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Old 27th August 2003, 09:18 PM   #1
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Default Dipolar sub- Magneplanar style

OK somebody posted something on doing a dipole subwoofer and it sorta got my juices flowing. My setup: Adire Tumult sub (site says its suitable for dipole use. My main speakers are already line source Magneplanar MGIIIA true ribbon/planar magnetic speakers. They stand 6 feet tall and are dipolar.

The trouble is right now I'm having a difficult time integrating my point source, omnidirectional monopole Tumult sub in a sealed sonotube sub with the speakers. It just doesnt sound "right". 50 Hz and below is boomy and there seems like there isnt much output above 60hz or so, so thers a hole in the response. I've tried both polarities of the sub in relation to the main speakers. I have a real time analyzer that helps especially when doing phase reverse tests. Integrating the subs just doesnt seem to be happening. Infact I had more luck with my 7 foot tall vented Cerwin Vega car sub in a sonotube for integration then this sub.

So, this brings me to my next thought. What if I made a dipole sub with the Tumult? This would be perfect as it is now my low end is already overpowering and I'm looking for some more upper bass response. That and the main speakers are already dipoles. This is my design plan. I would make it in the exact same shape as one of the Magneplanar speakers. I even have an extra grill cloth. I'd finish the trim and get extra feet so that it would look identical to the main speakers. The speaker is rather large, at 5'11 high and 2 feet wide. I havent done calculations but I think this would make an ideal dipolar sub. The woofer is rated to be suitable for dipolar use. I'm not really interested in doing a "box with an open back" design, but just a large board.

I've never used or experienced a dipolar sub. Ive read the links of the dipole sub FAQ here, but if theres any other readings or suggestions that would be wonderful.
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Old 27th August 2003, 09:24 PM   #2
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Hello -

In case you haven't seen it yet, one of the best places for information on dipoles is at:

www.linkwitzlab.com

Hope this helps,
Charles Hansen
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Old 27th August 2003, 09:55 PM   #3
SY is offline SY  United States
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Ditto Charles. Linkwitz's work is quite enlightening.

A conventional subwoofer can be mated with dipoles (I heard a nice combo last weekend, in fact), but it's a tough job. It takes a lot of moving of boxes around the room and time/phase compensation between the sub and the panels.
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Old 27th August 2003, 10:00 PM   #4
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Linkwitz- is this the same who was the HP engineer who did the Linkwitz/Riley Alignment?

Anyway, I was trying to read the technical articles and even though I'm an engineering student some of it blew me away still.
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Old 27th August 2003, 10:07 PM   #5
SY is offline SY  United States
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Same one.

Where are you in school? (UMBC, '77)
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Old 28th August 2003, 12:06 AM   #6
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You don't really need to build the panel that high (6ft) unless you are merely doing it for aesthetics. Something half that would be fine since the dipole roll-off starts at the shortest distance between the front and the back.

The roll-off should start at around 170Hz for your panel width.

You will need to equalize for this roll-off. Although there are other ways, which you can find here if you do a search for 'dipole' I think active EQ is easiest.

The cheapest active EQ is the Behringer DSP1124 Feddback Destroyer at around $100, or you could go to one of the combined crossover/EQ units like the Behringer DCX or the dbx Driverack PA.

I still suspect stereo subs will be better than one.

And you might want to consider how much movement a Tumult going full bore will give you when thinking of stands and supports etc.


Cheers

Steve
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Old 28th August 2003, 06:37 AM   #7
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Quote:
Originally posted by SY
Same one.

Where are you in school? (UMBC, '77)

Right now harford community college. I've been here a little too long, got my AS in business but now am trying engineering. Infact UMBC is where I plan to go to (try) and study mech engr. All these acoustics/waves things really fascinate me. I know of only a handful of acoustics based programs in the country though. I think would really enjoy working with speakers, sound and acoustics as a career.

Quote:
Originally posted by sfdoddsy
You don't really need to build the panel that high (6ft) unless you are merely doing it for aesthetics. Something half that would be fine since the dipole roll-off starts at the shortest distance between the front and the back.

The roll-off should start at around 170Hz for your panel width.

You will need to equalize for this roll-off. Although there are other ways, which you can find here if you do a search for 'dipole' I think active EQ is easiest.

The cheapest active EQ is the Behringer DSP1124 Feddback Destroyer at around $100, or you could go to one of the combined crossover/EQ units like the Behringer DCX or the dbx Driverack PA.

I still suspect stereo subs will be better than one.

And you might want to consider how much movement a Tumult going full bore will give you when thinking of stands and supports etc.


Cheers

Steve


Hmm interesting indeed. I am using an Audio Control Richter Scale, that has a graphic bass EQ and is a Linkwitz-Riley alignment crossover.

Other questions- placement of the sub I imagine is critical. Being dipolar, I imagine it needs to be a few feet away from the wall too?

Now thinking about it I might make the dipole baffle a sqare, probbly 2-3 feet with the driver mounted in the middle. Hmmm......
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Old 29th August 2003, 04:27 AM   #8
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The Audio Control crossover probably won't be ideal, but may be able to give you an idea of what you'll get with dipole bass.

As for positioning, you will need to keep them away from the back wall, but you can have them close to side walls.

I have heard somewhere that it is better to have the baffles slightly rectangular than completely square, but I could be imagining this.

Cheers

Steve
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Old 29th August 2003, 07:57 AM   #9
Kanga is offline Kanga  Australia
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Hi Eric

It sounds like your problem is problem one of positioning the sub in the room. I don't want to dissuade you from building a dipole if you really want to, but you may be able to fix the problem much more cheaply just by positioning the sub in a better location.

There is various info on how to locate a sub for a smooth response. A common suggestion is to put the sub where your listening point is and then walk around the room until you get the best response. Then put your sub in that location. I haven't used that method, but I have used CARA room optimisation software which is highly sophisticated, not expensive and gave very good response without having to lug speakers around.

If you're not sure if you want to buy the software you can send me the details of your room (if its relatively simple) and I can run a simulation for you. I'd be interested in your results.

Mick
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