AC = alternating current = AC = buzzer
DC = direct current = DC = horn
Many old vehicles without battery use as a AC power supply. This ones can only be connected to so-called buzzer-horn. These may be identified by the falling pitch depending on the engines speed.
Due to the non-constant frequency buzzer, sinc e the early 1980’s most German bikes have have been equipped with a battery and a DC electrical grid. This allowed to install real DC horns, recognizable by a constant pitch, regardless of the speed of the motor used.
There are some exceptions in which, despite AC electrical system, a DC horn was used. Piaggio used from year 1984 and later (with the introduction of Lusso/EFL series) special horn rectifier.
The AC Power supply is converted with a special, only for the horn, rectifier in DC. This rectifier can also be fitted on all vehicles with 12V AC electrical system, to fit a DC horn.
NOTE: Almost all German Vespa PK and PX models require a horn (DC).