Absorption, Reflection, Reduction And Flanking

When sound energy hits a flat surface one (or more) events will take place depending on what the surface is manufactured or constructed from.
Porous materials such as open cell foam or mineral fibre will provide absorption that assists in controlling reverberation.
Most surfaces will allow sound to bounce off (or reflect) back in to the space that again increases reverberation or can affect the balance of the room where uses include the need for critical listening etc.
Sound reduction is the process of adding mass to a system to prevent sound from travelling from one area to another (such as a wall). These materials are solid and can be in the form of a single material (sheet of plasterboard) or a composite material that offers both mass and absorption (wall with plasterboard both sides and a cavity containing mineral fibre).
Flanking transmission is created where sound bridges or bypasses the acoustic material or construction, usually as a result of gaps or weak spots within the installation or construction.
These four principles are shown in the diagram below:

Absorption – Highlighted by the red arrows, the sound energy passes into the material where some energy is converted in to heat and the remaining energy then bounces back into the room. If the material is hanging in free air then the sound energy may pass through the absorber instead of reflecting back in to the room.
Reflection – Where a hard surface such as glass is encountered the sound energy will bounce off this and radiate back into the room (as shown by the blue arrows) without the loss of any energy causing reverberation.
Reduction – If a mass layer such as plasterboard or concrete is used then this will prevent sound energy from freely passing through and the sound levels within the receiving room will be lower than that generated by the noise source (green arrows). The level of sound reduction will depend on the mass of the material used as well as the construction method used.
Flanking Transmission – This is the act of sound travelling around, or bridging, a building element by exploiting a weakness in the material, construction or installation (magenta arrow) and therefore rendering the acoustic treatment redundant.
When looking at designing acoustic solutions it may be necessary to include more than one of the above principles to achieve required performance levels. For example, when building lightweight internal partitions the use of mineral wool within the cavity not only provides the required thermal properties but also acts as an absorber for sound energy that passes through the plasterboard initially.