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Old 18th May 2009, 03:47 AM   #1
Bent is offline Bent  Australia
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Default Dipole Ribbon tweeter isolation from dipole mid-woofer array

Hi, I tried to get the title close....

I am building a dipole line source speaker consisting of a 1.2m tall ribbon tweeter and 8 x 8" woofers.

The panel width is ~ 550mm and completely dipole

Since the ribbon will be mounted next to the dipole woofers, will I need to place a "cutout" next to the ribbon (between the 8" and the ribbon) to prevent the woofer cancellation wave from "flapping" the ribbon around?

I have seen this cutout in another system and assumed it was for this reason.

I am wondering if the ribbon is directly connected to the amplifer (active system) the damping of the amp might "dampen" the flapping and if the "flapping" will even happen regardless as it is just a theory I had.


Another question, regarding reflections:

With such a large baffle, is it worth covering the baffle areas around the ribbons and woofers with damping materials to minimize reflections from walls etc being re-reflected of the front (or rear) of the speaker to the listener?

Any advice welcome. I could build the speaker with the cutout and blank it off later and build with the damping material, but I would prefer to design it optimally from the beginning for best athetics.

Thanks in advance (and apologies to all who read this before I edited out the poor grammar)
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Old 19th May 2009, 12:11 AM   #2
Bent is offline Bent  Australia
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57 views and no comments....either the answer is obvious and I should work it out myself or no-one knows.......the former I can accept but the latter seems unlikely...

I can build it with the cutout and aestheically "block off" later and test, however with such a wide panel, it utilises valuable real estate - I would prefer making the panels narrower.

Is this thread in the wrong forum - it just dawned on me that that is another possibility

Thanks in advance.
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Old 19th May 2009, 01:31 PM   #3
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I'm not sure that the answer is obvious, and may depend on how close to the baffle edge you are going to mount your ribbon and what spl's you anticipate. If mounted very close to the edge, I would think that short circuiting the bass dipole with a baffle gap would not be necessary. However, if the ribbon is mounted towards the middle of the panel, it may become necessary in order to avoid intermodulation -- I suppose that enough bass energy could even drive the ribbon from its gap.

OTOH, if this were a significant issue, monopoles would be susceptible to the same effects, and no manufacturer that I am aware of has made an effort to address it. In fact, the usual practice seems to be to mount the drivers as close together as practical.

As for the damping material on the baffles, I think 1-2" of compressed fiberglass should pretty much kill reflections >500Hz or so.
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Old 20th May 2009, 08:37 AM   #4
Bent is offline Bent  Australia
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Thanks David for the reply.

The woofers xmax is ~ +/-6mm and they will be equalised to drive to their maximum down to 70Hz.

The ribbons are 20mm diameter, gap ~ 22mm.

I do not have a depth of audi understanding to draw a conclusion on this. I could always try placing a piece of paper near the edge of a woofer mounted to a board and run a tone through it.....

At least that eliminates the engineering ;-)

Thanks again, Graeme
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Old 21st May 2009, 12:17 AM   #5
Bent is offline Bent  Australia
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I have decided to do just that - place a piece of paper at the edge of the baffle insert. I have already made a 18mm Jarrah Baffle insert (1835mm high, 240mm wide) and routered the 8 speaker recess to make the flush with the surface so all I need to do is dummy fit up a few drivers.

It looks awesome by the way!

Ultimately the Jarrah board will be glued to 25mm or 32mm MDF however for the purposes of the experiment 18mm will be fine.

I will post the results in case anyone else is interested.
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Old 21st May 2009, 12:10 PM   #6
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Default I will post the results in case anyone else is interested.

Hi,

Please don't hesitate to show us what you've made.

It's always good to learn from each other.

Best regards,


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