Great! We are getting somewhere good! I agree totally with the data you presented in the graphs. Those are based on normal incident sound, not diffuse field sound.
BBC 1992-11 in figs 1 and 2 show calculated vs measured diffuse field absorption for rw 2 using 6,000 Rayls as the resistivity of the material. Fig 3 compares theoretical to normal incidence measured.
Without quoted and getting into details that may bore people here, I agree with you. The difficulty was that short answers are almost always wrong because the specific are not explained. I am referring to my initial response.
Thanks for the reference to Bob's site. Have you studied the list at the very end of site?
Andre