The days of the quiet Toyota Prius could be numbered, with a new YouTube video that seems to feature the artificial sound that the hybrid is expected to make appearing this week.

The 'Vehicle Proximity Notification System' is designed to make the Prius and its siblings, the Plug-in Prius and Prius v, as audible as their gasoline counterparts in the absence of a loud engine noise.

Described by some as sounding like a spaceship, the futuristic sound is emitted from a speaker mounted underneath the hood at speeds of up to 15 miles per hour or 25 km/h (after which the noise of the wheels is audible).

Like a gasoline engine, the pitch of the audio changes depending on whether the vehicle is speeding up or slowing down, and it uses a blend of frequencies to help ensure that it isn't muffled by obstacles or background noise.

Under US legislation passed this year, all new electric vehicles will be required to make some kind of artificial sound to ensure that at-risk road users such as cyclists and the visually impaired are aware of their presence.

Similar moves are expected in Europe, where the Prius Plug-In will also go on sale in the first half of next year.

Listen to the sounds:

Toyota Prius:

Nissan Leaf:


Search for used cars