[richidoo]

Hey all, I am building large speaker cabinets out of some very nice, heavy and stiff Columbia brand American hardwood ply sold at Home Depot. The cabinet has internal horns that open at the bottom. There is a some bracing in the design, no unsupported span is more than 12". I may add a few more braces here and there. But the hardwood plywood I am using is very hard/stiff and has a ~600hz type ping when knocked. So I'd like to damp it on the interior surface. The designer suggests using 4mm thick bitumen pad. I am worried about the smell of asphalt inside the house. My sense of smell is bionic so I assume I'll be able to smell it, since the MSDS says "slight odor."  Raw bitumen is not available in US anymore, but there are modified bitumen products with other polymers added to improve use as roofing material. They still have asphalt in them. GAF Ruberoid is the one I was thinking of using, only $50 for a 100lb roll. It is very cheap for the coverage and mass. I probably need 40 sq ft per speaker.

The car audio realm has plenty of road noise stopping damping films and sprays to put on metal panels. They are extremely expensive, thin, and are concerned with light weight as much as damping. They may stop thin sheet metal vibration but I don't know about 3/4" plywood. I think a couple layers of heavy roofing membrane will stop mid freq resonance better than 1mm space age light weight foam. Another idea is Acoust-X sound absorbing coating. Anybody ever used that? It's still kinda expensive but designed for speaker interiors.

I know about Black Hole and similar audio specific cabinet damping foam. They are too expensive and too thick for this project. I'll be using Acousta-stuf and 10mm carpet felt for internal acoustic damping per the design.

Open to suggestions. Thanks in advance!
Rich

[WGH]

Here is my solution: Richlite Resonance Control Panels
http://www.audiocircle.com/index.php?topic=92198.msg914854#msg914854

Wayne

[jtwrace]

You might find this thread of interest.

http://www.audiocircle.com/index.php?topic=80882.0

[JLM]

I've seen some vendors (I seem to recall Pi speakers was one) that used expanding foam in the internal cavities. 

I also recommend HardiBacker (a cement board) be adhered to the backside to add mass and help deaden unbraced panels.

I'd brace the bass horns as much as possible (whatever you can lift).  12 inch panel spans are way too much IME for bass horns (you'll get colorations).

[walkern]

I've used vinyl floor tiles (peel and stick) to help reduce the panel vibes on some rather large speakers (that were relatively well braced, but still sang a bit) and that worked well.  I believe you can buy rolls of vinyl flooring (undoubtedly cheaper than peel and stick squares) and the smell would be limited to the type of adhesive you used.  The stuff is thin, light weight, and flexible to boot.  It does seem to shift the panel noise to a lower level, and a lower frequency.  Got the idea from Al at RAW acoustics (who designed the kit speakers I use).  He also suggested lining the internal surfaces with thin mattress pad material... but I wouldn't do that with the innards of your horns as that would probably impede the flow through the horn.

Neil

[DougSmith]

Constrained layer damping...  Use ~1/32" polyurethane sealer or melamine glue to afix panels of 1/2" MDF to the inside of the cabinet in the areas between the braces.

[richidoo]

Thanks for all the great ideas so far. 

More bracing is a definite, thanks JLM. Damping interior with MDF or Hardibacker is likely. I think cement's resonant freq and Q will be high like the wood, so MDF being lower freq, lower Q and heavier might be better. But MDF on it's own sounds so bad.

Doug, why melamine glue, is it more flexible (and thus more damping) than traditional wood glue? By polyurethane sealer do you mean wood finish sealer, or yellow foaming gap sealant, or gorilla glue type foaming glue? Thanks

Wayne, that richlite stuff looks awesome. I forgot about your thread about that. There is a cabinet shop in my town that is listed as a richlite dealer. I'll see if they have some scraps. It would be cool to build a speaker with it. Does it have any resonance of its own when you knock it?

jtwrace, if I had my druthers I would build the whole thing in 1/4 steel and vacuum bag on layers of wood for damping. Thick aluminum is pretty expensive, but it does work good. A couple aluminum speakers were at axpona.
Thanks again. Any more ideas are welcome.

[DougSmith]

Doug, why melamine glue, is it more flexible (and thus more damping) than traditional wood glue? By polyurethane sealer do you mean wood finish sealer, or yellow foaming gap sealant, or gorilla glue type foaming glue? Thanks

The key to CLD is the flexible layer between the two panels.  It is a very effective damping technique.  I used PL polyurethane 'door and window sealer' for my Abbeys.  It bonds to MDF very well and remains flexible.  It has some odor, but that will dissipate within a week or so.  According to Earl Geddes, melamine glue does the same thing (but perhaps with a thinner layer).  He also uses 2-part polyurethane adhesives, but these are expensive.

[jtwrace]

Thick aluminum is pretty expensive, but it does work good. A couple aluminum speakers were at axpona.

Like this: http://www.audiocircle.com/index.php?action=gallery;area=browse;album=1614

 

[planet10]

I think cement's resonant freq and Q will be high like the wood, so MDF being lower freq, lower Q and heavier might be better.

Toole shows studies that show that high Q resonances are less audiable then low Q resonances (there is also less energy under the curve to excite them)

Also energy to excite a resonance is at a minimum inversly proportional to the square of the frequency, one can argue that they are actually inversly proportional to the forth power of the frequency.

dave

[WGH]

Wayne, that richlite stuff looks awesome. I forgot about your thread about that. There is a cabinet shop in my town that is listed as a richlite dealer. I'll see if they have some scraps. It would be cool to build a speaker with it. Does it have any resonance of its own when you knock it?

I now have a video on YouTube that compares the resonant properties of three materials: 13ply 3/4" plywood, 1" thick Richlite, and 3/4" MDF. Each panel is suspended by a wire and tapped with a hammer. Check it out.
http://www.youtube.com/watch?v=SkSxH40x5SA



Wayne

[richidoo]

Toole shows studies that show that high Q resonances are less audiable then low Q resonances (there is also less energy under the curve to excite them)

Also energy to excite a resonance is at a minimum inversly proportional to the square of the frequency, one can argue that they are actually inversly proportional to the forth power of the frequency.

dave

That's why your designs try to raise the resonant freq as high as possible. Much easier to damp the higher freqs, and so easy to raise the freq with bracing. See, I was paying attention when I built your cabs. So the the higher freqs are easier to damp, but are more offensive as they climb up into the midrange frequencies if left undamped. Do you use internal damping?  Thanks Dave. 

[richidoo]

http://www.youtube.com/watch?v=SkSxH40x5SA
Wayne

Cool! The richlite is a single note of higher frequency, and shorter duration than the wood panels. Almost like glass.   

[richidoo]

I went to Home Depot today, scouting around for glue, rubberish putty, cement board, bitumen sheet, MDF, steel, whatever. All of the GAF asphalt products, even shingles have minimal smell right up close, more like rubber than petroleum, but inside a house is different than inside a warehouse. Another cheaper brand smelled very strong from 6 feet away. No melamine glue at HD, but plenty of urethane "tough" adhesives.   Thanks for the ideas guys

[WGH]

Cool! The richlite is a single note of higher frequency, and shorter duration than the wood panels. Almost like glass.

Notice how the Richlite panel rocks gently back and forth after being tapped and the MDF panel swings wildly. The Richlite is a lot denser than MDF and it takes a lot more force to get it moving, somehow this relates to Newton's Laws of Motion:

1) An object at rest stays at rest and an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force.

2) The acceleration of an object as produced by a net force is directly proportional to the magnitude of the net force, in the same direction as the net force, and inversely proportional to the mass of the object.

In other words - if you do place a force on an object, it will accelerate, i.e., change its velocity, and it will change its velocity in the direction of the force.

This acceleration is directly proportional to the force. For example, if you are pushing on an object, causing it to accelerate, and then you push, say, three times harder, the acceleration will be three times greater.

And finally this acceleration is inversely proportional to the mass of the object. For example, if you are pushing equally on two objects, and one of the objects has five times more mass than the other, it will accelerate at one fifth the acceleration of the other.

[planet10]

Notice how the Richlite panel rocks gently back and forth after being tapped and the MDF panel swings wildly. The Richlite is a lot denser than MDF and it takes a lot more force to get it moving, somehow this relates to Newton's Laws of Motion:

Of relavance on an open baffle, but in a box the edges are clamped. Note that the higher the mass the lower the resonance of the panel (all other variables constant).

dave

[WGH]

Thanks Dave, I plugged part of your post "higher the mass the lower the resonance of the panel" into Google and found some interesting information regarding sound transmission. I need to do a few more experiments with Richlite and a mastic with a combination of low stiffness, high mass and high damping, I'm thinking of Green Glue or an auto noise insulation mat that is sticky on both sides. Albert Von Schweikert is using a rubber adhesive layer for bonding and vibration damping between the stone and cabinet wall (link).

I found the article below informative:
http://www.kemt.fei.tuke.sk/Predmety/KEMT320_EA/_web/Online_Course_on_Acoustics/transmission.html

Wayne

[planet10]

Thanx for the link, i have added it to my references and will read in detail later.

A lengthy discussion that ends up being a defense of my box building philosopy

http://www.diyaudio.com/forums/construction-tips/98834-discussion-what-materials-build-speakers-out.html

Constrained layer is another approach and can be incorporated into this scheme, but (look at the chart in post 768) is the extra effort necessary?

dave

[JLM]

Deflection/vibration is an inverse function of the unbraced length (or the material's thickness) to the 4th power.

I vote for an inert cabinet (more mass, better bracing, and constrained layer damping).

If your cabinet sounds like say a violin, then everything will sound like a violin.  Some cabinets produce more spls than the driver (small driver/big cabinet).  Sealed or horn designs are the worse because they have higher internal pressures.  So my favorite mid/tweeter horns are made from solid blocks of wood.

[Photon46]

I now have a video on YouTube that compares the resonant properties of three materials: 13ply 3/4" plywood, 1" thick Richlite, and 3/4" MDF. Each panel is suspended by a wire and tapped with a hammer. Check it out.
http://www.youtube.com/watch?v=SkSxH40x5SA



Wayne

Wayne, not to nitpick, but wouldn't the test have been a more level comparison if the dimensions of the planks had been the same? Different sizes of even the same material would resonate at different pitches, the same as marimba bars do.