Every fibrous material has a property of its own called the flow resistivity , E, which gives the flow resistance per inch of thickness. (We are talking now about the material, itself, not a particular blanket of that material.)
Thus, if a certain type of glass fiber has a flow resistivity E = 60 cgs rayls/inch, then a 2" blanket of the material will have a flow resistance of R = 2 x 60 = 120 cgs rayls. And for this blanket the value of R/pc = 120/41 = 2.93. Remember: the flow resistance E is a property of the material, while the flow resistance R is a property of a blanket of the material with a particular thickness. The resistance ratio R/pc relates the flow resistance of a given blanket to the characteristic impedance of the air.
Now, at last, we are in a position to consider the maximum amount of sound absorption achieved at the resonance frequency of our tuned absorber. As we mentioned above, it depends only on the value of R/pc for the filling in the airspace: 1 IX = max Y2 + V4 (R/pc + pc/R) Table 3 gives values for IXmax (at the resonance frequency) corresponding to different values for R/pc of the cavity filling:
Table 3: Maximum attainable sound absorption (at the resonance frequency), as a function of the flow resistance ratio of the filling material.
As we said above, the maximum absorption coefficient at resonance in a tuned absorber is not very sensitive to the filling material: any value of R/pc from 0.5 to 2.0 will yield a value of amax of 0.89 or greater.
