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Old 5th January 2006, 03:24 PM   #1
Kensai is offline Kensai  United States
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Default Designing a fullrange dipole?

I have this new concept for a sealed, floorstanding speaker using a pair of 6"x9" fullrange drivers.

I want to mount them back to back, magnets pressed together. I'll mount the drivers tall instead of wide (I'm told they'll have greater off axis response that way). I'll wire them in series (that should get an 8ohm load out of a pair of 4ohm drivers, right?) and out of phase (so they're acting as close to a single, true dipole driver as possible).

I figure this would give me a tower about 8" wide (possibly a bit less depending on how the cabinet is constructed and exactly what driver I use), and possibly as little as 4"-5" deep (again depending on factors mentioned above). I'd want to put the driver center about seated ear height, so I'm guessing offhand that would make the cabinets 45"-50" tall. These are very much the "tall & slim" type, but the drivers I'm thinking of using get good response down to 40Hz or so mounted on a 16"x20" open baffle, using just the "loudness" correction function on a cheap Pioneer receiver, so I'm figuring that the fact these will be sealed (and possibly stuffed to some degree) and will use a pair of drivers running push-pull dipole would get enough reinforcement from the enclosure and the room to be mostly flat down to 40Hz and maybe a bit lower (these drivers are rated down to 28Hz by their manufacturer, but thats probably a -10db rating at terrible distortion under who knows what sort of unnatural conditions)

As for modification, I'm thinking to hand mod the drivers to include phase plugs. This should smooth and extend the upper end, correct?

Anyhow, I'm technically speaking a relative newb (I've only DIYed some drop in replacements to existing cabinets and some OBs), so I'm wondering who do you folks think this sort of thing will sound? How will it behave? Any particular concerns or design issues?

Also thinking of doing the same thing but just making the enclosure big enough to house the drivers, back to back (like 10"-12" tall, 7"-8" wide and 4"-6" deep) for use as bookshelves or centers. I'm sure that would cut bass response, but how much, really?

Kensai
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Old 5th January 2006, 03:33 PM   #2
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It won't really matter how big the box is (or how much you stuff it) if you plan on running the drivers out of phase. One driver will push and the other will pull and the pressure in the box will stay the same.

I'm no expert but I would imagine there would be little difference between this design and a single driver mounted on an open-baffle.

You would get much more bass with just the one driver in the sealed box!

Why not run the drivers in phase so that they compliment each others efforts rather than cancelling?
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Old 5th January 2006, 03:52 PM   #3
Kensai is offline Kensai  United States
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Why would running them in phase help? I would figure that would cause the cancellation. Seriously, I don't understand exactly what would be going on there.

My thinking was that if they were in phase, first off they would be fighting each other due to the internal pressure (both cones lifting or dropping at the same time either causing a vacuum that would make the cones difficult to lift or a compression making them hard to pull back in) and that since they would be transmitting the same signal into the room at the same instant, that would cause the in room cancelation of the identical waves bending around the cabinet.

Similarly, I was thinking that if they were out of phase, not only would the pressure and suspension inside the box be nearly constant, but the touching magnets would basically be working in tandem. Out in the room, one speaker would be transmitting power to the air while the other one was falling back at the same instant, so I'm not seeing how that is doing wave cancellation in-room.

I'm totally off in this thinking, aren't I?

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Old 5th January 2006, 05:30 PM   #4
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Quote:
Originally posted by Kensai
I'm totally off in this thinking, aren't I?
Quite definitely yes.

Think about water pumps. If you have one pumping in and one out of a reservoir the amount of water will remain the same. If you have two pumping in the same way you'll get double the amout of what one pump would move. It's about the same with speakers, although this example works only under the baffle-step frequency.
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Old 5th January 2006, 06:15 PM   #5
Kensai is offline Kensai  United States
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Thanks for setting me straight on that.

So if I have these two drivers running in phase, what sort of performance might I see out of this thing? How about out of the minimum size box version?

I don't know much about baffle step. I'm thinking that's the point below which the baffle isn't blocking the first wave length so you get cancellation as the wave bends around the cabinet. I had read that if you had a pair of identical drivers running like this that you didn't have to worry about baffle step correction. Eliminating the various needs for filtering are part of my emerging design philosopy. If I can avoid having any components between the amp and the drivers, that's what I want.

I also enjoy the way dipoles interact with a room. I've got a pair of these drivers already, set in 16"x20" baffles (I know, very small). If I could get a bit more bottom end out of them in a narrow enclosure, they could pass WAF and make it into the living room to replace the decent, but woefully mis-positioned bookshelves that are in there now (and if I could make a box that's not much bigger than a pair of these drivers, I might be able to get a center and/or surrounds in there, too).

I suppose, more simply (and more cheaply), I could just use one per speaker as standard sealed speakers. In that case, is there any way to guestimate good enclosure volumes. They're car audio drivers, so TS measurements are not forthcoming. Also, how bad would baffle step be with a baffle only 7"-8" wide?

Alternate thoughts on the matter would be to keep baffle the same and hinge mount them to the side of my 7' tall entertainment center so the baffle would be effectively infinite toward the center of each. At that point, I could use just one driver at ear (or TV centerline) height, a line array of them (3 would be taller than my screen, and 4 would be around 3.5' tall), or a single with an 8"-12" sub driver mounted mounted near the floor in the same baffle, set to cross at 40-50hz.

Using a single driver in OB would be cheapest (the most expensive part of doing anything this way would be getting WAF approved, matching OB panels made and mounted), but I would worry that the bass output would be disapointing (my current bookshelves have 8" woofers and are basically using the entire bottom panel of the entertainment center, and the 2" airspace underneath as a sounding board, so I get a very large amount of relatively high quality bass reinforcement).

I'm not terribly familiar with any of the physic or math of a line array. I'm not even sure that 4 of these per side would even technically be considered a line array. At closest mounting, the driver centers would be about 5" apart. Would this spacing cause beaming or lobing or any of the other derogatory terms I see bandied about in line array threads? These drivers are ~$50 a pair, so 4 per side would already be $200 in drivers alone. This method would probably up the cost of getting a proper baffle made.

Going with one with a sub driver to fill in the bottom 1-1.5 octaves would probably be fairly expensive, too, given that an OB appropriate driver with a low enough Fs to be worth using will be a big $$$ driver (and probably cause the baffle to need to be wider, thus costing more). This would also require either a crossover, or at the very least a sub plate amp.

Any guidance, thoughts, heckles or flames are welcome.

Kensai
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Old 5th January 2006, 06:47 PM   #6
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One important question is the final distance of the speaker from walls. Dipoles, bipoles and normal monopoles with BSC in the crossover must have space behind them, at least 3 feet, I would say, more is better of cource. If you want to have you're speakers close to a wall you'll have to have less BSC and finally with onwall/inwall speakers no BSC at all, because they have "almost" infinite baffle.

Here:
http://www.t-linespeakers.org/tech/b...tep/index.html
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Old 5th January 2006, 07:25 PM   #7
Kensai is offline Kensai  United States
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If we're talking about any of the boxed concepts, the mains would be 2' from the wall behind them (could easily be pulled out for critical use and then put away for standard) and 2'-3' from the side wall. They would be close to the corners of the big entertainment center and likely toed in somewhat, so the furniture would likely interrupt the BS on the center side of each speaker. Also, there is a 6' fake ficus in each of those corners. Center would be 2' from the back wall but also enclose in the space the TV rests in (maybe 1' above the speaker and 2' on either side). Then entertainment center is about 4" away from the wall in the back, so its not remotely a sealed chamber.

I know I could simply "try and see", but this type of thing would be impossible for me to prototype with my tools and the materials they allow me to work. Also, money is tight and time is money, so I'm fishing to see if any merit can be found in these designs to justify the upfront expense of trying.

Thanks,
Kensai

PS Thanks for the informative link.
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Old 6th January 2006, 03:51 AM   #8
poptart is offline poptart  Canada
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kensai,

putting the two speakers back to back and IN phase is called a bipole and there's lots of discussion here about that type of speaker to check out. It removes the need for baffle step correction and is reported to sound quite nice by many people here.

Putting the speakers out of phase will create a dipole, but with no real advantage over using just one speaker. The same amount of air is moved as just one speaker. If you wired them that way you would need a wide baffle/box just like a single speaker dipole or you wouldn't get any bass. You can use two speakers for a dipole but it's better to put them both on the same baffle so you get twice as much air moved. If these are fullranges you may need to roll the high end off on one of the drivers depending on how close they are, that's a whole different ballgame though...

Hope that helps some, try a search for bipole.
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Old 6th January 2006, 04:52 AM   #9
Kensai is offline Kensai  United States
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poptart,

Thanks for correcting my terminology. I don't think I knew the technical difference between di and bi pole configs and was just hung up on di since that's what I'd been working with on all my OBs. This should help me find a few more resources.

Removing the need for BSC is on goal here, to be sure, so I think I'm on the right track here. Now to research and figure out what benefits and challenges the bipole config will bring me in this situation.

Kensai
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Old 6th January 2006, 11:12 AM   #10
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Rule of thumb: a bipole will remove the need for baffle step correction. It will create a larger, but slightly more diffuse stereo image, which is probaly more realistic frankly (when did you ever hear perfect point-source imaging at a concert?), and a much deeper soundstage. See the other examples on Dave's (planet10) two sites.
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