Absorptive Layer Near A Hard Wall

We come now to a complication that we have already encountered (without an explanation) near the end of Part One: namely, it makes a difference where, within the air cavity, the sound absorptive material is placed (See Figures 18 and 19).

We realize that in order for the absorptive layer to work well, turning the sound energy into heat by the friction of the vibrating air particles within the fine pores of the material, there must be freedom for the air particles to move. If anything impedes this motion, then the energy conversion is less efficient and less sound energy is absorbed. And that is just what happens at locations near a hard wall: the wall itself, being rigid, cannot move with the sound wave, and this means that the nearby air particles also cannot move. Thus, any sound absorptive material placed against a hard wall is virtually useless, because there can be no air motion within the material to dissipate the sound energy. 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/pc 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/pc would have to be multiplied by 1/3, and the corresponding values of maximum absorption recalculated.