Absorption, Reflection, Reduction And Flanking

So what is absorption, reflection, reduction and flanking? What does it have to do with acoustics? How does it affect my project and what can I do?

When sound energy hits a flat surface one (or more) events will take place depending on what the surface is manufactured or constructed from.

Most surfaces will allow sound to bounce off (as a reflection) back in to the space that increases reverberation, affecting the acoustic environment of the room. This makes conversation difficult and uncomfortable.
Porous materials such as open cell foam or mineral fibre will provide absorption that assists in controlling reverberation.

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 a sheet material (plasterboard). Alternatively, a composite material that offers both mass and absorption (wall with plasterboard both sides and a cavity containing mineral fibre) can be used.

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 one.

absorption | reflection | reduction | Flanking Transmission

Diagram one: Illustration for absorption, reflection, reduction and flanking


Sound energy (red arrows) passes into the material where some energy is converted to heat. The remaining energy can then bounce back into the room as a reflection. If the material is hanging in free air then the sound energy may pass through the absorber..


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.


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.


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.

Failure to understand and design for these four acoustic principles is the most common cause of failing to meet the requirements of guidance documents such as BB93 and Approved Document E.