Whether in fog, in an emergency situation or to give sound signals, for example to open a bascule bridge - a signal horn belongs on board. In addition to permanently installed electric versions, there are very simple horns for blowing or horns with a gas cartridge. The latter usually use an environmentally harmful propellant gas, which is why pressurised air horns with a pump are also an option.
The Ecopower air pressure fanfare from Fisa has a hand pump that generates the necessary pressure for the horn. This means there is no need to replace the cartridge and the manual fog horn is always ready for use. The fog horn is said to reach a volume of up to 112 decibels and is available for 11.95 euros e.g. at Amazon .