Lighter than Sand but same properties to fill voids?

[Galu]

I mentioned the requirement for a viscoelastic material in post #3.

The most commonly known viscoelastic material in audio circles is Sorbothane.
What Is Viscoelastic Material? - www.sorbothane.com

Unfortunately, Sorbothane damping sheets are rather expensive!
https://www.amazon.co.uk/Sorbothane-Vibration-Damping-2-54mm-30-5cm-x/dp/B006YJ6SIO

 

[rex_johnston]

I mentioned the requirement for a viscoelastic material in post #3.

The most commonly known viscoelastic material in audio circles is Sorbothane.

Yes you did. Judging from some of the suggestions though, the viscous part isn't well understood. Unmodified silicone rubber for example, is elastic and is not a constrained layer damping option. Even modified silicone is elastic for small deformations (like that found within 2 layers of very stiff material).

Bitumen doesn't exhibit much elasticity at all...

 

[Galu]

Yes you did.

And I appreciate your backup and science input.

Now, if we could only source an affordable viscoelastic polymer compound!

 

[rex_johnston]

Now, if we could only source an affordable viscoelastic polymer compound!

Blutak would be the ultimate viscous polymer, but it's not that cheap either. There are some anti-rust oil based polymers that are set and are viscous, but they are a bit nasty, even the aqueous suspension based ones.

 

[rex_johnston]

that are set and are viscous

...that set and are viscous...

 

[prendrefeu]

Looking up alternatives for Sorbothane (because there is usually an alternative to something that is trademarked) - lead me to this:
Don't use Sorbothane Feet!? Now what? | Page 2 | Audiokarma Home Audio Stereo Discussion Forums
which lead me to this:
you can laugh if you want to | Audiokarma Home Audio Stereo Discussion Forums

Which lead me to this:
FUNoodles Foam Rod - Gopher Sport

Basically pool/float noodles. Cut up, insert into voids, removable as well (vs. foam) if the qualities are not ideal.