1 - 17 of 17 Posts

#### vann_d

·
##### Registered
Joined
·
887 Posts
http://www.silcom.com/~aludwig/Loudspeaker_construction.html#Panel_vibration_damping has a very good article on the subject and suggests using 30 lb. roofing felt.

I'm wondering if any of you have experiences with constraint layer damping in loudspeakers.

The gentlemen measured a 20db decrease in panel vibrations which I found impressive.
I've thought about this in the past. The company I work for develops passive isolation systems for spacecraft using constraint layer damping.

The problem is that to properly engineer the treatment, you need to know the shear modulus and loss factor of the visco-elastic in question. These two things are often very frequency and temperature dependent. You also need to know the panel mode (frequency) at which you want to maximize damping and you need to be able to use finite element analysis to properly model the behavior of the panel and damping treatment together. Then you can optimize the thickness of the both the face sheets and damping layer to achieve the optimum amount of damping for that panel mode. This is not simple stuff to properly design.

One could experiment, though, and possibly come up with something that works. It would just be trial and error, however.

The concept is to put the damping treatment at the neutral axis (center) of the panel. It is here that when the panel bends, the damping treatment will be in shear. The loss factor of the material and the amount of strain energy in the panel will determine the damping in the panel. It is important to have the damping treatment bonded to panels on both sides to maximize the strain energy in the material.

In regard to the article mentioned, it is not too surprising that he sees positive results even if the damping layer was doing nothing. He increased thickness of his 3/4" panel significantly by adding the 1/4" masonite and roofing material. Inertia goes up by thickness^3.

I would say most companies do not utilize this technique because of the engineering expense involved and a general lack of know-how. Why go though the trouble when you can just stiffen the panel for much less expense?

#### mdrake

·
##### Registered
Joined
·
1,648 Posts
I've thought about this in the past. The company I work for develops passive isolation systems for spacecraft using constraint layer damping. (Note: damping is the correct term, dampening means something like "getting wet".

The problem is that to properly engineer the treatment, you need to know the shear modulus and loss factor of the visco-elastic in question. These two things are often very frequency and temperature dependent. You also need to know the panel mode (frequency) at which you want to maximize damping and you need to be able to use finite element analysis to properly model the behavior of the panel and damping treatment together. Then you can optimize the thickness of the both the face sheets and damping layer to achieve the optimum amount of damping for that panel mode. This is not simple stuff to properly design.

One could experiment, though, and possibly come up with something that works. It would just be trial and error, however.

The concept is to put the damping treatment at the neutral axis (center) of the panel. It is here that when the panel bends, the damping treatment will be in shear. The loss factor of the material and the amount of strain energy in the panel will determine the damping in the panel. It is important to have the damping treatment bonded to panels on both sides to maximize the strain energy in the material.

In regard to the article mentioned, it is not too surprising that he sees positive results even if the damping layer was doing nothing. He increased thickness of his 3/4" panel significantly by adding the 1/4" masonite and roofing material. Inertia goes up by thickness^3.

I would say most companies do not utilize this technique because of the engineering expense involved and a general lack of know-how. Why go though the trouble when you can just stiffen the panel for much less expense?
Good response vann_d but now my brain hurts!! :T :dumbcrazy:

Matt

#### Anthony

·
##### Elite Shackster , HTS Moderator Emeritus
Joined
·
2,216 Posts
Remembering . . . things . . . from Solid Mechanics 210 . . . AND . . . Materials Science 206 . . . :explode:

#### vann_d

·
##### Registered
Joined
·
887 Posts
I also think this guy's conclusions are a bit off.

The black curve is with just adding 1/4" masonite. The green and cyan are with his CL damping treatment. I don't see much difference here. It is hard to tell, however, because his measurements clearly lack enough resolution to capture the response peaks.

#### Anthony

·
##### Elite Shackster , HTS Moderator Emeritus
Joined
·
2,216 Posts
Yeah, looks like he only took 10 data points per (base 10) octave.

I've used Room EQ Wizard to lower tighter peaks than that.

#### vann_d

·
##### Registered
Joined
·
887 Posts
Remembering . . . things . . . from Solid Mechanics 210 . . . AND . . . Materials Science 206 . . . :explode:

:rofl: sorry, this just happens to be an area of expertise for me...:nerd:

#### Anthony

·
##### Elite Shackster , HTS Moderator Emeritus
Joined
·
2,216 Posts
Also, WAAY back when, I used Dynamat on the insides of a subwoofer cabinet. This is heavy and adhesive and worked okay in a subwoofer application, but was far better on the backs of pictures, mirrors, and other "light" resonators throughout the room.

Hadn't thought to concentrate it on the panel node points, though.

#### lsiberian

·
##### Registered
Joined
·
2,625 Posts
Discussion Starter

#### lsiberian

·
##### Registered
Joined
·
2,625 Posts
Discussion Starter
Yeah, looks like he only took 10 data points per (base 10) octave.

I've used Room EQ Wizard to lower tighter peaks than that.

#### vann_d

·
##### Registered
Joined
·
887 Posts
20 bucks for roofing material is a far cry from the cost of the amps and crossovers.
I'd use Dynamat or a comparable product. It won't smell bad like that asphalt based roofing stuff does.

The treatment should work best if you place it between two layers of equal thickness. That puts the damping material right at the location of max shear in the panel, maximizing effectiveness.

#### Anthony

·
##### Elite Shackster , HTS Moderator Emeritus
Joined
·
2,216 Posts
It's probably not on the interwebs anymore, but back in the 90's when I first got into subwoofer building, there was a guy who would make his own composite panels and then build the box.

Basically it was 3/4" MDF -- roofing felt -- 3/4" MDF sandwich. He would then miter the edges or carefully design a box in a box so that the felt layer was never short circuited by contact with an outer layer of MDF. It was a lot of extra work, but he swore by the damping effects. Not sure what adhesive he used, it's been a while.

I was researching NHT1259 sub designs at the time, might have been the old Bass-list.

#### vann_d

·
##### Registered
Joined
·
887 Posts
I've also thought about using honeycomb sandwich panel construction but the cost seemed to outweigh the benefits. Maybe a foam core sandwich panel could be constructed DIY and really help reduce the weight of the cabinets while maintaining excellent panel stiffness. The foam core may act as a damping material in the process.

#### lsiberian

·
##### Registered
Joined
·
2,625 Posts
Discussion Starter
I'd use Dynamat or a comparable product. It won't smell bad like that asphalt based roofing stuff does.

The treatment should work best if you place it between two layers of equal thickness. That puts the damping material right at the location of max shear in the panel, maximizing effectiveness.
Peel-n-seal is what one of my friends uses. The exterior structure will be 3/4" plywood and quarter round on the edges to help with baffle step and make finishing quick work. I'm quite sure this is overkill, but that's the point of this build. :R

Constraint Layer Damping, A bracing matrix and thick Rockwool treatment is more than my current speakers have.

#### Lucky7!

·
##### Registered
Joined
·
790 Posts
http://www.silcom.com/~aludwig/Loudspeaker_construction.html#Panel_vibration_damping has a very good article on the subject and suggests using 30 lb. roofing felt.

I'm wondering if any of you have experiences with constraint layer damping in loudspeakers.

The gentlemen measured a 20db decrease in panel vibrations which I found impressive.
I remember the old Ludwig build, but hadn't read it in ages. My first thought reading your initial post was 'what the is 30lb roofing felt?'. Turns out we have a similar product here called Malthoid dampcourse which is about \$A13 for a 100mm wide 20m roll so I think I'll give some a try in the speakers on the interior walls. Not sure what to glue it to the walls with though. I've emailed and enquiry over the w/e and hope I'll get a response.

Vann's post 2 is my understanding of CLD (better expressed too).

I've also thought about using honeycomb sandwich panel construction but the cost seemed to outweigh the benefits. Maybe a foam core sandwich panel could be constructed DIY and really help reduce the weight of the cabinets while maintaining excellent panel stiffness. The foam core may act as a damping material in the process.
Many years ago, about the time of the SL600 Celestions, a friend built some speakers and subs with a Styrofoam core and Laminex skins. It was very light and stiff and seemed to work well but we had no way of measuring back then. Each skin was added over the 50mm thick styro and left to dry. Panels were cut and mitred and glued to each other to form the enclosure. The baffles had marine ply behind the drivers to give extra support.
The speakers were huge, a big 4 box sort-of line array a la Infinity IRS.

#### Syd26147

·
##### Registered
Joined
·
58 Posts
I'm wondering if any of you have experiences with constraint layer damping in loudspeakers.
Lots of DIY stuff.
Found the roof felt too cumbersome and difficult to work with.
I also tried the layer technique in SpeakerBuilder 3/89 of layering veneer plywood with drywall sandwiched in between. Again cumbersome and difficult.
Another article in SB 5/96 ( Hajo Prodan "The Opposite Moduli(OM) Speaker Cabinet" ) lead me to OM compounds and try laminating layers of plywood with silicone RTV.
While messy to work with it created panels that were much more inert than layered ply.
On my later subs I did not want to resort to a layered "brute force" technique, so I made an acoustic mud compound to grout on the inside of the sub panels.

Syd

#### sunnyduce

·
##### Registered
Joined
·
2 Posts
Yoo All

Currently building curved speakers using 1/6" sheets of PVC bonded with Neoprene to a thickness of 3/4"
Best enclosures I have build in 30 years of building
Cheers
Sunny

1 - 17 of 17 Posts