[glynnw]

While this is my first posting to this site, I have built several speakers over the last few years and I am a happy member of the Bottlehead group.  The last large speakers I built were a set of BassZillas, which got me interested in open baffles.  Attending VSAC 2008 I heard and loved the Lowther Tiny Tot open baffle speakers, so I used that basic design for these new speakers, substituting some Fostex 206E drivers which I already had for the Lowthers.  I had acquired 2 pair of the Fostex drivers and experimenting with them led me to preferring the sound of 2 of these drivers per side - just sounded warmer and better to me.  Bass driver is the 12" Tone Tubby Alnico.  I also have a couple of 18" Bagend Infrasound powered subs for deep bass.  Initially I had some modified Selenium tweeters, but they really weren't necessary with the Fostex drivers. While listening one day I was considering that many designs use rear firing tweeters, so I simply reversed the bottom pair of Fostex drivers, wiring them so that the diaphragms of both still moved in unison.  In my room, sitting about 9' from the speakers I prefer the drivers like this.  When both face forward they give a much larger sound and would be best if I were in a very large room sitting 20' or so from them.  The picture is oif them mounted in 3/4" MDF, my initial build.  Now I am planning to go with a couple of 3/4" pieces glued together to get a stiffer mount.  So here is my initial question.  Do you guys think I will be better off sticking with MDF or should I go with plywood?  I am also thinking of decoupling the top driver, as in the Linkwitz Orion, but I am not sure if it would really make a difference since there will still be 2 drivers exciting the baffle.  I'll probably try it just to see, since it an easy change to make.  Any input would be appreciated.  I will try to attach a picture, but I am not sure I understand the directions.

[glynnw]

[matevana]

Can you explain the significant difference in sound you hear by reversing the 2nd fostex driver, when both cones are moving in unison, vs. both drivers facing forward? it doesn't seem like there should be a large difference.

With regard to baffle materials, you may find that the same thickness plywood will require additional bracing to firm it up when compared to MDF. Possible advantage of plywood may be easier to finish. It will also be lighter whcoih can be either good or no good depending on baffle movement issues.     

[glynnw]

First let me be clear that my observations are purely subjective - none of that nasty science stuff for me.
When one of these drivers is reversed, the magnet structure is so large in relationship to the cone that you see none of the cone when seated.  So there is very little direct sound from the reversed driver.  By the same token, when both face forward, there is not nearly as much sound directed behind the speaker.  So when one driver is reversed you receive less directly radiated sound and a great deal more reflected sound.  Also, each driver has a whizzer cone which acts very effectively as a tweeter.  When facing forward these "tweeters" are a foot apart, which might affect precision of imaging.  When one driver is reversed, you only receive a direct signal from one tweeter and you get a lot more relected treble sound.  At least this is how my train of thought goes.  What I hear is two different sound pictures.  Both drivers facing the listener gives a larger, less precise, more diffuse soundfield, which seems for no good reason to be a bit higher in the room and more forward.  When one driver is reversed I get more a sense of the body of a person being where the voice emanates, and the point of origin of the sound seems to be a bit behind the plane of the speakers and a bit lower in the room.  In truth this is a soundfield I personally prefer and  I normally tweak my speaker positions to get this effect.  Liking this effect is probably a matter of personal preference. Hope that helps.

[SteveRB]

I've done a few different open baffles over the past couple years. I really think MDF vs Ply Wood is splitting hairs. Although I have no way of measuring this assumption...

The biggest baffle concerns for me are stiffness, mass, a speaker bracing. I've had good luck with 1" MDF.

[glynnw]

Thanks for the input.  I think I wil go with 2 pc of 3/4" MDF glued together with a nice veneer on the front.

[JohnR]

Very interesting concept 

[scorpion]

Although I have had some views with Rick Schultz here, I think he has a main point for baffle construction by glueing two sheets together with damping glue, like the Green Glue. This will silence the baffle much more than just glue sheets together for a given thickness. I think MDf is perhaps not even so good as Chipboard made up like this. Birchply would definitely be better. And please like you suggest decouple the fullranger. 

/Erling

[glynnw]

Thanks for the info.  This is exactly the kind of guidance I am looking for.

[planet10]

I've done a few different open baffles over the past couple years. I really think MDF vs Ply Wood is splitting hairs. Although I have no way of measuring this assumption...

The biggest baffle concerns for me are stiffness, mass, a speaker bracing. I've had good luck with 1" MDF.

My opinion is that MDF has little role to play in building really good speakers.

You value stiffness? 18mm BB is about the same stiffness as 1 1/4" MDF (and doesn't have near the same level of energy storage issues)

If you want really stiff, treat yourself to some stranded bamboo plywood.

dave

[matevana]

Can you comment on the AlNiCo Tone Tubby's? I played my Strat through them and they sounded amazing. Really had that brown sound.  Very little xMax so I'm wondering how they perform as a bass driver.

[glynnw]

I am not sure if I can provide a good answer about the Tone-Tubby drivers.  They are really just a small part of the sound and a part I have ignored in the couple of months I have had these speakers running.  A couple of days ago I was beginning to notice just how pretty the upper bass sounded and I wondered if the drivers were just getting broken in. These are supposed to go down to about 70HZ while I am simultaneously running a pair of 18" self-powered Bag End subs, each running it's own channel of Dspeaker DSP, and I tend to have this turned up a bit so much of the bass from the open-baffle is just lost in the mix.  If I notice anything special about them in the future, I'll try to post it here.