newbie questions on OB design for MTM speakers

cporada · « **on:** 3 Aug 2007, 06:05 pm »

Hi everyone
For several years I have been enjoying my ACI alpha limited DIY project (back from the days when ACI sold only DIY kits. This kit employs a tweeter and dual AC5 midranges plus dual AC10 woofers per channel). My self-built cabinets are starting to show signs of aging (and signs of my newness to woodworking at the time), so I'm getting ready to build some new cabinets. What I'm wondering is if it would be possible to build an OB for my mid-tweeter-mid "enclosure" and keep a sealed box for the 2 woofers (AC10's). The drivers employed in the MTM are : Vifa H26TG-35-06 tweeter, and AC5 drivers for the mids with parameters:
Fs 69 Hz
Qms 1.784
Qes .722
Qts .514
Vas 7 liters
I think in my current configuration, the mid to woofer crossover occurs at somewhere around 125Hz or even higher, so the mids are not really being called upon to produce much bass. Does anyone know whether it would be possible/advisable to try using an OB design for the MTM portion of these speakers, and if so, can they give me some idea as to what dimensions I should try as a baffle? Should I use more or less the same size/shape baffle as the drivers had in their closed box configuration?
Thanks so much
Chris

Vapor Audio · « **Reply #1 on:** 3 Aug 2007, 06:33 pm »

Those driver specs look good for OB use. As far as baffle width, I honeslty wouldn't over-analyze it. If you're front mounting the 10" drivers, the front baffle should be plenty wide to bring bass response from the MTM down to around 200hz.

You are planning on redesigning the crossover as well aren't you? You have to voice an OB speaker differently than a closed box system, there's just more midrange energy put into the room - and taking nearfield measurements won't give you an accurate representation of that.

Unless you need the sensitivity of that horn loaded Vifa tweet, I'd dump it and go for something that can cross lower. A Seas TDFC would be a good inexpensive choice, as would the Vifa XT25TG-30.

cporada · « **Reply #2 on:** 3 Aug 2007, 06:37 pm »

Thanks Turbo
It seems now I'm getting in way over my head. How does one re-work the crossover to voice the drivers for OB design? The crossover I'm using it pretty much exactly what was designed by ACI (other than some major tweaking on cap's and resistors), and all I know is that it's a second order crossover. I wouldn;t even begin to know how to start modding that to use the drivers in OB. Any suggestions would be greatly appreciated.
Thanks
Chris

Vapor Audio · « **Reply #3 on:** 3 Aug 2007, 06:55 pm »

Quote from: cporada on 3 Aug 2007, 06:37 pm

Thanks Turbo
It seems now I'm getting in way over my head. How does one re-work the crossover to voice the drivers for OB design? The crossover I'm using it pretty much exactly what was designed by ACI (other than some major tweaking on cap's and resistors), and all I know is that it's a second order crossover. I wouldn;t even begin to know how to start modding that to use the drivers in OB. Any suggestions would be greatly appreciated.
Thanks
Chris

How to voice them? Easy, by ear

Honestly, you could try just building an OB enclosure for them and use the original crossover ... it might work, who knows. Of course, it could easily be made better from that point. If you were close to St Louis I'd be happy to help you design a new xover.

cporada · « **Reply #4 on:** 3 Aug 2007, 08:58 pm »

Hi Turbo
Thanks for the offer and the advice. St Louis from Reno is kind of a long drive though...
Would you be able to make some suggestions if I e-mailed you a schematic for the crossover, or does it have to be done just by listening?
Chris

markC · « **Reply #5 on:** 3 Aug 2007, 09:10 pm »

I'd start by trying the existing x-overs and listen for any problems. If you do identify problems such as a hole in the x-over point or too much energy @ a certain frequency, I'm sure folks here will help out.

cporada · « **Reply #6 on:** 3 Aug 2007, 10:08 pm »

Thanks for the helpful responses. So, if my existing closed box baffle is 13" wide and the MTM are offset to the inside of the baffle placing them towards the center of the space between the speakers, should I start with that as my baffle, or should the MTM be centered on the baffle? Does the width of the baffle affect imaging like with closed box speakers, and if so, would it be better to build narrow front baffles that have hinged sides that can be bent back to achieve the required width yet maintain a narrow front baffle for imaging?
Thanks
Chris

Vapor Audio · « **Reply #7 on:** 4 Aug 2007, 01:03 am »

There are tons of ways you could go with this, it all depends of your level of skill and/or desire to learn.

To design a from scratch crossover, one needs a phase/impedance graph of the drivers, and frequency response graphs preferably taken with the same power input and distance with all the drivers mounted on the final baffle. That will get you close, then the final trick ... the art, is the voicing by ear.

If you want to try the original crossover you can. Chances are it's going to sound quite thick though using it, and there's a simple reason - Open Baffle doesn't require baffle step compensation, and the original crossover probably has some built in.

The width of the baffle and driver layout on it primarily determines the diffraction effects, as does the amount of roundover or chamfer you put on the front baffle. Again, these these things if changed will affect final crossover values.

But even with all those things considered, if you just want to experiment, go ahead and build some new cabinets and slap it all in there. Build in a way to easily access the crossovers for later changes, either an external box or an access panel. Give it at least a 1.5" thick front baffle for strength in the open part, and who knows, it might sound great?