# ADHT: OB Center (theater) build



## Anthony (Oct 5, 2006)

Well, with a lot of testing, experimenting, hair pulling, more testing, and crossover design, I finally came up with the design for the new OB HT.

I came to a few conclusions during my testing:

1: OB's do not like a horizontal baffle, period. In order to get enough U baffling to support the bass, you do nasty things to the midrange and tweeter.
2: Midrange and tweets like as narrow of a baffle as possible. No need for bass reinforcement there, the baffle just colors the sound.
3: Woofers should be much more efficient than your mids. Why? Because you have to eat that efficiency for the sake of extension. Because of the OB low frequency rolloff and reinforcement hump, you end up having to cross much lower than your actual selected point. Put another way, if you want to cross at 500 Hz, you end up actually picking an electrical crossover closer to 200 Hz. The hump, as it was, fights the crossover and extends the response higher. Sadly, there's no other way around it. You end up eating about 9dB of efficiency to get the extension down around 100 to 80 Hz. So plan on an efficient woofer!
4: Asymmetric baffles help smooth out response, but only to a point.
5: Large baffles for the woofer help extend the bass, but only to a point. Eventually you start fighting effecst at the top end of what the woofer will play.

But enough about observations, it's time to build something!

I built an OB sub, a la Linkwitz design. Once my camera comes back from the shop, I will take pics. It is a dual 15" Tempest IB (limited edition for IB/OB use). The ideal spot for this was off center, which opened up the floor for a floorstanding "shorty" speaker for my center. No more horizontal center. It won't be traditional, but I think I'll like the sound better.

So attached are some pics done in Solidworks for my speaker build. Right now the design is going to be made of laminated ply/mdf. 3/4 MDF with 1/4" ply veneer (undecided as of yet) and of course, solid wood edging for strength and to hide the ply edges.

I'm sold on the Dayton 225 (8", 8 Ohm, metal cone). I'm mostly sold on the Neo3PDR for the tweeter, in dipole mode. Right now I'm deciding between the HiVi B3N and a TangBand Bamboo 3" for the midrange. If I can get by with the lower efficiency of the HiVi, I will likely stick with it because it has better power handling around 500Hz, where the crossover will be.

Let me know if you see any potential problems, or to just chime on which potential design you like best. I can finally see the light at the end of this project and it looks nice! 

Oh, and I'm calling it the Anthony Dipole Home Theater (ADHT) Project, in case you wonder what that acronym meant


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## Anthony (Oct 5, 2006)

Here's one of the soundstage with the three speakers


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## Sonnie (Apr 11, 2006)

I don't know anything about OB to be giving you any advice, but the models sure look nice.


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## Anthony (Oct 5, 2006)

Right now we (wife has a say in the finish) are leaning towards the dark cabinet with the light stripe. I think that would look thinner and disappear into the room better. My theater has a full green wall behind the screen (dark, sandwash, so very diffuse).

The only thing that worries me is the center's drivers will be closer to the floor which will add bass and may upset the balance. The center may need a slightly different crossover.

Construction will be laminate 1/4" hardwood ply over 3/4" MDF. I have read many debates/flame-wars over plywood versus MDF for speaker construction. For me the answer was simple -- use both. The alternating layers of ply add bending stiffness in both directions (as will the edge banding I will use to hide the MDF). But you also get the damping and mass of the MDF as an advantage. 

I plan on making the prototype out of MDF this weekend. Which means it will be lucky to be complete by April


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## Brian Bunge (Apr 21, 2006)

You'd be much better off using 3/4" Baltic Birch ply; possibly two layers. It's much stronger and rigid than the MDF. I don't think a 1/4" sheet of ply applied to 3/4" MDF is gonna add much strength. If you want to save a little money, then use a 3/4" layer of each.


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## Anthony (Oct 5, 2006)

Well I was planning on using 1/4" ply on both sides with a minimum of 3/4" hardwood T edge banding (using a spline) for bracing. I was more interested in the MDF for its damping properties than its strength.

I see so many commercial speakers using 1" MDF for their construction, including OBs. I'm planning on having 1-1/4" total, with at least 6 layers of alternating ply. I was planning on testing a few different sandwich layers before I did my final build. Something like a 1' by 2' section with one B3N or RS225 in the middle. Basically try the same driver in a bunch of different baffle materials to see where the peaks and nulls are. It will also pinpoint any manufacturing problems that might arise.

Unlike most of my projects, I plan on only building these things once


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## Brian Bunge (Apr 21, 2006)

Oh, great! It sounds like you've got all the bases covered. :T


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## JCD (Apr 20, 2006)

When you say the U baffling messes with the mid and bass, are you implying that you are using a U baffle with the mid and tweeter as well as the bass?
Will the woofers be used in a 1.5 arrangement? Or will you be running the same frequency range to them?

In either case, I can't wait to see how it turns out!

JCD


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## Anthony (Oct 5, 2006)

I meant that the baffling that works best for the bass really messes up the midrange and tweeter. You get a weird combing effect and at least one large peak and dip in the response for the mids and tweet. When the baffle gets smaller around the mid and tweeter, then things smooth out.

What I am most worried about is the extra floor gain for the center. I may have to make the baffle a little smaller or something to compensate, but there is not way around it, the center is as tall as it can be.

Woofers both get the same signal. I thought about some sort of half way scheme to compensate for the ob rolloff, but I decided to keep it simple for now.

Thanks for the encouragement. I promise to post pics along the way.


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## 1Michael (Nov 2, 2006)

FWIW I used 3/4 Birch or Oak ply on top of 3/4 MDF for my OB speakers and that seemed to work quite well for rigidity. Then use solid oak on the sides to hide the ugly edges.


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## Anthony (Oct 5, 2006)

That's sort of what I had in mind. Except that cherry and mahogany ply is really expensive for 3/4" and 1/4 is much nicer (and it is easier to work with than veneer).

But that and solid edge banding (which is probably a good supplier of strength right there) is the general plan. I just have to work out the details.

Thanks for the input.


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## Zembonez (Mar 17, 2008)

Very nice looking design. Are you going to post a build thread?


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## Anthony (Oct 5, 2006)

This is sort of the unofficial one.

I got a letter from Canon today that they have my digital camera and should have it back to my next week. So I can actually start taking pics and documenting the process.

First step is to probably get my mic calibrated. That's been the main source of the problem in this build. Basically, I was compensating for peaks that weren't there and the overall level of the tweeter was not to be trusted. 

Now of course, this project is competing with our landscaping project -- so rainy days I play with speakers, sunny days I work outside.


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## BoomieMCT (Dec 11, 2006)

Anthony said:


> Right now we (wife has a say in the finish) are leaning towards the dark cabinet with the light stripe. I think that would look thinner and disappear into the room better.


I don't know what matches your HT better but the light cabinet with the dark stripe actually looks thinner. A dark stripe is often used in fashion to make people seem slimmer. Black outlines make things look wider. 

Just my HO - both look great!


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## BoomieMCT (Dec 11, 2006)

Look how they did the edges here. Kinda cool maybe?


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## Zembonez (Mar 17, 2008)

Anthony said:


> This is sort of the unofficial one.


Thanks. Looking forward to seeing your progress in here soon. Appears to be a really good project in the making.


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