The woofers will each have their own vented enclosure as big as I can get it, with the depth of the box being 400mm

Ideally with this design I'd like to make it bi-amped and only have passive xo to the tweeter and mid, but with my 3.5 way idea this might not work! Ideas?

Questions:
* Is the open back an advantage or disadvantage if the speaker is placed close to the rear wall (say 100mm clearance behind for the rear vents) ...
* Is there a better way to deal with box coloration for the midrange?

Vented design to get bass down to 40 Hz if possible although due to size it might only be 50 Hz. The idea is that the centre speaker should be able to handle midbass well and not rely on the LFE channel. I think this may be better than going sealed and rolling off below 100 Hz or so ...

Alternative: use 3 x P17 to match my mains in looks, slightly larger. I thought the P13 is easy to use and should match the mains, while I can use the CSX for the woofers where timbre matching is not really critical. I like the look and I think they have a slightly better motor for low distortion.

Any other comments, feedback, suggestions welcome.

[johninCR]

Beautiful design. I have just a couple of ideas.

First, the resulting open chamber behind your midrange is going to result in some serious resonance coloration, probably even with a lot of polyfill stuffing. Since you're only going down to 300hz with it, very little baffle width is required. A better approach (at least sonically and visually you seem quite creative) would be a common front baffle with boxes on each side for the woofers, and maybe a few inches of "wings" for the mid unit in case you need some polyfill in back of the driver due to close wall proximatey. This would also give your dipole portion more breathing room.

Second, I personally find no need for anything below 100hz in a center channel. I'd say seal those woofers or go dipole with them as well. Blending vented bass with dipole mids will prove difficult and you don't want a shallow rolloff of your woofers going and "mucking up" your dipole midrange.

Good luck with it. You may even want to get the drivers and plywood and do some experimenting first before deciding on a final design.

[paulspencer]

Another thought I've had is perhaps I should use 3 of the Vifa P17 to match visually and avoid overexcursion problems with the open back. I'm not sure if this would be a problem or not. I can live with vented bass for the centre channel. If I seal the woofers then that gives me very little extension and less of a reason to use 3 way. I'd be using 2 woofers for 100 - 300 Hz, hardly worth it. Suppose then I do an MTM on its side dipole down to 100 Hz. Then I have the sideways MTM orientation that I was trying to avoid with this design.

The 1/4 wave resonance of the open back chamber would be at around 200 Hz where the woovers have taken over. I would think that the stuffing would take care of this if it were a problem. I'm more concerned about what happens to the sound that gets through the open back and re-radiates to the room. I wonder if this is any better than just having a stuffed sealed enclosure. Or perhaps a terminated line (inverted horn) as used by Norh and B&W is a better approach.

Another concern is that the sound may not match the mains that well. I believe my TL's have good midrange due to the TL enclosure. It has a lot of stuffing to kill the back wave and when you put your ear in the "vent" at the end, there is very little midrange that gets through.

Point taken - good idea to experiment. I'm very much into making a quick test box and trying things.

I'm not sure about the wings. If I make them shorter then I end up with dipole radiation which it not what I want. Also being shorter the resonance will shift upwards into the range the mid is covering, unless they are so short that it shifts above the range covered. Also with placement it will be difficult not to have large wings. The Tv will act as a wing on one side if placed on top.

[johninCR]

Paul,
It sounds like you've go things well under control. You'd have to test that long narrow stuffed chamber vs a very small baffle with a little stuffing in the back to see if you can get the dipole quality of mids with very little rear radiation. I think it would become a short TL playing only frequencies out of the tuning range. Please let us know how it works.