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Multi-Way Conventional loudspeakers with crossovers

constrained layer damping with MDF and Ply
constrained layer damping with MDF and Ply
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Old 16th October 2009, 10:51 PM   #11
preiter is offline preiter  United States
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Sounds very much like an open baffle I made.

The construction is: plywood - green goo - MDF - green goo - plywood

Drivers are mounted to the MDF layer, and coupled only through the green goo to anything else.
The MDF layer is cut in half, so the upper and lower drivers are mounted to separate pieces.
The MDF layer comes about 1 cm short of the edges of the baffle, so it does not contact the trim.

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The speakers sound very nice, and if you knock on the front baffle with your knuckles, it sounds very dead.
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Old 17th October 2009, 01:02 AM   #12
rcw is offline rcw  Australia
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What is obvious I am not sure.
A natral material like marble for instance shows hysterisis for the very small displacements we are considering, and then becomes close to linear after that.
People who should know tell me that epoxy adhesives have a similar property, and yet nobody in this forum would consider these as candidates for a constrained damping scheme, but at the displacements we are dealing with they are.
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Old 17th October 2009, 02:10 AM   #13
DualTriode is offline DualTriode  United States
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Hello all,
I have often considered making a sandwich that includes Gypsum wallboard. The architects of the world and their suppliers like UG Gypsum or Domtar have spent lots of time and developement brain power into using wall board as sound dampening material. They even make a specical grade of wallboard that is better at attenuation of the db's than the standard Home Depot grade. Thoughts?
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Old 17th October 2009, 02:15 AM   #14
rcw is offline rcw  Australia
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A wall board layer between two steel plates would be worthwhile perhaps.
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Old 17th October 2009, 02:24 AM   #15
planet10 is offline planet10  Canada
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constrained layer damping with MDF and Ply
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Originally Posted by bmwman91 View Post
why not use a single sheet of 25mm MDF? I understand that you are hoping to achieve some elastic damping from the green-goo interface, but I think that the most important thing to go for in bass enclosures is wall rigidity
Then MDF is not your best choice. Plywood is much stiffer.

MDF is not as well damped either, and certainly not as well damped as a well done constrained layer.

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Old 17th October 2009, 02:27 AM   #16
MJL21193 is offline MJL21193  Canada
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Quote:
Originally Posted by planet10 View Post

and certainly not as well damped as a well done constrained layer.
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Old 17th October 2009, 02:29 AM   #17
planet10 is offline planet10  Canada
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constrained layer damping with MDF and Ply
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Originally Posted by rcw View Post
The so called constrained layer damping used by speaker builders is in fact mass damping.
Why so?

2 dissimilar materials laminated together with something that remains flexible (constrained layer) is intended to create a dramatic impedance difference across the boundry to reduce transmission.

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Old 17th October 2009, 02:53 AM   #18
rdf is offline rdf  Canada
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Quote:
Originally Posted by rcw View Post
....People who should know tell me that epoxy adhesives have a similar property, and yet nobody in this forum would consider these as candidates for a constrained damping scheme, ....
Errr.... I'm considering exactly that for aluminum sandwich CLD cabinets for a pair of Jordans. I might add a sheet of very fine cloth or paper as a carrier and lossy fill.
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Old 17th October 2009, 05:07 AM   #19
rcw is offline rcw  Australia
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Any dramatic change in acoustic impedance causes reflection as well as transmission.

For the constrained layer to be effective the layer must be pushed far into its hysteresis region.

The type of displacements you get in plasterboard, especially when its plate modes are excited causes sufficient flexing of the constraint layer to push it well into its hysteresis region and provide damping, you would have to make your enclosure from something like eighth inch hardboard for it to flex enough for such a layer to be effective.
For a loudspeaker enclosure the best solution for the lowest colouration is the stiffest, you don't need to damp panel flexing if there isn't any.

The point is that a flexible damped panel still transmits more sound than a stiff undamped one, its resonance modes are lower in q that's all.

For instance you might be able to reduce a particular resonance by 10db. This is achieved by broad banding the resonance and increasing the displacement at other frequencies, if you make the panel twice as thick it increases its stiffness by eight times, this reduces the main resonance by ten db. and every other frequency as well, and a general 10db. reduction at all frequencies is better than the same at just at just one.
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Old 17th October 2009, 05:38 AM   #20
planet10 is offline planet10  Canada
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constrained layer damping with MDF and Ply
Quote:
Originally Posted by rcw View Post
Any dramatic change in acoustic impedance causes reflection as well as transmission
I don't see a reflection as a problem. Keeps energy in the box.

Quote:
the layer must be pushed far into its hysteresis region.
do you have a reference we can read that explains what this means?

Quote:
For a loudspeaker enclosure the best solution for the lowest colouration is the stiffest, you don't need to damp panel flexing if there isn't any.
That is my approach. Push panel resonances as high as possible (less energy to excite them), and as high a Q as possible (less energy to excite them and less audible if they do get excited). So i won't use MDF because it isn't very stiff.

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