any compromises in using Linkwitz W-frame dipole? - diyAudio
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Old 5th July 2006, 08:31 PM   #1
taloyd is offline taloyd  United States
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Default any compromises in using Linkwitz W-frame dipole?

Hello,

I'm building a dipole subwoofer for use strictly below 80 Hz, crossed 4th order. Drivers used are the AV15 15" aluminum subwoofer built by TC Sounds, 2 per side. EQ is a Behringer DEQ2496. Active crossover is a Rane AC23B.


Are there any compromises in using Linkwitz w-frame as seen here:

http://www.linkwitzlab.com/images/graphics/d_woof1.gif

as compared to a U-frame? I know the cavity resonance is much lower, and that reduces the useable frequency response, but that's not a problem in my application. My main issue with the design, and I'm not sure if this carries over in reality, is that both woofers are firing at a physical barrier directly in front/behind them that is parallel to the woofer, ie: perpendicular to the direction of the sound pressure.

Does this cause any problems? Is a U-frame superior in any noticeable way? The main reason I'd like to use the Linkwitz style box is:

1) the even-order distortion cancellation
2) the force-reduction due to mounting the woofers against each other (in terms of pressures applied)
3) achieving a smaller total size as compared to a U frame containing 15" drivers. A U frame with two 15" spaced as closely together as is reasonable is probably at least 35" x 18" (H x W) whereas a W-frame can be perhaps 20" x 20", which is much preferable.

Alternately, there is this configuration:

http://www.mfk-projects.com/images/dipole2.gif

But it wouldn't allow for the even-order distortion reduction, or the force cancellation. Again, worth it?

Just want to make sure before I take the plunge...


thanks in advance,

-Tal
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Old 5th July 2006, 09:53 PM   #2
Davey is offline Davey  United States
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Tal,

The dipole configuration on Monte Kay's web page would indeed allow for even-order distortion reduction. The wiring of the drivers would be such that one cone would be moving towards its magnet and the other cone away from its magnet. There is also still force cancellation at work, although it's not a full 180 degrees in direction as in the "W"-baffle concept.

The cavity resonance freqquency from a W-frame configuration would generally be above 200Hz for typical dimensions. That would seem to be well above your area of possible usage.

I like the W-frame configuration primarily because of the force cancellation aspect. This will impart significantly less vibration and buzz into listening room floor construction than the alternative. Of course, if you listening room floor is a slab this won't be an issue.

Your project sounds viable to me and it sounds like fun.

Cheers,

Davey.
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Old 6th July 2006, 06:05 AM   #3
sqlkev is offline sqlkev  United States
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I built the w frame dipole with the drivers recommended in the original Phoenix design. What would be the advantages, if at all, when I go with a H-frame dipole?
I want a bit more output from the w-frame.
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Old 6th July 2006, 08:02 AM   #4
Rudolf is offline Rudolf  Germany
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Tal,

before delving into force cancellation et al. you should think about the main difference between U- and W/H-frames:
While H-frames are true dipoles and W-frames come close, the U-frame is by definition a cardioid, but no dipole!
Both have different advantages/shortcomings mainly with regard to room response.
So I recommend to find out first, if you want/need a dipole or a cardioid subwoofer.

Rudolf
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Old 6th July 2006, 02:14 PM   #5
Davey is offline Davey  United States
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sglkev,

The H-frame setup won't yield any more output than the W-frame. The drivers excursion and Sd are the limiting factors in SPL capability.
If you need more output your best bet would be to build an additional two woofers and stack on top of the originals. That will give you a full 6db more output. The original Madisound drivers are no longer available unfortunately, but there are some alternatives.

An example stacked setup is shown at the bottom of this page:

http://www.linkwitzlab.com/woofer3.htm

Davey.
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Old 6th July 2006, 04:05 PM   #6
taloyd is offline taloyd  United States
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Default dipole vs. cardioid

hello,

In regards to that discussion, I'm mainly interested in a velocity-source vs. pressure source subwoofer. Additionally, as I don't have too much room to play with, a cardioid may perform better when closer to the wall.

But the main thing I'm going for (as I think all implementations will sound fantastic) is a compact box, within reason.

thanks,

-tal
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Old 6th July 2006, 05:08 PM   #7
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Quote:
Originally posted by Davey
sglkev,

The H-frame setup won't yield any more output than the W-frame. The drivers excursion and Sd are the limiting factors in SPL capability.
I think you're missing the effective path separation.
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Old 6th July 2006, 06:14 PM   #8
Davey is offline Davey  United States
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No, I didn't forget about that. But I was making an assumption that it would be the same or nearly the same.

Davey.
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Old 6th July 2006, 08:06 PM   #9
Rudolf is offline Rudolf  Germany
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Default Re: dipole vs. cardioid

Quote:
Originally posted by taloyd
But the main thing I'm going for (as I think all implementations will sound fantastic) is a compact box, within reason.
Tal,
I would go for a U-frame. Some reasons:
For the same output the U-frame builds half as deep as the equivalent H-frame.
If you need the sub closer than 1 m to the front wall, the U-frame will profit from a wall gain, the dipole will not. Worse: the dipole will suffer from early reflections, that cancel much bass energy.
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Old 6th July 2006, 09:57 PM   #10
ScottG is offline ScottG  United States
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Here is my take on this:

1. A cardoid "subwoofer", U frame or not, acts much like normal "omni" design above about 50 Hz. This could give you a bit more "punch" in the cavity resonance region. Around 50 Hz to about 25-20 Hz the cardoid acts like a cardoid. Below this region the cardoid starts transitioning to dipole radiation.

2. Dipole "subwoofer", suffers from reduced spl across its bandwidth whereas a cardoid is about 5-6 db louder in its cardoid and omni operation region.

3. Both designs operating in free-air are velocity type as opposed to pressure type designs - which should help considerably with linear decay.

4. Proper rear wave dampening of either an H or U frame will give not only a cardoid character (as mentioned in #1), but will also effect system compliance - lowering the impeadance "bump" of the driver's fs (..like an aperiodic box). This, and the velocity character, is probably the most benefical aspect of this design at least in the freq. range you desire. Doing so allows greater amplifier control over the driver at and near fs.

Notice what GaryP has to say about rear-wave dampening here:

dipole stereo bass?


5. The H frame allows for non-linear 2nd order reduction (and the U doesn't). Note however that the Linkwitz H frame is compromised for both even AND odd order non-linear distortion. The "fix" to this is to "tightly" couple the drivers frames together, AND do NOT couple the H frame baffle to the drivers.

Notice what GaryP has to say about baffle vibration and non-linear odd order distortion here:

dipole stereo bass?

Though it was a U frame - an H frame will still have some of these problems if it isn't decoupled from the driver.

With particular emphasis on #4, AND that #5 can include #4 - I'd suggest an H* frame cardoid with the appropriate rear wave dampening. It should give you the best of everything with only a modest increase in H frame size (where the rear is slightly increased to allow room for a few spaced fiber partitions from some modest "batting").

*Note that you do not have to keep equal path lengths in this design, i.e. you might just as well "clip" the front extension of the H frame.
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