# Dipole Center Channel



## Anthony (Oct 5, 2006)

Okay, I wanted to wait until I had pictures, but I figured I'd start my official thread here.

The project is to build an open baffle center channel speaker for primarily HT use. If successful, I will build a full run of matching speakers. However, since center channel speakers offer the most challenges, I wanted to get it working first and figured the vertical MTM would be much easier.

Design is MTM. First cut is using a parts express Dayton Ref. Series 8" woofer pair, 8 Ohm for the bass and a BG Neo3PDR for the tweeter. Back plate removed on the Neo3. Crossover frequency will be around 1800 Hz. This is a tad low for the Neo3, but thanks to Dan Wiggins and his DDR design, I know it can take it. The real question is, how will the pair of woofers handle a crossover that high. 

While not easy to spot on the frequency response chart, apparently there's some cone breakup distortion around that crossover frequency. There's actually a lot in the treble, but I'm using 4th order crossovers so most of it is down 20 to 40 dB.

Crossover design will be 4th order electrical Linkwitz-Reilly slope. Quasi 6th order when you factor in the acoustic rolloff.

So I built the test baffle a test crossover and have started breaking it in.

First impression:
Not bad. The Neo3 really sings on the highs. The Daytons give some mid-bass punch (not even broken in yet). There's definitely a problem at the crossover, though. You can tell when voices hit a certain note or music plays in that region that there's some "fuzz". Not full on distortion, but some weird handoff effects.

I plotted a frequency sweep:

You can see a dip-peak-dip right at the crossover. I'm thinking one of the two isn't centered where it should be. Since the crossover is a mess right now (twisted together and linked with alligator clips) I could be getting a lot of weird effects. Resistance from bad connections, induced inductance from bad orientation (cross-talk), inductance changes from being atop my subwoofer, etc.

Plus I made the rookie mistake of designing from the spec, not the test, so the woofer and tweeter might roll off differently than I expected. Well, I can't test until they're broken in anyway, so no loss. I'll get to that later.

And finally, it's not in an ideal location. Too close to the front wall and directly atop the subwoofer. Ideally it would be out from the wall another 6 to 12 inches and a little higher.

To Do:
Finish break in and test raw woofers and tweeter.
Prototype and solder up current crossover with proper inductor alignment.
Redesign crossover based on tests.
Test different baffles.

So I've got a ways to go, but hey, it's a start.


----------



## Anthony (Oct 5, 2006)

One more thing for the to do list:

Figure out what's a room mode in that response so I don't try to design around it with great futility! 




Oh, and take that response with a grain of salt. Turns out I was using the wrong cal file for my ECM8000. More measurements tomorrow night!


----------



## alan monro (May 9, 2006)

Anthony , seems like a very interesting project . Having x-over at that high frequency you would have to have the woofer very close within 6" of the other spkrs . There is no such thing as break-in for speakers . It is a lot of hype invented by some speaker salesmen who dont know what they are talking about . Kind regards , Alan .:wave:


----------



## Sonnie (Apr 11, 2006)

How about some pics Anthony?

Alan... I might have to disagree with you on the break-in comment. I can guarantee you that I've had speakers that changed in their sound characteristics after having played them for 40-50 hours. I could label that as break-in. I wish I'd taken some measurements. But, this is not really the thread to discuss it, so I'll leave it at that.


----------



## Anthony (Oct 5, 2006)

I think where the myth of break-in occurs is at a factory production level.

Most speaker drivers are tested to verify their T-S parameters and frequency response before assembly. This requires some testing at the factory. Plus, once a speaker is built, there is usually a test period to verify the crossover and drivers are all working as planned. Granted, this is not a full 40 hour burn in, but the drivers have at least seen some use before going to the consumer.

But when you buy the drivers new, it's a **** shoot. Some might have been tested at the factory (spot checked) and seen some playin, but most are fresh off the assembly floor.

I'm expecting the Neo3 to not break in much at all. Very little in there is elastic and the displacements are VERY small.

But I expect to hear a lot more detail in the bass woofers as they open up. The spider and surround are very elastic and will loosen up after a few hours of playing.


As for design, the MTM is as tight as I can get it. The surrounds of the speakers are almost touching the face plate of the Neo3. That's about a 4" separation, edge to edge. If I decide to route out a waveguide in the final baffle, I can probably get them another inch closer, but that would be tight and hard to do well.

Had a setback on the pictures. Wife can't find the cable so I need to borrow one from my day job and upload them tonight. Along with new tests (with the right calibration file for the mic).


----------



## alan monro (May 9, 2006)

Sonnie , Do you ever agree with what I say .:rofl::rofl::rofl::rofl::rofl::rofl::rofl::rofl::rofl::rofl::rofl:Alan


----------



## Sonnie (Apr 11, 2006)

If I don't disagree with you, then yes... and you have said quite a bit. :T


----------



## ktaillon (Apr 3, 2007)

All speakers need a certain amount of breakin. If you purchase a new woofer and run a frequency test before and after running the drivers for a certain amount of time. You WILL see the difference in the test, also, you will hear a difference. 

Open backed speakers are a little tricky, you might want to try using a three-way. Or maybe use a smaller woofer to crossover to the tweeter.

Is the phase correct from woofer to tweeter? Maybe your getting cancellation at the crossover point..


----------



## Anthony (Oct 5, 2006)

The design has changed a bit.

First off, I now have a test rig set up for reliable impedance, frequency response, distortion, etc.

I've verified that the woofers I have distort right at the crossover frequency, so a 3-way was in order. I have two HiVi B3Ns on order. The plan now is an MTMWW or WMTMW. First crossover is at 500 Hz, second at 3kHz. I"m still not planning on getting anything below 80Hz on this setup.

I also have two 15's on order for either stereo OB subs or a quasi-4way for the Left/Right speakers.

More details as the final design emerges.


----------



## Bob in St. Louis (Oct 21, 2006)

Anthony said:


> More details as the final design emerges.


So how's that coming along Anthony?

Bob


----------



## Anthony (Oct 5, 2006)

Ugh, it hasn't.

Life took over. I was getting ready to start, but realized I had a lot of routing to do and that my shop was overrun with scrap wood and junk. So I cleaned that out, which led to moving stuff, which led to rearranging, which led to me screwing through a power line (GFCI to the rescue), which led to rewiring behind two pegboard walls. 

So the shop was reassembled, better than ever. But then I realized I had other woodworking projects that had been shelved that needed finishing first. Then the wife wants the backyard landscaping finished over the holiday weekend.

And before you know it, there went my August 

The good news is that I have all the drivers and 1st cut crossover components. Right now it's about number 3 on the priority list though 

Sorry for no news yet. I'll keep you all posted.


----------



## Bob in St. Louis (Oct 21, 2006)

I understand all too well Anthony. 
I refer to such things as, "life's little intrusions". ha ha

Bob


----------



## Anthony (Oct 5, 2006)

Sadly, no pics yet, but I did cut the baffle for the prototype yesterday.

1' tall, 3' wide, 3/4" thick "show" pine ($5 at HD). Not ideal, I know, but I plan on making the final speaker out of bubinga with maple accents, so I'd rather start with cheap wood first 

This configuration is the W "MTM" W with the MTM vertical. 

I'm wiring up the crossover tonight and will start to break the thing in. Pics and measurements by the weekend, I promise.


----------



## Ayreonaut (Apr 26, 2006)

Anthony, I'm thinking about building Linkwitz Orions in the next few years, so I'm following your progress with great interest. Did you see the page where Linkwitz built a two-way center? http://www.linkwitzlab.com/surround_system.htm#need for a center speaker


----------



## Bob in St. Louis (Oct 21, 2006)

Sounds good Anthony, can't wait to see some photos. :T



Anthony said:


> This configuration is the W "MTM" W with the MTM vertical.


Whatever you do Anthony, don't tell these guys what you're doing: :rofl2::rofl:

http://www.hometheatershack.com/for...lague-toppled-mtm-center-channel-speaker.html

Bob


----------



## Anthony (Oct 5, 2006)

first: good luck with the Orions. They look like great speakers. The only thing I don't like is that they are active crossover and triamped. Too complicated. I figure the more things in the signal path, the more things I can screw up.  So I'm going for an all passive arrangement (down to the sub, of course).

As for the "toppled", I'm doing this one right. It's a toppled design, but the MTM section is vertical, which eliminates almost all of the nasty lobing effects that plague sideways MTM speakers. Granted, I will have some negative effects below 500Hz (where the mid-woofs are horizontal), but it's much less prevalent there. 

Status update:
Fixed the crossover last night and continued break-in. I only have one speaker now, so it's hard to judge things like soundstage  But overall these sound fantastic.

They are only lacking a tiny bit in bass. The rolloff starts at 100Hz and is very flat shallow down to 80Hz. Adding the "U" wings to my baffle should help. don't let anyone tell you that a 1' baffle can't play low.

The midrange and mid-woof section is phenomenal. Tom-tom drums and tympanis sound crisp and lifelike. I heard male backup vocalists that I never knew were on certain tracks (I gave the speaker a heavy dose of ELO, Styx, and Dire Straits last night)

The only problem seems to be transition to the Neo3 and it's range. I get an early spike where it shouldn't be playing, a dip at the crossover frequency, and a steep rolloff after 10kHz. I'm hoping this is a break-in issue. As for sound, it pretty much follows the FR: occasionally a higher male or female voice would get loud, a guitar note would disappear, and things like cymbals and ambiance effects sound hollow.

So I'm going to remove the notch filter and see if that clears up the hollowness. Definitely some more crossover tweaking to be done. Once that's done, I'll experiment with baffle design some and make plans for the LR speakers (with massive active 15" bass sections :buwahahaha: )


----------



## Bob in St. Louis (Oct 21, 2006)

Sounds like things are shaping up rather nice dude. I'd recommend some cardboard and duct tape as wings. Trim off with scissors until you get the FR you want/need. "Quick, easy and free". Then you can install permanent wings (that mimic the cardboards shape) after the testing is finished.
Yes, I know a 1' baffle can get decent bass. I'm using a 2 _INCH_ baffle and get a bit lower than yours. :bigsmile: (but of course they're 15" drivers, so technically I'm cheating) :whistling:

You may find open baffle will "re-invent" your music collection. I've recently heard things for the first time on CD's that I've owned for 20 years. 
O.B. is THAT good. :yes:
(Hint: Pink Floyd "Wish You Were Here" is scary







)

Personally, I wouldn't sweat the small dips and peaks at this point. Everything is new and not burnt in yet (I'll get flamed for that).

......still need photo's man! :waiting: :yes:

Bob


----------



## Anthony (Oct 5, 2006)

Well the prototype crossover is done. It still needs some baffle testing done and some tweaking of the high end, but for the most part, it is done.

It sounds great, is more efficient than my maggies (not fully calibrated yet). But I had to cut the level by 7dB to match the volume.

Attached are a photo and a FR curve. Mic was about 18" away, on axis with the tweeter. Simple 20 to 20k sweep, no gating or anything.

Still to do:
Experiment with baffles to get a little bit better bass response.
Rebuild crossover with "singles", i.e. no combined capacitors and inductors if at all possible. 
Redo notch filter and/or pad tweeter (it's still a bit bright, but not overly so).

Then once I have a true prototype, I'll design a pretty baffle, get some hardwood, and build three of them. 

More to come.


----------



## Bob in St. Louis (Oct 21, 2006)

_*NIIIIIICE !!*_
Dude, that's cool.
I wouldn't worry about what's below 80Hz (THX standard), but a nice looking graph. 
A wise man once told me that the shortest distance from the rear of a baffle shouldn't be over the top of it.
........Just putting a bug in your ear so when you do "wing experiments" you'll have one more tidbit of information.
I wouldn't think it'd take much to get that graph a little more level.

But I gotta admit, that a sexy lookin' arrangement Anthony! :yes:

Bob


----------



## Geoff St. Germain (Dec 18, 2006)

Looks great Anthony. I recently finished a center speaker with dual 8" Dayton Refs and they are very nice drivers (especially for the price IMO).


----------



## Anthony (Oct 5, 2006)

Bob, do you remember who or why they mentioned that? I'm talking about the shortest path not being over the speaker top.

I've been playing around with EDGE to try some different baffle shapes and such. I seem to get the smoothest response by having the drivers very close to the top edge of the baffle. It seems weird.

Also, the trapezoid shape seems to also make for a very smooth response. Still fighting that rolloff, though. I'm envisioning the final design being a folded U trapezoid with a smaller surface on top (which is why I'd like some more info).

Thanks for the support, guys.

a


----------



## Bob in St. Louis (Oct 21, 2006)

Hey Anthony,
It was Dave ("Hurdy-Gurdy") on the Hawthorne forum (and Decware forum, and AudioCircle forum, and Lenco Lovers forum and.....)
I'll see if I can find where he said that, and _WHY_ he said that. I've got some extra time here at work. LOL

Bob


----------



## Bob in St. Louis (Oct 21, 2006)

OK Anthony,
I've found one post from Dave that mentions a top plate on a baffle. His comment here appears to be more drivers specific than a general OB "rule". I know he's mentioned it a couple times over the past couple of years, but I can't find them. I can't find the EXACT post I remember though. If memory serves, he _was_ speaking 'in general' for OB and not for the Hawthorne products exclusively. I also recall the conversation being about the addition of a verticle extension of the baffle, not a horizontal plate.
But the key here is that he's saying "....._extending the top so there is a greater distance from the driver to the top than to the sides_."

To 'qualify' Dave and his knowledge, he is the crossover builder for Hawthorne Audio.

Here's his post from December 2006.
http://www.hawthorneaudio.com/forum/viewtopic.php?t=438&postdays=0&postorder=asc&start=150

Bob


----------



## sfdoddsy (Oct 18, 2007)

I suspect you will find yourself on a slippery slope once you complete your dipole center. I have a full 5.1 system using dynamic dipoles based on the Linkwitz Orion/Phoenix and it is very addictive.

www.doddsy.net/steve6_008.htm

Your implementation is different in that you have the panel horizontal, but I would suggest you also do some measurements from the listening position before finalising the crossover. Are you going to EQ the woofers? You already seem to be getting dipole roll-off and this will become more apparent from further away.

Best of luck with the speakers.

Steve


----------



## Anthony (Oct 5, 2006)

Thanks Steve.

Right now the plan is for no EQ. The woofers are more sensitive than the rest, so I am hoping to "jack" them a bit to compensate for the rolloff. I'm only looking for extension down to 80 Hz. After that, I will hand off to some OB 15" drivers in a separate enclosure that will cover the rest. That will be EQ'd.

I'm also not done with baffle design. I'm hoping to get just a hair more lower extension by making a wider folded U baffle, but right now I'm competing with resonances, so there's some trial and error there.

I've already bought the stuff to make a full run of front speakers. I'm still not sure how I can use the existing driver configuration for surrounds. Right now I have magnepan MGMC1's back there on the wall for the surrounds, so I like the dipole rears.

I like having equipment in the back of the room and don't have any more conduit capacity to run more speaker leads up front, so I am going for single-amp'd passive X-over speakers.

I need to do a lot of measurements, I've just been very short on time lately.

thanks for the support.

Anthony


----------



## sfdoddsy (Oct 18, 2007)

Hey Anthony,

Whether you use EQ or not you'll have to compensate for it. You can see from your trace that the response of the speaker is sloping up to 20kHz instead of down from 20 Hz. Ideally you'd want to flip this around since I suspect that when you start measuring from further away the bass response is going to dive.

Not all of this is dipole related. 

The horizontal layout may confuse things, and I'm a tad lazy to check my baffle width/cancellation program, but I'd say that your tweeter and mids are crossed high enough that dipole cancellation isn't the problem, they're just set a bit hot.

However, I'd be really surprised if you get useable bass down to the 80-100Hz required to cross to a sub without EQ. You could fudge things by placing the speaker on a TV and thus increase one boundary, but by the looks of it your baffle is about 16" wide which usually means a 6dB octave slope down from the Fequal position of 250Hz.

(OK, so I checked.)

You can boost the woofers to counterract this, but it requires a lot more EQ than folk think. The Orions have 20dB of boost at 100Hz,for example, and double that in the bass.

To do this without active EQ, you need to lower the level of the mids and tweeter by the equivalent amount.

This why those who prefer passive crossovers for the mids still tend to go active down low. 

So I reckon you can get away with the crossover you have now for the mid/tweeter, but you'll need to find a way to give a 6dB/octave boost starting at 250Hz, or a similar cut before allowing for the crossover.

As for surrounds, I seriously considered just using the same Maggies you have and kind of wish I had. Six channels for the fronts is bad enough, but the extra six for the surrounds and center (plus the extra crossover) does get a bit wearying. Were I to do it again I'd probably just do the front three and use any decent dipole for the rears. 

Cheers

Steve


----------



## Anthony (Oct 5, 2006)

Thanks for the comments. Yeah, I know about the mids and tweet being hot. In fact, I was estimating about equal sensitivity to the Magnepans (MG10.1), but when I put this center in,I had to cut it 7dB, so I think the B3N and Neo3 were a bit more sensitive than their specs (which in any other case would probably be a good thing).

As for the baffle, right now it is a full 3' wide (horizontal) and 12" tall with U wings of 11 and 13 inches. So unfolded, it is roughly 36" square. The wings are only clamped on so I can try a bunch of different designs before production.

The two major issues I'm working now:
rising tweeter/mid response
A resonant energy storage in the mid to upper male voice range. Sounds like a slight echo where one shouldn't be. It is not anywhere else.

Once I get the measurements done, I should be able to see that in the gated response and the impedance plot. I'll post all those as soon as I measure them.

Thanks again for the help. Great website, BTW.


----------



## Bob in St. Louis (Oct 21, 2006)

Hey Anthony,
Just a thought; Do you guys think the highs are hot due to the mic being so close and on axis to the tweets? How about tryng a FR graph with the mic backed off several feet? Measuring OB from such extreme close distances seems like it has the potencial for frustration.

Bob


----------



## Anthony (Oct 5, 2006)

Okay, an update of sorts:

I wired the crossover wrong and there were two major problems: one was a lack of crossover at the woofers, another was the wiring of the midrange.

When fixed, the bass response was much better and I lost some of the distortion I was hearing. The bad news was that the mids and tweeter are now MUCH more efficient than the woofer section so I'm getting an overly bright tweeter. This was all measured from the listening position.

The baffle extentions I clamped on were resonating, so that was a source of distortion as well and to be honest, they didn't extend the bass all that much. Right now it's back to just the flat plate with the drivers.

I have been playing around a lot with measurements in REW and Speaker Workshop. I also keep hearing a resonance or distortion at certain frequencies, so I am trying to nail them down. I know the B3N has one at 10kHz, but I'm crossing at 3kHz, so it should be way down by the distortion point. I may throw in a notch filter anyways to completely nail that down (for more info on the B3N and some speaker designs, check out the Zaph audio writeups on them)

I also heard more distortion when I played things loud this weekend, so I may also have a power handling issue. It's so tough to tell whether error you hear is in the speaker or source material.

So it seems close, but if I can't locate and elimate these distortion points, I may have to go back to the drawing board.

Right now, here's the plan:
1) tame the levels of the tweeter to get it back inline with the rest.
2) Rewire the crossover to eliminate the alligator leads. They were great for initial testing and breaking in, but I have to know that the power handling issues aren't from a bad connection or wire that's too thin.
3) Detailed test and evaluation. I may even try to get a rig together to go way outdoors and test in true open field. That should eliminate the room as a source of error.

So it's not a lost cause yet. This has definitely been a learning experience.


----------

