A foghorn is a device used in maritime locations to signal other vessels and to prevent collisions. The tones are deep and penetrate the fog better than higher pitches. If you've never heard one, imagine a train whistle or horn except sounding more like Barry White crooning how much he loves your girlfriend in 1975.
Ships use foghorns to send short blasts to help other vessels locate them by sound. This is helpful for small craft that do not have RADAR to scurry out of the way of the lumbering behemoth about to run over them. Some ports and lighthouses use them as well when the fog rolls in.
Iron Noder 2017