Structure-borne sound is sound that spreads in a solid body. This is not the same as airborne sound and plays an important role in buildings. For example, the airborne sound of a loud music system is converted into structure-borne sound in a wall. This structure-borne sound is not audible. This changes, however, when it is converted back into airborne sound, for example when it is emitted from a wall. This process is responsible for the “impact sound”.