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how to brace a speaker cabinet?
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Old 29th January 2015, 03:39 PM   #201
twinter is offline twinter  United States
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Bushmeister, do you know the modulus of elasticity and the damping factor of the bitumen damping material?
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Old 29th January 2015, 04:01 PM   #202
twinter is offline twinter  United States
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Quote:
Originally Posted by bushmeister View Post
I have found this thread very interesting. Last year I built some active three way sealed speakers with high quality drivers (scanspeak and SB), high quality active crossovers, and MC2 amps.

I am extremely pleased with the outcome, but am continually dismayed by the 'stethoscope test' on my cabinet walls.

They are made from 18mm MDF with substantial oak and MDF cross-bracing, and 8-12mm of bitumen/aluminum (silent coat) internal damping. Over the bitumen is a 2cm layer of high density wool felt, and finally they are densely stuffed with glass fibre.

I was hoping with a well braced, well damped, well stuffed enclosure I would be safe - but it appears resonance is hard to tame!

I have even tried external application of tiles with 'green glue' type damping adhesive, which didn't seem to achieve much. So I am left with two options:

1. Either start again with the cabinet. ( I would now probably try either 9mm ply with 12-18mm bitumen damping and minimal bracing, or a full complex CLD enclosure with chipboard/plywood.)

2. De-couple the drivers from the current cabinet.

Given the complexity of the first option and cost, I have decided to try decoupling the drivers first - particularly as this has been shown to have a massive effect on panel resonance if done properly.

To decouple the drivers I am using two techniques:
1: I am making new baffles from stabilised hardwood which will be epoxied onto the current MDF baffles - if I understand things correctly this should provide greater damping and stiffness through the use of the two different materials bonded with epoxy - ? form of CLD, as the new hardwood baffle will only be tied to the rest of the cabinet via the epoxy.

2. 3mm sorbothane gaskets for all the drivers (given results shown demonstrate that the mid and tweeter can excite just as much resonance as the woofer) which are held to the new baffle with thin countersunk head screws and rubber O-rings - to prevent the metal screw from touching the speaker frame.

I hope that with these two measures I should get a good degree of decoupling of the drivers from the cabinet.

I will report back on my success - hopefully with photos and measurements in the next few weeks!
I understand from my experience too. Sometimes what we "know" just ain't so.



A link to a paper concerning driver decoupling from Andrew Jones, Director of Engineering, TAD and designer of the TAD Reference 1. I believe he posts on this site as AndrewJ. They used EAR type E-610-1 isolation grommets in the study.

http://www.linkwitzlab.com/Driver%20Decoupling.doc
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Old 29th January 2015, 04:24 PM   #203
twinter is offline twinter  United States
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Quote:
Originally Posted by twinter View Post
Bushmeister, do you know the modulus of elasticity and the damping factor of the bitumen damping material?
Is the bitumen layer stiff enough for optimum damping (modulus of elasticity)? For most effective extensional damping the damping material should be:

1) 1/2 to 1 times the thickness of the base panel, even greater thickness is more effective, (this you have done)
2) optimally, the modulus of elasticity of the damping material should be similar to the base panel (surprisingly), within +-50% is best, though 1/5th works
3) the damping factor of the damping material should be greater than 0.1, with 0.4 to 0.6 best, damping factor = 1/Q
4) the damping material should be firmly attacked to the base panel, no flexible adhesive here, you want the damping material to flex when the base panel vibrates

The isophthalic polyester resin (available at serious boat repair shops and FRP supply stores) mixed with bentonite (cat litter) is very effective. See the Qualia website.

Some informative application information on damping tile effectiveness, see the damping factor tables on the right hand side of the page"
http://www.pyroteknc.com/download/De...P%20310713.pdf
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Old 29th January 2015, 04:32 PM   #204
bushmeister is offline bushmeister
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I only have REW measurements as per this thread:

http://www.diyaudio.com/forums/multi...i-start-7.html

I do have on access waterfall etc. But it all looks good. It is the stethoscope test that disappoints.

Is there any measurements I can take with rew and my umik that might assist?

Wow missed all your other posts! It is 4mm silent coat, which is one of the better butyl based car dampening products with an aluminium constrained layer.

Last edited by bushmeister; 29th January 2015 at 04:36 PM.
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Old 29th January 2015, 04:35 PM   #205
sayrum is offline sayrum  Germany
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Hey
Quote:
Originally Posted by twinter View Post
... Green Glue or other adhesive made for constrained layer damping ...
What adhesive for CLD can be recommended considering a closed cabinet? -> So with no fumes or harmful chemical emitions like bitumen (for the suround, cone etc).

Last edited by sayrum; 29th January 2015 at 04:38 PM.
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Old 29th January 2015, 05:04 PM   #206
Baseballbat is offline Baseballbat  Germany
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I've already hinted you to this one: tesaŽ Powerbond INDOOR - Adhesive Tapes - tesa SE

Then you won't need a damping layer any more, just put them between two thin (6 mm) plates of MDF.

But if it is applicable in a closed cabinetI can't say.
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Old 29th January 2015, 08:42 PM   #207
bushmeister is offline bushmeister
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Why not a simple stiff ply box, lined with this:
iKoustic Mute Board 4 | Wall Soundproofing | iKoustic

Would seem to tick all boxes in one?
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Old 29th January 2015, 09:01 PM   #208
twinter is offline twinter  United States
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Quote:
Originally Posted by bushmeister View Post
Why not a simple stiff ply box, lined with this:
iKoustic Mute Board 4 | Wall Soundproofing | iKoustic

Would seem to tick all boxes in one?
Here's a related thread, with input from Brian Ravnaas, co-developer of Green Glue.

Green Glue in my speaker?!?! - AVS | Home Theater Discussions And Reviews
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Old 29th January 2015, 09:21 PM   #209
robotc is offline robotc  Australia
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Location: Melbourne
A very interesting discussion. I have been following it from the beginning. Like a lot of people here I am on a similar journey.

Here is a link to a page describing one persons findings on the subject of resonance control using a double wall cabinet with CLD.

This quote from the article might be relevant here.

"The best overall choice for building cabinets is composite layer construction. The fundamental resonances are raised an average of 1-1.5 octaves and reduced by 10-15dB (based on my tests with an accelerometer), and the enclosures are *dead*, without any messy goop (that may be hazardous to driver adhesives) or roofing felt. The trick is the glue that constrains the two layers.

I recommend you try this for yourself, but I spent a month and about $30 trying various types of adhesives, and found an excellent, cheap, high-damping, aggressive adhesive that is readily available...acrylic latex caulk plus silicone. Specifically, DAP ALEX Plus. The latex caulks don't have enough bond strength to the wood, and the pure silicones have no damping whatsoever.

Rigid bonds, such as wood glue, are practically worthless in composite constructions, providing strength but no damping. Try it yourself.. .buy a tube of ALEX Plus and glue a I' square piece to another. Do the same with wood glue. Let them dry overnight, and then knock on them. The ALEX Plus is dead, but the wood glue still almost as resonant as the single thickness piece' (This is supported by accelerometer testing that I have done, and I may be able to have the data here for you to look at at some point.ˇ..) I tried several other latex In' silicones from other manufacturers, but none of them had the right balance of adhesion, damping, curing time and viscosity (and a reasonable pH so that "acid evolution" won't destroy the bonds over a couple of years)."

Robert
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Old 29th January 2015, 10:06 PM   #210
bushmeister is offline bushmeister
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Excellent links chaps. I love the idea of coating the inside of my current speaker with green glue then slapping wet plaster all over it...to produce a very high performance cld....

Would be achievable and I would only have to remove the current lining but no braces. More reading...
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