Audible aids navigation appeal to the sense of hearing of the navigators,
In cases of poor visibility due to fogs or snow，sound signals are sent to vessels indicating the location of the source of the sound．
The facilities are known as fog signal stations．
Each station gives out sounds of fixed different periods．
The diaphragm system is the main method applied to send fog signals．
The distance to be reached is short and is weakened by the wind direction，wind speed，noise on the vessel and other obstacles．
The direction to be determined by reflection is not accurate, and as the distance to be reached is not fixed, the number of audible aids is decreasing．
Fog signal stations are mostly attached to coastal lighthouses．