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Old 21st December 2003, 05:06 AM   #1
MtnBob is offline MtnBob  United States
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Question HT surrounds, Dipoles, Zobels

I am new to all this, as will be obvious as you read this. I am planning the 3 back channel speakers for my HT. I have 6 4 in. "full-range" drivers and Would like to use these in pairs, wired in series, in dipole/open baffles. I would prefer them to be as unobtrusive as possible. I am not particularly worried about bass, as they will never see anything lower than 80-100 Hz. I'm also not worried too much about the top end, since my hearing rolls off pretty sharply after 8000Hz (Too much Hendrix back in the 60's) Driver Qt is .76 BTW. I have read a lot in this and other forums, and I am confused on several points. Please excuse the crude drawing, but I think you can get the point.

1. Alignment - Some say the drivers should be in the center of the baffle, as close together as possible. Others say to offset the drivers, but how and how much? Which of these 3 alignments would be best, or is there another?

2. Baffle shape - #1 is goimg to be just too large. #2 & 3 would be better I guess. I haven't seen anyone talk about a curved baffle, but it seems to me it would work just as well, if not better. How about alignment #2 on a curved baffle? Too diffuse? Remember these are for the surround channels.

3. The info I have seen on baffle size says that to get 80 Hz, you need a 40 in./16 cm baffle, which seems huge, especially for a couple of 4 in. drivers. Is that measured from the center of the driver front to back? Does it really have to be that big? I assume you also measure over the top and around bottom too. There seems to be a lot of very technical discussion of this, but it seems to center on how to get bass.

4. Now I really show how little I know - I am thinking of a Zobel for these. So if they were in series, and Re is 3.5 and Le is.6, then I would calculate the Zobel values based on Re 7 and Le 1.2 and wire it from + of spkr 1 to - of spkr 2? Don't know if it will help, but what could it hurt?

5. Does wiring (series or parallel) multiple drivers affect Qe, and therefore Qt? If there's a formula, do I have to be an EE or Physicist to figure it out? Will it change enough to really matter?
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Old 22nd December 2003, 07:44 AM   #2
Rudolf is offline Rudolf  Germany
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Hi Bob,
as nobody else seems to care, Iīll try some answers:

1. Alignment - Which of these 3 alignments would be best, or is there another?
You donīt want to have full-range drivers work in parallel on the baffle because their output will interfere and you will get some nasty "combing" in the higher frequency range. Better try a "1.5 configuration" where one driver only supports the range up to 200-300 Hz. Alignment 3 should be prefered.

2. Baffle shape - #1 is going to be just too large. #2 & 3 would be better I guess.
Baffle #2,3,4 are all acceptable. A curved baffle would be nice. Have a look at this one for some inspiration:
http://www.audiodiskussion.de/user/Stixx/Saba-SWR.jpg

3. The info I have seen on baffle size says that to get 80 Hz, you need a 40 in./16 cm baffle. Is that measured from the center of the driver front to back? Does it really have to be that big?
As a rule of thumb: the circumference of the baffle should be roughly equal to the wavelength of the lowest frequency. Thatīs 4.3 m for 80 Hz. If you position the driver on the baffle according to the golden rule, youīre nearly perfect :-)

Iīm afraid I canīt answer Q 4 and 5 sufficiently, but I recommend searching this forum for everything regarding open baffles and dipoles.

Happy X-mas
Rudolf
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Old 24th December 2003, 04:18 AM   #3
MtnBob is offline MtnBob  United States
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Thanks, Rudolf. I guess everyone else thinks these have already been covered in other posts. I have read about 200 or so related to OBs and that's how I got so confused. But then most people aren't building what I want, they want real full-range. Guess I'll scrounge around some construction sites for scrap plywood and just see how it works out.

Merry Christmas to you
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Old 24th December 2003, 08:01 PM   #4
diypole is offline diypole  United States
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Hello MtnBob,
Much of the design theory you are seeking is here: http://www.linkwitzlab.com , click on the "Pheonix dipole speaker" link, and at the bottom of the page, click on models.

To figure out how low they will play on a give size baffle, use Mr. Linkwitz's spreadsheet spl_max1.xls on the same site. You will need to enter the effective cone area, typically listed in the driver specs as Sd; the driver x-max; and the effective path difference (D), which is the shortest path from the center of the driver to the edge of the baffle (on a flat baffle). When using multiple drivers, sum the Sd, but keep x-max the same.

A Qts of .76 will work well on an open baffle, probably rising to ~.78 when mounted, but this will drop to half with series connection.

"1. Alignment - Some say the drivers should be in the center of the baffle, as close together as possible. Others say to offset the drivers, but how and how much? Which of these 3 alignments would be best, or is there another?"

JY: Center alignment will produce the most uniform horizontal off axis frequency response. Offsetting will reduce the peak that occurs when the frequency produced by the front and back are in phase, but reduces the path difference (D). IMO always align the driver centers vertically. You may want to low pass one driver at the frequency which has a wavelength that is twice as long as the distance between the drivers to reduce comb filtering.

"2. Baffle shape - #1 is goimg to be just too large. #2 & 3 would be better I guess. I haven't seen anyone talk about a curved baffle, but it seems to me it would work just as well, if not better. How about alignment #2 on a curved baffle? Too diffuse? Remember these are for the surround channels."

JY: The only way to know with an open baffle is to build and measure or listen. While wings can reduce baffle width for the same effective D, which may improve imaging, the box formed in the rear can introduce resonance. The nice thing with an open baffle is the forces are small, and you can just use cardboard. I like sheets of foam insulation myself.

"3. The info I have seen on baffle size says that to get 80 Hz, you need a 40 in./16 cm baffle, which seems huge, especially for a couple of 4 in. drivers. Is that measured from the center of the driver front to back? Does it really have to be that big? I assume you also measure over the top and around bottom too. There seems to be a lot of very technical discussion of this, but it seems to center on how to get bass."

JY: Sorry, it's worse than that; it's center driver to front edge. Getting the bass is the great dipole problem, but IMO it's the best bass you can get.

To find your answers to #4 and #5, look at FAQ # 12 on the Linkwitzlab website.

Good luck and Merry Chistmas!
Jason: Drinking old #7 in northern Vermont
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Old 27th December 2003, 08:28 PM   #5
MtnBob is offline MtnBob  United States
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Thanks, Jason - one question - You said
"You may want to low pass one driver at the frequency which has a wavelength that is twice as long as the distance between the drivers to reduce comb filtering."

If the drivers are only an inch or so apart, wouldn't that be a pretty high frequency? (Sometimes I wish I'd paid more attention in Physics)
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Old 28th December 2003, 11:50 AM   #6
diypole is offline diypole  United States
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Hello MtnBob,
I should have said the center to center spacing. For example, if your 4" drives are 4" apart center to center, than you might want to try a lowpass at 4*2=8" wavelength, speed of sound~ 13500"per sec/8"=1687.5hz.
This gives a rough estimate of where combfiltering begins, so lowpass somewhere under ths point. That being said, try it with one and see if it makes a difference to you. Maybe the best place to put the Lowpass with 2 fullrangers on an open baffle would be at the baffle step, which is the same point that dipole rolloff will begin. A 6db/oct lowpass one octave below the rolloff point would be a simplistic way to counter some of the rolloff without adding stress to the small drives. This will make for a warmer presentation. Best luck.
JasonY
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Old 29th December 2003, 03:20 AM   #7
MtnBob is offline MtnBob  United States
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Thanks, Jason. I figured it was something like that, but didn't know exactly what.

MB
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