Air-borne sound is detected by its ability to cause vibration in some component of the detector. In the same way as direct mechanical impacts on a surface, air-borne sound is able to cause vibrations in any surface with which it comes into contact. The sound energy can then be carried further as structure-borne sound to re-radiate air-borne sound further. Such indirect paths of sound are known as flanking transmission. These include any areas of low transmission loss, such as open windows creating outdoor paths from window to window. Other air-borne paths lie through connecting duct-work, ventilation grilles, openings under doors, gaps at walls, poor seals at floor edges, back to back electical sockets, false ceilings and open troughs in floor-ceiling structures.