Nevertheless, it is common practice to mount sound absorptive
layers directly against a wall, because it is very convenient to do so.
We must, however, realize that, in such cases, only the outer one-third of the thickness of the layer is effective in absorbing sound.
The rest of the material is simply acting as a convenient support!
Therefore, the values of R/rc for the 1/2" layer of material
given in Table 4, and the corresponding values of amax, assume that
this 1/2" layer is mounted near the perforated metal screen with, say,
an inch of empty airspace behind it. so that the entite 1/2" layer is
effective.
If the layer were mounted directly against a hard wall, the
tabulated values of R/rc would have to be multiplied by 1/3, and the
corresponding values of maximum absorption recalculated.