fibrous damping of diaphragms - diyAudio
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Old 30th November 2012, 02:18 AM   #1
Few is offline Few  United States
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Default fibrous damping of diaphragms

In my efforts to distill some of the best of what's been developed in planar speakers (electrostatics, simple ribbons, planar magnetics, etc....) I've come across a few examples of builders---DIY and commercial---using fabrics, felts, and absorbers of various kinds, to improve their speakers' performance. Usually absorbing sound or damping diaphragm resonances is the goal. I'd like to determine which approaches have been demonstrated to be most effective.

Bohlander-Grabener uses non-woven fabric between their diaphragm and magnets to damp resonances. I've not seen a clear statement about whether the fabric is in contact with the diaphragm or is just acting as an absorber of air-borne sound. I would be very interested if anyone has inside knowledge.

Infinity used fibrous layers to improve (by their measurements) the performance of their EMIM and EMIT drivers. They used it to damp diaphragm resonances and control directivity:
[URL="http://www.davidsaudio.com/Infinity_Epsilon_WhitePaper.pdf"]http://www.davidsaudio.com/Infinity_Epsilon_WhitePaper.pdfi/URL] It looks to me like their diaphragms were rather thick (heavy) but they certainly spanned the intended bandwidths and then some. I've never heard their cost-no-object efforts so I don't know how their performance stacks up against newer speakers, especially those that have caused me to stop in my tracks and think something truly extraordinary is going on. How do the best of the old IRS speakers stack up against the slightly less old Apogees, recent Magnelanars, or the best of the electrostatic speakers?

VMPS developed a fibrous front panel add-on that absorbs high frequencies radiated by the perimeter of their planar magnetic drivers while allowing the high frequencies to pass unattenuated through a central vertical slot. They claim very low insertion loss (meaning the overall sensitivity of the driver isn't adversely affected too much), more uniform directivity, and better imaging as a result. I've not heard any of their speakers, and I just read Brian Cheney is having health troubles so the future of VMPS is in question; perhaps I've missed my chance to hear them!

I'm aware of some of the diyaudio threads on using fabrics to damp low frequency ESL resonances, and not overly tensioning ESL diaphragms to minimize the magnitude of their resonances. Has anyone done any experiments using sound absorbing fabrics or foams to control directivity or damp ribbon (or "quasi-ribbon") diaphragm resonances?

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Old 30th November 2012, 05:19 AM   #2
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You might also have a look at the RAAL ribbon graphs with/without the foam pieces that they put over the ends of the ribbon.

Haven't done any experiments myself yet, I'm afraid.
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Old 30th November 2012, 03:14 PM   #3
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Right--thanks. I forgot to include that example in my non-comprehensive list. I should probably review any information they have online.

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Old 30th November 2012, 06:14 PM   #4
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most of the camping you are really needing with planar design, electrostatic or magnetic or with ribbons will be the cavity directly behind the film. I have damped esl panels with non woven material but I find wool felt the best. I am also damping the insides of my compression driver mid and tweeter drivers to very good affect with Dynamat and SAE rated F-11 acoustical felt. Why don't you play with some and I think that you will get a handel on it very quickly. I don't think that you want to have any damping material in contact with ant diaphragm as you will simply either cause buzzes and add mass. The idea is to use diaphragm which can be mass controlled by the air load. It is the cavity and reflections of the structure which cause the problems. Good luck and best regards Moray James.
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Old 30th November 2012, 06:47 PM   #5
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As Moray James noted, be careful with things like felt. It has rather thick strands or hair which(and even one!) will buzz if it gets to contact the film.
At least I have experienced this while in long in and difficult journey in making the ESLs quiet while doing sine sweeps
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Old 1st December 2012, 08:48 AM   #6
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Quote:
I've not seen a clear statement about whether the fabric is in contact with the diaphragm or is just acting as an absorber of air-borne sound. I would be very interested if anyone has inside knowledge.
I've dismantled both Neo3 and Neo8, the fabric is glued to the magnets and is not in contact with the membrane.
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Old 1st December 2012, 02:53 PM   #7
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As far as I know, use of fibrous material for damping membrane modes is always done by placing it close to, but not touching, the diaphragm.

However, in an interview with Ross Walker, there is reference made to damping with a coating applied to the diaphragm.
I could imagine this helping perhaps with the "crinkly" nature of Mylar, but not with the diaphragm modes.

"...Secret ingredient X is used as a damping layer on the Mylar..."
QUAD Interview
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Old 2nd December 2012, 11:18 PM   #8
Few is offline Few  United States
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Thanks, all, for the helpful replies. I'll report back with my own damping experiences once I get that far.

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Old 6th December 2012, 04:16 PM   #9
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Add Acoustat to your list of manufacturers that used damping on their ESL's. Three 1/4" thick felt pads were applied to the rear of each ESL panel, one about 2" wide in the center of the panel, and two 1" pieces above and below the wider piece. The intent was to dampen diaphragm resonances.

If memory serves correctly, the felt used by Acoustat was a hyrid of wool and rabbit fur. Sounds exotic, but apparently rabbit fur is a common component of some felts. The felt was adhered to the rear of the plastic stator supports with a contact adhesive, and I do not recall any reports of problems with felt fibers migrating into the ESL gap. I think the contact adhesive helped with 'nailing down' any loose fibers on the panel-side of the felt.
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Old 6th December 2012, 07:16 PM   #10
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The felt that Acoustat was used intended to add a resistive air load to the diaphragm to help improve stability which the panels lack due to too little tension too wide a diaphragm span and too great a diaphragm to stator spacing. Since Acoustat used DuPont HS65 which is a heat shrink Mylar film they were limited to the amount of tension that they could generate. Their fix was the felt pads. Masking tape or duct tape works about the same as the felt does. If you want to get fancy you can you silk screen mesh. Best regards Moray James.
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