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Aengus 15th May 2006 05:43 PM

Open-baffle design question
 
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First, if this topic has been covered elsewhere, I apologize. I did a search on “open-baffle” and didn’t see anything close to this question – if I missed it, please provide a link. Thanks.

I’m planning to build an speaker with an open-baffle midrange. At first I considered something like this (this is the plan view):

[attached file]

but with any realistic size front-to-back (12-18”, say 30-45 cm.) this only get down to about 500 Hz. before the wavelength exceeds the rear-to-front distance and interference takes place. So I thought, why not extend it this way (and offset the speaker at the same time):

[see next post]

My thinking is, as long as the slot at the back is wide enough, it should still look like an open baffle at lower frequencies. At higher frequencies, it would probably need some felt on the inside of the panels to keep reflections onto the back at the speaker reasonably low. This seems to mean that it would act as a partial open-baffle, but I’m not sure what the practical consequences would be. Has anyone experimented with this, or does anyone have any comments based on wider knowledge than mine? Almost any degree of knowledge will be wider than mine, BTW :)

Aengus 15th May 2006 05:44 PM

Added image
 
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Sorry, I'm having trouble embedding the images. Here's the second one.

Kensai 15th May 2006 05:53 PM

The wings are okay. They don't help as much as true baffle surface area, but they don't generally cause any issues, either, unless something about their construction rattles. whenyou start adding extra walls folding back toward the center though, you almost automatically start having issues. You'll be getting something similar to bass reflex, but with completely unrealistic port dimentions. Its not just the slot to the back, but the open top which starts looking like a cylinder. Its not really possible to model this behavior either, but every instance of it I've heard has sounded very noticeably bad.

So, what midrange are you using, and what are you hoping to cross it over to on the bottom end?

Kensai

Aengus 15th May 2006 06:29 PM

Hi Kensai

Thank you for the input. The point about turning it into a pseudo-cylinder is a good one - I should probably have seen that just by looking at it.

On the weekend I bought from a friend 4 each of 6" and 7 1/2" (say, 15 and 19 cm.) Audax drivers. Unfortunately, I can't remember the model numbers and can't check at the moment; I know that they're a model that's now discontinued, with paper cones. My friend had them individually mounted in Corian tubes in a W-M-T-M-W arrangement, with a homebuilt ribbon tweeter. Driven by a pair of custom 300B amps, they sounded very nice.

I'm thinking of using the 7 1/2" drivers in an Isobarik configuration, but I may experiment with open-panel for them as well (but the panels would be getting pretty big at that point). The advantage of that would be that I could fairly quickly put together a test panel just using some shop-grade birch plywood I have in the garage.

Don't know yet about tweeters; my friend and I are still discussing the ribbons, but knowing what he paid just for the magnets, the price may be prohibitive for me.

Kensai 15th May 2006 07:37 PM

I've never seen an Audax that had ideal parameters for OB. I'm sure they can be made to sound good, but they're likely just going to roll off early and quickly on the bottom end.

I've been thinking alot about building bipoles lately. Build a very slim cabinet that's just wide enough for the driver to mount nicely and just deep enough to mount two, facing in opposite directions, with their magnets pressed together (likely with felt or something else between to prevent any rattle). Sorta like isobaric, but I've never really understood that config very well (I mean, what's up with having the driver's good radiating surface inside the box? maybe I'll figure it out eventually).

Anyway, building a bipole should (I say should since I haven't done it yet to see myself) give you somthing very OB like in a skinny package, remove any need for BSC and give you like a 3-6db efficiency increase so you could easily mate it with a good planar or even a cheap piezo up top.

As for the ribons, well, I don't really like actual ribbon drivers so much (yeah, they sound cool, and I'll take them over cones of any sort and most domes, but not my favorite). I really like planars, though. I can super highly recommend B&G Neo3PDRs from personal experience. They're great in dipole, too (you've got to strip the back cup they come with off, but that's not hard). The PDR variants of the B&G planars do this thing where the higher the frequency, the narrower the radiating area is in use, so as the frequency goes up, they get closer to true ribbon behavior. In use, to me, it just gives it better dispersion than most planars (planars can tend to be a bit directive), but since its still connected to the whole diaphragm, they seem to be alot more restrained where a true ribbon seems to get all nuts on the high end. The Neos also play alot lower than most ribbons if you need them to, so it gives you alot more freedom in your crossover design.

So, I guess I'm saying, build a sealed'n'stuffed skiny bipole for your Audax (put the 7.5"ers up front and build surrounds out of the 6"ers, maybe?) and then just extend the front baffle up 5"-6" above the top of the box and mount Neo3PDRs in dipole (you can buy faceplates for them for like $5-$6 a pair; they come in surface and flush mount; I've used the surface mount ones in various configs with no issues at all, though it seems more high end to try to flush mount them). Most of the smaller Audax woofers I've seen specs on seem to be pretty well behaved through most of the spectrum, so you could start crossing them out at 2kHz or even maybe closer to 4kHz.

Regardless, Audax drivers tend to be pretty nice, so I'd be interested to see what you come up with for them.

Kensai

DIY_newbie 15th May 2006 09:54 PM

What you're describing is very similar to a U-Baffle.

If you look at the polar response of a simple open baffle it will have a figure 8 radiation pattern at low frequencies.. If you look at a U-Baffle it will radiate in a cardiod fashion..

Pure open-baffles are just flat surfaces, what you're trying to build is a "compact open baffle" or a "folded baffle" ... There are already several designs for folded baffles that optimize surface area for a given space..

Try searching for the following terms:

W-Baffle / W-Frame
H-Baffle / H-Frame
U-Baffle / U-Frame

Regards,

--Chris

el`Ol 17th May 2006 06:37 PM

45cm is perfectly alright for 200 Hz. But if an even response is desired you go better with high Qts. Here starts the problem: Hard to find such a midranger. Are you sure you don`t want to go fullrange?


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