All foghorns use a vibrating column of air to create an audible tone, but the method of setting up this vibration differs. Some horns, like the Daboll trumpet, used vibrating plates or metal
reeds, a similar principle to a modern electric
car horn. Others used air forced through holes in a revolving cylinder or disk, in the same manner as a
siren. Semi-automatic operation of foghorns was achieved by using a
clockwork mechanism (or "coder") to sequentially open the valves admitting air to the horns; each horn was given its own timing characteristics to help mariners identify them.