TMbaSQ: High Density is Good but Concrete is Bad?

I think this is a good question but I don't have a ready answer... so let's try to think this through.

We do know how "porous absorbers" work: Sound waves (vibrating air molecules) encounter a mass of fibers of fiber glass or mineral wool etc. The vibrating air molecules cause the fibers to vibrate and transform acoustic energy into heat energy. OK, we all know that. Now what happens when the density of the fibrous increases? Here's where I need some help...but... what I have come to accept is that as the density of the porous absorber increases, it's ability to absorb lower frequencies also increases (at equal thicknesses). This turns out to be a good thing for bass traps because we can save some space in our already small rooms.

But your question is, what if we take this to extremes? What if we consider that our porous absorber is replaced with a slab of concrete?

Well, we already know that concrete is also porous...and that it's also very dense. So why is it not used for bass traps? Here's where some may say your question is stupid. We know empirically that pretty much all sound bounces off of a concrete wall. It's porous but also rigid. So now I can only make a SWAG that it's got something to do with the frequency in question, either absorbed or reflected. I'll bet that all audio band frequencies are reflected and possibly some sub audio band frequencies are absorbed and converted to heat.

So it seems your assumption is valid. My suggestion is you compare the acoustic test data of various materials and determine what's appropriate for your application and move on. Maybe someone else here can give you a more definitive answer.

Pasim, which dense porous materials do you mean? Concrete? If so yes, I agree, I'm not suggesting anyone use concrete for bass traps. I'm sure you're correct the the "gas flow resistivity" is the reason concrete is not a bass trap. I realized that some time after I shut down last night but thanks for pointing that out.

Technically, almost all materials are porous, since there is some sort of void in everything. But in reality, it's a matter of degree.

This question is obviously theoretical since I would hope anyone (even the OP) should realize that sound bounces off hard materials.

And denser insulations materials, have higher GFR numbers.

It's not a case of 'more is better' but of the 'golden mean'.

No GFR: no absorption - pass through unimpeded.
100% GFR: no absorption - reflected.
Middle GFR: frictional action aka "absorption" occurs.