A porous absorbing material is a solid that contains cavities,
channels or interstices so that sound waves are able to enter
through them. It is possible to classify porous materials according
to their availability to an external fluid such as air. Figure 1 shows
a schematic cross-section of a porous solid material. Those pores
that are totally isolated from their neighbors are called “closed”
pores. They have an effect on some macroscopic properties of the
material such as its bulk density, mechanical strength and thermal
conductivity. However, closed pores are substantially less efficient
than open pores in absorbing sound energy. On the other hand,
“open” pores have a continuous channel of communication with
the external surface of the body, and they have great influence on
the absorption of sound. Open pores can also be “blind” (open only
at one end) or “through” (open at two ends). A practical convention
is used to make a distinction between porosity and roughness,
which assumes that a rough surface is not porous unless it has
irregularities that are deeper than they are wide.12