What best Bitumen to use in a sandwhich?

[rcavictim]

I wish to try making a composite consisting of 3/4" MDF on the outside with a layer of Bitumen and then a 1/2" sheet of cement board on the inside for speaker cabinet wall construction. I have been looking at the roofing tars and driveway fillers/sealers at Home Dumpo wondering what might be a suitable filler that will not run away in a mess after laminating and will provide the viscous damping needed. Most of the sealer products want to be exposed to air dry or cure. Anyone have a brand or type they had good experience with in this sort of application? I would prefer something that works at room temperature but could heat the Bitumen if necessary to apply it.

[GM]

50lb roll roofing works well.

GM

[rcavictim]

GM,

Will this act as an adhesive that will keep the cement board from falling away from the surface of the MDF?

Can you describe "50 lb. roll roofing" in better detail? I don't know what you are talking about. Thanx!

[Jonathan Bright]

I think this sort of thing is good fun and one of the big advantages of DIY but just a word of caution with the bitumen. I seem I recall reading in Collums book on High Performance Speakers (or maybe some where else) that if the bitumen is not allowed to dry sufficiently there have been cases where the solvent vapour ruined the adhesive to the centering spider. Grief ensured. Given any thought to mixing sand or some other aggragate with the bitumen?

[Artmaster]

hello!
sandwhich is a compromise solution. you want to meet half-way result as well. unpredictable damping factor, logarithmic decrement.
what do you think about more resolute decision?
let's make speaker cabinet, using real marble coffers
it's realy not expensive, but you'll completely satisfied of quality and by sight

[rcavictim]

Originally posted by Yury
hello!
sandwhich is a compromise solution. you want to meet half-way result as well. unpredictable damping factor, logarithmic decrement.
what do you think about more resolute decision?
let's make speaker cabinet, using real marble coffers
it's realy not expensive, but you'll completely satisfied of quality and by sight

Yury,

Good idea for compact sized speakers ....but. Have you seen the large speakers in my avatar? They weigh 300 lbs each! That is with 125 lb. of sand in filled L & R side surfaces. In marble these behemoths would easily weigh 1200+ lbs each.

I don't know anybody that has forklift and loading dock to their living room. I used to but that is another story.

[Artmaster]

you're right, but I made big cabinet ones. I'd assembling it on the place, in the room of customer. it's one way.
second one is - makes it by thin panel. paste it on the wood cabinet. 6mm-10mm. you'll get nice design, best acoustic parametres and not very heavy.
ok. I see. it's not easy, but the same as sandwhich....
good luck.

[BillFitzmaurice]

Use rubber roll roofing; this is about 1/16 inch thick, adhesive on one side, typically used under shingles for prevention of ice-dam leakage. Being adhesive on one side only you'll have to use screws to bond the two outer layers, and lots of them. But it won't deteriorate or de-gas.

[rcavictim]

Originally posted by Jonathan Bright
I think this sort of thing is good fun and one of the big advantages of DIY but just a word of caution with the bitumen. I seem I recall reading in Collums book on High Performance Speakers (or maybe some where else) that if the bitumen is not allowed to dry sufficiently there have been cases where the solvent vapour ruined the adhesive to the centering spider. Grief ensured. Given any thought to mixing sand or some other aggragate with the bitumen?

Johnathan,

I also had a concern for the outgassing of solvents that would be an issue if store bought roofing treatments were used as bitumen. The untreated surface of the MDF would absorb some of this in time and it could end up lifting the paint or outer finish while potentially making an eternal bad/hazardous smell to anyone with sensitive olfactories or solvent intolerance.

I've seen municipal road crews undertaking asphalt road crack repair with a small trailer that has a propane fired heat source to melt the tar they use and they just pour it into the cracks as a liquid which then cures as it cools. This stuff would probably work very well but to use it I suspect one would have to set up a time consuming and costly to prepare industrial process in the shop.

I do not know that adding sand to the bitumen would be helpful as one wants this zone to be a semi-viscous shock absorbing layer.

I'm sure two part silicone electronics casting compound from Dow Corning would be a hot trick to use in this application but it is about $50/pint last time I bought some about a decade ago.

[rcavictim]

Originally posted by BillFitzmaurice
Use rubber roll roofing; this is about 1/16 inch thick, adhesive on one side, typically used under shingles for prevention of ice-dam leakage. Being adhesive on one side only you'll have to use screws to bond the two outer layers, and lots of them. But it won't deteriorate or de-gas.

Bill,

How about using contact cement on the non adhesive side of the roll roofing layer against the cement panel applied liberally to both surfaces being joined after the roll roofing has been pressed onto the MDF?