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Old 30th September 2004, 10:07 PM   #1
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Default Have TUMULT ... will Di-pole

Greetings


For the last year and a half, I've been happily enjoying my single tumult in a sealed 120L enclosure.

There seems to be a 'cult-like' following indicating that di-pole configuration is the way to go for bass nirvana.

I would like to try this out for myself.

I am looking for a design to build a di-pole sub with one tumult driver.

I have read countless threads, visited linkwitz's site and I am now ready to start the process. Hopefully with some help from this forum I can build an awsome di-pole.

First question, the 'H' design. Can one use a cylinder with the driver mounted in the centre (half the length) to obtain a '3D' H ? Effectively the driver would 'see' the same 90-degree baffle.

Also, how do you determine the ideal dimensions for the H ?

Will I need an EQ?

Thanx in advance

Dan
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Old 1st October 2004, 03:29 AM   #2
Bill F. is offline Bill F.  United States
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Hi Dan,

A Tumult sounds like a good main ingredient for your first dipole recipe.

However, I'd say ixnae on the ylindercae. By positioning your driver at a cylinder's midpoint, you're pretty much getting the worst of both or however many worlds. Not only is the front-to-back path length identical in all dimensions, guaranteeing you a sharp peak followed by deep nulls, you'll also be begging for cavity resonances unless you lowpass it early.

These are the same reasons why round baffles with centrally mounted drivers are generally to be avoided, as are unnecessary cavities--again, unless you're planning to cross very low and steeply. To spread Fp, you want a variety of front-to-back path lengths. This is why people on this forum often arrange their drivers off-center in flat baffles. I've often thought a driver mounted in the center of a star-shaped baffle would be pretty ideal since the deep cuts and long legs of the star would present such a great range of front-to-back path lengths.

Obviously, you don't have to go to extremes, but just don't forget that there is wisdom in a multitude of path lengths. ...Did that sound a little Buddhist to anyone else?
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Old 1st October 2004, 03:51 AM   #3
ghemink is offline ghemink  Netherlands
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Quote:
Originally posted by Bill F.
Hi Dan,

A Tumult sounds like a good main ingredient for your first dipole recipe.

However, I'd say ixnae on the ylindercae. By positioning your driver at a cylinder's midpoint, you're pretty much getting the worst of both or however many worlds. Not only is the front-to-back path length identical in all dimensions, guaranteeing you a sharp peak followed by deep nulls, you'll also be begging for cavity resonances unless you lowpass it early.

These are the same reasons why round baffles with centrally mounted drivers are generally to be avoided, as are unnecessary cavities--again, unless you're planning to cross very low and steeply. To spread Fp, you want a variety of front-to-back path lengths. This is why people on this forum often arrange their drivers off-center in flat baffles. I've often thought a driver mounted in the center of a star-shaped baffle would be pretty ideal since the deep cuts and long legs of the star would present such a great range of front-to-back path lengths.

Obviously, you don't have to go to extremes, but just don't forget that there is wisdom in a multitude of path lengths. ...Did that sound a little Buddhist to anyone else?

I think you should use the dipole woofer only at frequencies below the resonance point. Linkwitz is doing that as well. Myself have a dipole woofer in a H-frame and it has total dept of 40cm, so 20cm at both sides. It has a nasty resonance at 200-250Hz or so but if you filter well below that, say 120Hz or so with sufficiently steep filters, then you don`t see that resonance. Of course you need EQ to correct for the 6dB/Oct roll-off that you get with a dipole.

Have fun

Gertjan
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Old 1st October 2004, 04:09 AM   #4
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Thanx for the replies


I find this baffle stuff somewhat baffling.

Bill F.

I would only have a baffle diameter large enough to mount the driver onto. The cylinder enclosure I envisioned I read from the Linkwitz site He uses a cylinder for his analysis of the H-frame dipole woofer. I guess it could be a square. Linkwitz states: “L=d1+d2 and width and height W”. When width = height the cross-section is either a square or a circle. From his graphs with his dimensions he shows a +6db peak at 283Hz. If I use similar dimensions I should receive similar results (in free space that is). And as quemink suggests, the sub will be crossed over 24db per octive below 80Hz so the peak shouldn’t be an issue.

So, the only filter requirement would be to replace the 6db/oct that i will lose because of the configuration? A BFD could do this, no? or I guess i could modify my AVA250 plate amp.

Dan

Dan
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Old 1st October 2004, 07:42 AM   #5
tg3 is offline tg3  United States
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You should also look into the U-frame dipole design by John Kreskovsky.


Click the image to open in full size.


And the Tumult should be just great for this application.
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Old 1st October 2004, 08:29 AM   #6
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Is infinite baffle an option for you? If you *do it right* you can get all of the performance or a dipole without losing output.

* ie. room treatment and / or room eq and experimenting with positioning

Have you tried the tumult out of the box for turbulence? I find it hard to imagine that you could use all that excursion without a lot of turbulent airflow.
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Old 1st October 2004, 10:57 AM   #7
ghemink is offline ghemink  Netherlands
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Quote:
Originally posted by tg3
You should also look into the U-frame dipole design by John Kreskovsky.


Click the image to open in full size.


And the Tumult should be just great for this application.

Yes, the U-frame is very interesting as well, I was thinking of doing an U-frame for my next project. The advantage of the U-frame is as you can see that you radiate less to the back so the reflections from the back-wall will be less disturbing. You also should get a maxiumum of 6dB more output for the same cone movement. So more soundpressure before bottoming.

To get the best effect, you have to make sure that at the back of the speaker, sound from the back side of the woofer and the front side at the same time. You can see that the path length from from front of the woofer to the back of the enclosure and from back of the woofer to back of the enclosure is longer. So I guess it would be best to place a sort of labyrint in the enclosure so that the back sound arrives at the same time as the front sound. So basically you create a very short transmission line at the back.

At least, the above was my plan, hane not build it yet.

Best regards

Gertjan
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Old 1st October 2004, 03:29 PM   #8
tg3 is offline tg3  United States
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Quote:
Originally posted by paulspencer
Have you tried the tumult out of the box for turbulence? I find it hard to imagine that you could use all that excursion without a lot of turbulent airflow.
From the Adire Audio web site:

All of this is attached to a 12-spoke cast aluminum basket; combined with the 1.375" diameter pole vent the Tumult has no audible air noise in operation. The ultimate speaker for dipole use (where high excursion is mandatory).



Click the image to open in full size. Click the image to open in full size.
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Old 1st October 2004, 03:35 PM   #9
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Paul

The IB has definite appeal to me. The problem is where do you cut the hole for the driver, ceiling? floor? wall?. And at which location on the ceiling/floor/wall? Is this an important consideration?

I guess I do have a old intake vent on the floor in the corner I could experiment with....anyone using the basement won't particulaly like it though


The U-Frame seems like a good choice as well.

Damn too many choices.
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Old 1st October 2004, 04:45 PM   #10
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Quote:
Originally posted by tg3
From the Adire Audio web site:

All of this is attached to a 12-spoke cast aluminum basket; combined with the 1.375" diameter pole vent the Tumult has no audible air noise in operation. The ultimate speaker for dipole use (where high excursion is mandatory).
I have heard similar comments about my sub driver - the Stryke AV12 but I can tell you it isn't silent when moving with 10 Hz test tones, far from it. Normally with music this won't be a problem as turbulence will be masked, but I would prefer "silent." The AV12 has a similar basket design - very open with a decent size vent. I doubt that the Tumult could be silent given it has so much more excursion.

You can cut the hole for infinite baffle in any part of the envelope. Your choice will be determined by:

*how it couples to room modes in that position
*aesthetics
*practicality
*sound isolation (if you have a tiled roof space behind it and neighbour issues then you might consider your options)
*vibration issues

You can actually make a very large sealed box in the room - if it is large enough relative to the VAS then it may be considered IB still.

IB drivers are normally mounted so that they cancel the mechanical vibration that is transferred to the structure to a certain extent, so you may consider using two Tumults.

IB is a very interesting area and I suggest you do some searching both on google and on this forum.
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