3-5 mm bitiminous felt ? - Page 3 - diyAudio
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Old 6th March 2008, 05:52 PM   #21
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Thanks Blue

I think I can get this wrapped up now well before my St Patricks Day Party ! You have all been a big help.
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Old 7th March 2008, 12:56 PM   #22
sreten is offline sreten  United Kingdom
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Hi,

As stated contrained layer damping works best with thin walls and
generally these are not braced, the point being to drop the main
resonances as low as possible.
Trying to constrian damp ever thicker walls is an exercise in futility.

The design calls for 5mm panels. 2/3 layers of bitumen roofing felt
could be used, attached with water based contact adhesive and
possibly some staples for a belt and braces approach, but the
adhesive between layers must not be omitted.
Usually for a vented box around 25 to 30mm of acoustic foam
lining is then used. BAF bulk stuffing can be used in areas away
from the port for fine tuning of the bass end. I would add two
layers of acoustic foam to the brace to help damp the main
cabinet vertical resonance.

It is probably not a good idea to increase the 18mm sides to
25mm,there is probably some resonance spreading designed
in, this would bring them closer together.

FWIW the panels do not need to completely cover into the corners,
a border around each one makes fitment a lot more straightforward.

/sreten.
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Old 7th March 2008, 12:59 PM   #23
sreten is offline sreten  United Kingdom
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Quote:
As to the bitiminous felt, if you must, then find a thick dense layer of felt, but skip the bitiminous part.
Hmmm.... seems you have no idea what the purpose of this layer is... /sreten.
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Old 7th March 2008, 01:41 PM   #24
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Hmmm.... seems you have no idea what the purpose of this layer is... /sreten.

Well, I would think that would be fairly obvious.

Here is part of my problem, is this bituminIZED felt, meaning felt impregnated with a degree of bitumen, or does in simply mean dense thin felt meant to be used with bitumen?

It is bituminized felt that I mostly object too; in other words 'tar paper'. I don't see how you can glue something that is impregnated with a petroleum product to wood. And, I don't really think I want a petroleum impregnated substance inside my speakers.

I'm also unclear what you mean by 'damp the panels' or 'constrained layer dampening'. If it simply means keep the sides from vibrating, I can see other less toxic ways of doing that.

I can see how dense 'bitumin felt' could be used as a sound proofing layer. It seems dense enough to not allow sound to penetrate, but again, there seem to be less toxic ways of doing that.

Still, the original poster seems to have made a decision, and is proceeding forward, so hopefully our discussion has help him along that path.

Cheers.

Steve/bluewizard
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Old 7th March 2008, 02:59 PM   #25
sreten is offline sreten  United Kingdom
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Hi,

Classic bitumen damping panels are multiple layers of heavily impregnated
bitumen felt layers, a 1cm thick panel having about 11 felt layers.

The original BBC research paper is here :
http://downloads.bbc.co.uk/rd/pubs/reports/1977-03.pdf

Often misinterpreted as much as being understood.

/sreten.
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Old 7th March 2008, 03:58 PM   #26
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Quote:
Originally posted by BlueWizard
Well, I would think that would be fairly obvious.


sreten has done a nice job of explaining the more technical aspect of it.

Quote:
Here is part of my problem, is this bituminIZED felt, meaning felt impregnated with a degree of bitumen, or does in simply mean dense thin felt meant to be used with bitumen?
Here's where I can help. Impregnated felt (what it's called in Canada) is just that, an organic felt made from old clothes and other things impregnated with asphalt (most common) or coal tar pitch. Used almost exclusively for flat roofing.

Quote:
It is bituminized felt that I mostly object too; in other words 'tar paper'. I don't see how you can glue something that is impregnated with a petroleum product to wood. And, I don't really think I want a petroleum impregnated substance inside my speakers.
It is not tar paper. Paper and felt are two different things. It can be glued or stapled to the inside of your box. There are a few adhesives that will work.

Quote:
I'm also unclear what you mean by 'damp the panels' or 'constrained layer dampening'. If it simply means keep the sides from vibrating, I can see other less toxic ways of doing that.
Constrained layering is making a sandwich (usually) with an absorbent material in the center to do the damping.

Quote:
I can see how dense 'bitumin felt' could be used as a sound proofing layer. It seems dense enough to not allow sound to penetrate, but again, there seem to be less toxic ways of doing that.
It will absorb the sound and convert it to heat. It is stable, has virtually no fumes and really should be considered non-toxic unless you plan to eat it. This is not the same as the peel and stick membranes which have a high solvent count and will gas off over time possibly having a detrimental affect on your speaker guts.

Quote:
Still, the original poster seems to have made a decision, and is proceeding forward, so hopefully our discussion has help him along that path.
Let's hope.
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Old 7th March 2008, 05:45 PM   #27
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Thanks for that clarification.

I am reading the link Sreten pointed me to also.

I guess on general principle the idea of putting a petroleum product in my speakers gives me the creeps.

I can see how it works, but I think I would find another way.

Always productive talking with Sreten, even if we don't agree.

Thanks again.

Steve/bluewizard
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