damping the rear wave of a dipole woofer - diyAudio
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Old 10th December 2004, 08:49 AM   #1
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Default damping the rear wave of a dipole woofer

Probably this forum is more the place for a dipole specific question -

If damping – significantly - the rear wave of a dipole woofer, doesn’t much benefit directivity or efficiency, may as well do a M or W baffle over a U frame,

http://www.diyaudio.com/forums/showt...&postid=529961

anyone have a view on the trade-offs of heavy rear damping?
Other related thoughts welcome . .
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Old 11th December 2004, 07:52 AM   #2
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Default range of absorption coefficients at different Hz

I’m also now wondering about the benefit of damping the low end of a dipole's *midrange, as an alternative to active equalisation (a la SL designs).
Denser and thicker fibreglass absorbs more at low frequencies eg Fibreglass Blanket 32-100 (obvious really).
From http://www.bobgolds.com/AbsorptionCoefficients.htm I learnt there’s a *huge range of materials with different patterns of absorption coefficients over the frequency range, from around the world.
After massaging the data in Excel, I compared the absorption at 250 Hz with absorption at 1000 Hz - the range of relative absorption coefficients of products listed there, is from 13% to 333%!

The table on Bob’s web site give much more confidence that a baffle can be built then measured, and later tuned to reasonable flatness with a degree of accuracy of finding the right material, to give an appropriate degree of *passive equalisation.
If anyone wants the spreadsheet making analysing the data in Bob's many tables, let me know (or tell me how to upload it!).
Cheers
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Old 14th December 2004, 04:10 PM   #3
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(sorry for my english)

very interesting tabulate, but I think it is "optimistic"
4-5 inch can't stop the 125 Hz wave!
Look at the anechoic chambres: there is 3-4 feet minimum of glasswool -rockwool .
Very pragmatic : put a minimonitor into a "box" made of these absorbing materials and listen the result. It's hard to stop a tweeter!
Again , do you remember the first 4 way NAUTILUS B&W ?
The idea is great but people show to understand nothing ( audiophiles loves dipoles ......)
Regards
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Old 15th December 2004, 02:16 AM   #4
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The Dahlquist DQ-10 had an open midrange unit (about 4-5" as I recall) with a heavy felt backing. It can be done, at least for higher frequencies. For woofers, it's a more difficult proposition, due to the wavelengths involved.

Grey
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Old 15th December 2004, 02:24 AM   #5
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Default damping the rear wave of a dipole *mid

John k’s NaO woofer gives more efficiency by damping the rear wave (and a U-frame gives a longer path for the sound than a H frame).

I’m waiting to find out why he did not also use a damping + U frame for the *mids, to boost their sensitivity too?

By comparison, Linkwitz uses EQ of 6 dB/ oct roll-off, of the midrange dipole response, with no damping. For a simpler passive midrange, careful choice of damping material could approximate the EQ.

Thanks
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Old 20th December 2004, 02:27 PM   #6
Davey is offline Davey  United States
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rick57,

I think you're missing the whole point of these designs. One of the key design objectives of the Linkwitz systems is to create a much more even power response than a conventional speaker would have. If you start damping the backwave of the mids you're on your way to creating another box system. Sacrificing even power response for a sensitivity gain is not a trade-off Siegfried would want. I am assuming this was John K's objective with the mid-panel of his system as well.

The U-frame approach to the woofer system is a slightly different subject and is well documented on John K's website.

Cheers,

Davey.
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Old 24th November 2013, 11:06 AM   #7
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Anyone tried something like this in the past 10 years this thread is alive?

I was thinking of making an enclosure for a dipole sub-woofer with a 15 inch driver that is suitable. The enclosure should be 50 liters (1.7cuf) in a U shape. The rear of the box, the full 50 liters should be filled with 2-4kg (4-8lbs) of dampening material, and the rear will be half-open with a wire mesh to hold the material.

This should be a box that is between IB and dipole. The Box of 50 liters should virtually have a pressure of at least 500 liters or close to IB. But the rear wave will escape (although dampened) from the rear.

The box should look like this:
Click the image to open in full size.

I am sure that someone has thought of this, but I cant seem to find anything on this subject.
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Old 24th November 2013, 12:15 PM   #8
sreten is offline sreten  United Kingdom
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Music and Design

Hi, damped U frames are covered in some detail on the above site, rgds, sreten.
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Old 24th November 2013, 10:57 PM   #9
peril is offline peril  Switzerland
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Hi
do you now this speaker?

ME Geithain RL 901K Test::Bonedo
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Old 25th November 2013, 11:51 AM   #10
Juhazi is offline Juhazi  Finland
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Kimmosto has done extensive studies with cardioid bass
DIY archive of Kimmo Saunisto

He is now a fan of aperiodic enclosures. This craphics tells how much a cardioid's response changes in a room with different placement! I would say that this is as good as it can be!
Click the image to open in full size.
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