Sound Speed in a Porous Material...

Concrete is also porous. So if you have a solution for concrete, you can perhaps apply it to fiberglass

Here are some good websites that may help

http://www.t-linespeakers.org/projects/tlB/appendix/bradbury/index.html

http://www.soundes.com/

Just thinking out load....

A porous material, say swiss cheese, has both cheese and air pockets. I think it is safe to calculate for the cheese only, as all of it is inter-connected.

If I take a solid iron bar 1 meter in length, I can easily calculate the time a vibration of a given frequency will travel through it, if I then drilled some holes in any direction through the bar, would I really be changing things (other than the reduced mass etc.)?

Of one takes the same solid iron bar and strikes it with something heavy, though not violently enough to fracture it, the bar will "ring" with a fundamental frequency and harmonics related to its shape and where it was hit.

Drill a load of holes in the bar and try again, and the "ringing" is not so true.

Could it be something to do with the speed of sound in the bar and in its surrounding air being different?

Can this analogy be applied to inferring the speed of sound in Swiss cheese, bearing in mind that it is preferable to strike an iron bar; striking Swiss cheese sounds somewhat wasteful...