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bitumen sheets for panel dampening
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SaSi
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PostPosted: Fri Jan 29, 2010 3:44 pm    Post subject: bitumen sheets for panel dampening Reply with quote

I am currently shopping for a roll of bitumen (tar) sheet to dress up the panels of a test enclosure.

I can only find the variety with small stones impregnated in the tar. I was actually looking for the plain bitumen sheets, but appear to be hard to find.

I am wondering if the sheets with small stones will be better or worse for the dampening application. They probably have more weight for the volume they occupy but perhaps the stone fragments have their own nasty resonances.

I would appreciate your insight.
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T.O. Chef
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PostPosted: Fri Jan 29, 2010 8:41 pm    Post subject: Reply with quote

Don't know ... these may be too far away for you Sasi.
Heavy stuff. Expensive to ship.
Perhaps there's a Solen distributor closer to you who stocks it?

http://www.solen.ca/pub/cms_nf_catalogue_fiche.php?id=1607&recherche=&numRows=&manufacturiers=&niveau1=1&niveau2=2&niveau3=50

It is however very close to what I've seen in 70's KEF systems.
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SaSi
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PostPosted: Fri Jan 29, 2010 9:45 pm    Post subject: Reply with quote

I have seen equivalent product on bitument sheet manufacturer sites and catalogs. Plain tar adhesives with a 3mm thickness. They go for 9 euro /sqm. Problem is find a yard that stocks them. I only need a roll of 25 meters long by 1 meter wide and it will last me I don't know for how many units.

BTW, I have noticed that the bitumen sheets inside a R105.1 bass enclosure are very hard and fragile. One of the pads was deformed and an edge was hanging. Upon trying to gently push it back against the panel a small piece of the edge broke off.

I am suspecting that these are not doing what they are supposed to be doing.

Do these pads have a life span? Has anyone replaced bitumen pads in the process of restoring a 35 year old enclosure?
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T.O. Chef
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PostPosted: Sat Jan 30, 2010 3:20 am    Post subject: Reply with quote

I'm about to change some Sasi. In some 104aB's.
The stuff does seem dry out with time. I've experienced exactly what you have.
I can't say if there's much if any effect though, unless it no longer attached at all.
I'll let you know when I've done it whether there's any audible change.

As for Solen, I'm sure they'd ship to you but for that size and weight you want expect to pay big for it.
Ouch.
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Lee in Montreal
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PostPosted: Sat Jan 30, 2010 3:42 am    Post subject: Reply with quote

Car audio uses a lot of damping tar. Imagine what it requires to make automotive sheetmetal more neutral... Therefore, one more source for sticky damping material could be anyone's local car audio shop.

Look for products such as Acoustimat or Dynamat.
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SaSi
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PostPosted: Sat Jan 30, 2010 11:50 am    Post subject: Reply with quote

Lee, thanks for the idea. I have two car audio shops in close proximity to the office. One is "high-end" the other one vanila flavour. They might stock the stuff or point to a supplier.

Chief, keep us posted on these 104ab. I just fit new foam inside the 105s and now work on the mid/hi freq enclosures. And I have a pair of Calindas that I re-veneered (they were painted yellow).

Please let us know how to remove the old pads. There has to be glue and I can also see staples. Removal can become messy business.

As an alternative, some 30 years ago, with my father's help, we built a pair of CS1s with the 105 heads looks. Used 20mm birch ply for the enclosure and 10mm rubber sheet glued to the inside for dampening. They were - and still are - pretty neutral sounding.

And enclosure colorations are very audible. When checking everything on the first 105 under restoration I only half tightened 2 of the 8 screws holding the back of the B110 enclosure to the head. The sound was horrible to the point I thought the driver was faulty. When all 8 screws were tightened the sound changed dramatically (to the better).

This makes me wonder if my 104ab, Calindas and especially 105s can sound a lot better if the dampening is corrected.
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T.O. Chef
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PostPosted: Sun Jan 31, 2010 1:23 am    Post subject: Reply with quote

Ahhh ... how I've struggled with this cabinet resonance thing.
Some Ditton 66's that I thought would benefit from the addition of panel damping resulted a negative effect. They seem to NEED to vibrate all to heaven.

I'll be sure to let you know how easily the 104's damping comes off ... but removal of the 66's stuff took a layer of veneer with it in spots.
If faced with that scenario I'll abandon the idea.
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proffski
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PostPosted: Sun Jan 31, 2010 4:26 pm    Post subject: Reply with quote

The damping panels may well reduce resonances at a band of frequencies, but you are still adding mass, mass = inertia = energy storage.

So removing annoying artefacts at one frequency may well shift them to another and perhaps more annoying.
Rigidity is but one answer, oh the black art of loudspeaker design...

KEF designers would not have added something that costs money if it was not needed!
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SaSi
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PostPosted: Sun Jan 31, 2010 5:11 pm    Post subject: Reply with quote

From my reading and research, I understand there are two schools of design for enclosures.

First approach is to make an enclosure as stiff and rigid as possible. Thick walls and ample bracing are the tools. The panels are not allowed to resonate at low frequencies. But the stiff construction moves the resonance frequency higher in the band.

Second approach is to use rather thin enclosure panels, basic bracing and dampen the enclosure vibration using tar pads. The resonance stays low but is impeded by the addition of dampening.

There is an argument that the first approach results in heavy and large (for their effective volume) enclosures that resonate at higher (midrange) frequencies where the human ear is more sensitive. The second approach is (IMHO) more elegant.

The counterargument is that moving the resonance frequency of the bass enclosure in the midrange band and mechanicaly isolating the midrange driver results in a system where the resonance of the bass enclosure won't be triggered by the midrange.

However, the midrange is there to produce sound so no matter how well the enclosures are isolated, they produce acoustic output in the same room, so the trigger remains.

So, the bottom line is, there is nice engineering knowledge available but the art of putting things together is what makes a good loudspeaker system.
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proffski
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PostPosted: Sun Jan 31, 2010 5:21 pm    Post subject: Reply with quote

Get the latest top of range Quad Electrostatics, problem solved! Smile
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SaSi
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PostPosted: Sun Jan 31, 2010 8:16 pm    Post subject: Reply with quote

proffski wrote:
Get the latest top of range Quad Electrostatics, problem solved! Smile

This is cheating Wink
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DrBoar
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PostPosted: Thu Feb 04, 2010 9:40 pm    Post subject: Reply with quote

Bitomen increase mass but not stiffness so the resonance frequency has to go down. Used by Spendor and LS3/5A where the midrange is the holy grail.

The price to pay is a more resonanses in high bass lower midrange. So first braces then perhaps some pads Very Happy
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proffski
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PostPosted: Fri Feb 05, 2010 12:07 am    Post subject: Colouration... Reply with quote

I'll keep dreaming about the top of the range Quad Elecrtrostatics or the Martin Logans then!
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iso
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PostPosted: Sat Oct 08, 2011 1:25 pm    Post subject: bitumen pads Reply with quote

I used ordinary roofing felt. The one used as first layer, as these have less sand attached on these. You can glue it with bitumen glue that is intended for gluing it around pipes etc... or use heat gun or torch to melt one side of felt and then attach felt on wood or or or other layer of felt, if multiple layers are used. I used 3 layers according BBC bulletins.

Availability of this stock sholuld not be problem and it is quite economical to use. It is of course quite messy to use compared to self adhesive sheets.

I believe it will aadhere better than selfadhesives as it resist sun and ice on roofs for 15-25 years.
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audiolabtower
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PostPosted: Sat Oct 08, 2011 7:53 pm    Post subject: Reply with quote

From the BBC monographs the damping pads had a small effect of shifting panel resonance frequencies downwards, but a very large effect of reducing the Q of the resonances - effectively to below audibility in the midrange. This meant that very much more energy was required to stimulate those frequencies with compared to without the pads, perhaps orders of magnitude. The low bass resonances were lower in subjective colouration according to their many "tweaking" listening tests of monitors versus live orchestras. The midrange (and male voice) colouration was critical for an uncoloured sound and pinpoint image and depth in otherwise identically matched speakers.
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