All new electric and hybrid vehicles are now legally required to have an external sound generator so they can be more easily heard by pedestrians and cyclists.
The EU-mandated system is called an Acoustic Vehicle Alert System, or AVAS. It becomes law on 1 July 2019 after years of campaigning by groups such as the European Blind Union.
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AVAS systems need to work at speeds of up to 13 mph, and the volume must be at least 56 dB (similar to the volume of an electric toothbrush).
The volume must not be louder than 75 dB, equivalent to the noise of a conventional car.