Absorption

The importance of each part is depending on (#1) the material's hardness, (#2) the surface roughness and (#3) the porosity. There is always a combination of all three effects.When seen form outside the room, or from behind the surface, there is still (#4) transmission to take into account and depending on the configuration, (#5) vibration of the whole surface element. But let's look at the three first effects to understand absorption.If the surface in question is a thick hard concrete wall, then there is almost 100% reflection with a bit of diffusion depending on the surface roughness. The absorption is close to 0%.Let's give it the index 0.If the surface is an open window towards an infinitely empty space, the reflection will be 0% and the absorption will then be 100%.Let's give it the index 1.For a specific material, the absorption is not the same at every frequency. For example, a piece of cloth will absorb mainly high frequencies and an element of suspended ceiling (ceiling tile) will absorb medium and high frequencies but almost no bass frequencies.It also depends on how the element in questions is mounted.The example below shows a graph of absorption that could also be summarized as a table.