Open Baffle musings

[charlielaub]

Re: Open Baffle musings

He did the HOSS and others think?

I think on the HOSS he had a monopole tweeter and dual omni midranges... how is that any better than what I am proposing???

I have to admit that I don't quite grasp the consequences of mixing monopole, dipole, omni, etc. I know that JohnK has some material on his web site about it, but it's been awhile since I read it. Are the problems mostly as a result of the power response not matching?

-Charlie

[Pete Schumacher]

Re: Open Baffle musings

D
Why are y'all's panties in a bunch over this?

What is the big deal with going from dipole to omni, just for the top end sparkle?

-Charlie

You asked for it, that's why . . . :p

I want to bounce some ideas around here, so please feel free to deconstruct and criticize the following:

Open Baffle Project WAF-1
Drivers:

• 12" Eminence Driver: 60Hz - 300Hz
• 6" AE Speakers TD6M: 300Hz - 2500Hz
• up-firing SB Acoustics SB29RDCN ring radiator

[jhollander]

Re: Open Baffle musings

…Deward summed it up in his last few paragraphs

But you should go for it. You’ll learn more about dipoles than you thought possible. Seriously, have some fun with it.

[Deward Hastings]

Re: Open Baffle musings

Why are y'all's panties in a bunch over this?

You asked for it, that's why . . . :p

First principles . . .

There are two lines of thought that lead to dipoles:

“Open baffle” . . . the goal of eliminating box/panel resonances and colorations, so that all you are working with are the inherent problems of the drivers and baffles, and

“Constant directivity” . . . the goals of uniformly illuminating the room with simultaneously flat (uniform) on-axis and power response and minimum distracting interaction, and of presenting the ear with a consistent (angular) pattern of first reflections to aid in localization and soundstage formation (what SL calls the “audio scene”).

Within those lines of thought all the other “rules” of speaker design (frequency response, distortion, driver spacing, crossover response, diffraction control etc.) and all the expected constraints of acoustics and physics still apply . . . but you are faced with a whole new set of design considerations and limits. That all the good (successful) designs seem to converge on a small set of common features should tell you something . . . dipole design is very constraining. You don’t get there with a “grab bag” approach . . .every element of the design has to be considered in light of *all* the issues and constraints that govern dipoles. And while “pick up some drivers and try to make them work together” may be a fun approach to a “speakers in a box” contest the chances of it leading to a successful dipole are awfully close to zero.

99.99% of dipole design is rejecting good ideas that satisfy (or even excel) in one area but fail in another. The remaining 99.99% is making what’s left work together . . . balancing all the necessary compromises, figuring out which rules you can (will have to) “bend” without deviating too much from the overall design goal. Start with a piece of paper. Draw the classic “line and two circles” dipole pattern on it. The line is your speaker. The two circles are your goal. Test every single ”idea” against that diagram . . . will putting the “idea” on the line facilitate or detract from creating those circles.

That, of course, is just the beginning . . .

[johnk...]

Re: Open Baffle musings

Hi Charlie,

Don't let all the hype about narrow baffle dipole systems deter you. Trust me on this, I know!

33" x 39" baffle, 2 SLS 12" woofer + 2 10" SLS woofer per side + ScanSpeak 21W8554 + SB Acoustics SB29RDC. I would recommend 1/2 carpet felt over the back side of the midrange to limit rear radiation.

[fdieck]

Re: Open Baffle musings

If nothing else it would make the mother of all open baffle subwoofers:applause: Any measurements and or low frequency EQ advice for this beast. I have a single SLS 12 on slightly smaller flat baffle and it sounds great.

[johnk...]

Re: Open Baffle musings

Nothing hanging around. The system is basically +/- 3 dB from 30 to 20K. Bass could be extended further by changing the EQ. A broad dip of about 3dB between 400 and 800 that I never eq'ed out. It's a toy. BIG image but very precise. Very powerful. Bass eq is by ear in my room. It's just too hard to get good measurements with a woofer system that large and the free field doesn't translate to in room. It's a completely different animal from my other systems, as you can imagine.

[charlielaub]

Re: Open Baffle musings

JohnK, thanks for the encouragement.

Others: thanks for the criticisms and Deward for explaining some points about design goals WRT OB systems.

I'll just have to go off and try this I guess. I think it's mostly nailed down except for the tweeter... I could do a back-to-back dipole, a monopole front firing, or an upfiring omni with some adjustment for each to the midrange transfer function.

-Charlie

[JasonP]

Re: Open Baffle musings

I could do a back-to-back dipole

This is on my todo list for next year. I really want to hear a dipole with a better balance of rear and forward radiation. I assume you are meaning having drivers on the front and back of a sealed cabinet? The only issue with this I imagine is that as the wavelength of the sound decreases, eventually the speaker will see 2x Monopole loading and have weird cancellation issues. Though that would only be like say, 1.5khz and up for a 4" deep cabinet. I dunno. I'm still thinking this will be an issue that needs addressed.

[charlielaub]

Re: Open Baffle musings

This is on my todo list for next year. I really want to hear a dipole with a better balance of rear and forward radiation. I assume you are meaning having drivers on the front and back of a sealed cabinet? The only issue with this I imagine is that as the wavelength of the sound decreases, eventually the speaker will see 2x Monopole loading and have weird cancellation issues. Though that would only be like say, 1.5khz and up for a 4" deep cabinet. I dunno. I'm still thinking this will be an issue that needs addressed.

I mean mount two units of a small tweeter (like the Vifa OX20) literally back to back and then mount those in the baffle at the top (there is no "box").

Anyway, I could get HF dipole operation if I use the Tang-Band W2-800SL:


It's a full range, but the radiating surface is about 1.5" in diameter IIRC. I've used it as a tweeter in a recent project. Mounted on top of the proposed baffle, I would get a dipole pattern. Add a rear chamber and it's monopole.

-Charlie

[JasonP]

Re: Open Baffle musings

Oh, interesting, ok, I was going a totally different way in my head

[diy speaker guy]

Re: Open Baffle musings

Anyway, I could get HF dipole operation if I use the Tang-Band W2-800SL:


It's a full range, but the radiating surface is about 1.5" in diameter IIRC. I've used it as a tweeter in a recent project. Mounted on top of the proposed baffle, I would get a dipole pattern. Add a rear chamber and it's monopole.

-Charlie

Not really. You won't get a dipole pattern (especially at high frequencies, but even in the mids) due to the fact that there's a big motor and basket on one side of the cone, right in the middle where the high frequencies usually emanate from. If you look at a pic taken from the back you probably won't be able to see any of the cone at all. Sure it will be a lot more dipole than a low fs closed back tweeter, but not fully dipole, and less so as frequency increases. If you want a real dipole tweeter, use what John uses, it's a lot more practical and appealing than back to back mono tweeters.

TD6M centered side-to-side in baffle, very close to top edge

Sorry, this is what messed me up, I read that as dual mids side by side. My mistake.

Anyway, baffle width is all about power response. But as MJK has proven with his wide passive OBs, lots of people don't mind a less than ideal (on paper) situation.

[charlielaub]

Re: Open Baffle musings

Not really. You won't get a dipole pattern (especially at high frequencies, but even in the mids) due to the fact that there's a big motor and basket on one side of the cone, right in the middle where the high frequencies usually emanate from. If you look at a pic taken from the back you probably won't be able to see any of the cone at all. Sure it will be a lot more dipole than a low fs closed back tweeter, but not fully dipole, and less so as frequency increases. If you want a real dipole tweeter, use what John uses, it's a lot more practical and appealing than back to back mono tweeters.

OK, I agree, for high frequencies the motor is kind of in the way. Only an open-back planar could work then, I guess.

Anyway, baffle width is all about power response. But as MJK has proven with his wide passive OBs, lots of people don't mind a less than ideal (on paper) situation.

So this is what I would like to know a little more about. Why does a wide baffle interfere with or preclude dipole operation (at higher frequencies) or reduce power response there? Is this related to the wavelength, and is there some kind of rule of thumb for that? Please expand on this...

-Charlie

[diy speaker guy]

Re: Open Baffle musings

So this is what I would like to know a little more about. Why does a wide baffle interfere with or preclude dipole operation (at higher frequencies) or reduce power response there? Is this related to the wavelength, and is there some kind of rule of thumb for that? Please expand on this...

-Charlie

I don't remember the details, John K probably knows all about this stuff.

All I remember is that several years ago Skorpion or Rudolph or one of the OB guys in the audiocircle OB forum pointed this out with graphs and measurements and the rule of thumb that followed was to use baffles less than 2x the driver diameter, which obviously gets pretty small for tweeters. I forget the details and might not even be remembering this correctly.

What I do know for sure is that if you use very small baffles (following the rule of thumb above), if you go with active crossover, it is very easy to use each driver within it's completely linear range (linear but rising response) below the first dipole peak. This requires equalization to flatten the rising response but other than the rising response there is no diffraction to worry about (no rippled response).

Anyway, the trend for active OB is for narrower baffles, and gainphile at diyaudio is a pioneer in completely baffleless OB, I think he's got a website or blog so you should probably look for his opinions on OB, I assume he's got some good reasons (and explanations) for doing something so radical.

[Deward Hastings]

Re: Open Baffle musings

I could get HF dipole operation if I use the Tang-Band W2-800SL:

Over at least part of its range anyway . . . it is certainly a plausable candidate. With a bit of baffle you can get decent dipole behavior (and output) down low enough to permit crossover at 2kHz. The cone will be beaming (forward) at a (roughly) dipole-equivalent pattern by around 4kHz., above that it will be progressively narrower (beaming). The rear lobe above 4k will simply be a mess because of diffraction and occlusion by the frame, spider and magnet, but the overall reflected sound may not be all that bad. To avoid vertical lobing it will want to be as close as possible to the mid (without introducing baffle interference) . . . that, combined with the decidedly non-dipole pattern you're going to get from the mid mounting position (as proposed) poses problems. My suggestion would be a stepped sub-baffle (super-baffle?) like you-know-who does above the slab for the combined mid and tweeter that would provide the close spacing required and overall dipole behavior that includes the midrange as well.

The initial fr curves will not be pretty at all, but the necessary dipole equalization and crossover will be easily handled by the miniDSP.