damping the rear wave of a dipole woofer

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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%!
:cool:
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
 
(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
 
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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Joined 2001
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.
 
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:
lEWh6Az.png


I am sure that someone has thought of this, but I cant seem to find anything on this subject.
 
Thanks Juhazi - am interested in Kimmo Saunisto's work - but the knowledge level for entry is probably a little high for me at the moment, also very few people seem to have built from his information - I think I would only attempt once the design had been validated by someone else (i.e. a build and review here on DIY Audio).
 
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