Great idea for a thread. This is something I'd like to learn more about.

I've done some reading and have discovered some things which I find very surprising. Perhaps I've misunderstood something. Either way, I would appreciate comments from Andre and/or other experts on this forum.

As I understand it, the speed of sound in air at room temperature is around 344 m/s. This is under adiabatic conditions. Inside a porous absorber at low frequencies, conditions are isothermal. This should result in a speed of sound which is lower by a factor of approximately 1.18 if no other factors come into play. This would give a speed of about 291 m/s.

To find out whether there are other factors to be considered I had a look through some papers and books and discovered the following:

According to Delany and Bazley (and others), the speed of sound in a porous absorber is given by the angular frequency divided by the real part of the complex wavenumber. If we take a porous absorber with flow resistivity 10,000 rayls/m, at a frequency of 100Hz the Allard/Champoux model predicts that the real part of the complex wavenumber will be 5.65 . Other models (eg. Miki, Delany/Bazley) give values in the same ballpark. If we divide the angular frequency (628.3) by 5.65 we should get the speed of sound in the porous absorber. This calculation gives us a value of 111 m/s. This is dramatically less than I expected, and if correct, has some interesting repercussions.

It has been shown that if the thickness of a porous absorber is at least 7% of the wavelength (in air), then we can get close to 100% absorption. Some people have found it difficult to believe that an absorber that thin can be so effective. If my calculations above are correct, then the wavelength inside the absorber is much shorter due to the reduced speed of sound. For a porous absorber with flow resistivity 10,000 rayls/m, at a frequency of 100Hz, 7% of the wavelength in air corresponds to about 22% of the wavelength inside the absorber. Very close to the 25% that many people feel is necessary for full absorption!

Any comments are welcome.

Demetris