Recipe

Hi,

I was hoping that someone with some experience and background beyond my own would be able to give me some advice. Perhaps this is a bit off topic, but I hope you'll think the stretching your your mental muscles a bit stimulating.

I'm interested in casting a loudspeaker enclosure from concrete in order to remove any panel resonances that there may be. The enclosure will be a good size and will be for a home subwoofer. I estimate that it will have about 6 ft^3 of material. I'm hoping for it to weight 200 lbs or less, which means the density would have to be around 30 lbs/ft^3.

I've been looking at the 3M glass bubbles and have thought that the K37 was a good choice and also from speaking with a 3M rep through e-mail.

Would this be possible to get this low a density? Also, How should the mix be reinforced? Perhaps add some poly-pro fibres? What would you recommend for proportions of materials and water?

I've seen some references to recipes that create a dry flaky result. I obviously don't want this.

I've also seen some perlite recipes that have a great density, however, their compressive strength is very poor. The compressive strength of the K37 is 3000 psi. I would imagine that this could be a good solution. I don't know about the bonding that could/would(?) occur betweent he cement and the aggregate.

Should there be some polymer additive? Could you recommend one?

One benefit that I've thought would be good about the glass bubbles is that since they're separate cavities, they would help to absorb resonances and also diffuse the waves.

Thanks!


Brendon

 

[RiBeneke]

An interesting question. Perhaps you have already found your answer, but anyway I have a few thoughts ..

To absorb the sound on the inside, you correctly note that the the surface should be as textured as possible, with a range of intermixed texture depths. A textured outside surface with rounded corners will help to reduce radiated sound.

For the structure of the cabinet to avoid resonance it should be accoustically dead. For this it requires as much mass as you can handle, and it should have low resilience (low elasticity or springback) and should not be able to ring like a bell. I suspect that glass is not good in this regard. You need a cork-like material instead, but one that will bond to the cement.

You should probably use some fibre reinforcement, and as for other admixtures, I suggest you try experimentation.