Some vehicles are known for the sounds they make, Harley Davidson motorcycles for example. Now, Toyota's Prius V has its own signature sound as well.
The Prius is a hybrid vehicle that runs on a combination of electricity and gasoline power. When the vehicle is using only its stored electrical power, it can be almost completely silent. Without the noise of an internal combustion engine, there's only the noise of tires on the pavement. At slow speeds, even that can be hard to hear. That is the problem that the Toyota Prius V's new proximity sound system was designed to solve.
When a hybrid is navigating city streets at slow speeds, it can be difficult for visually impaired or distracted pedestrians to hear it coming. Regulators, fearing that this new breed of quiet car could lead to accidents, passed the Pedestrian Safety Enhancement Act which became law at the beginning of this year. It basically says that any electric or hybrid vehicle must announce itself with an artificially generated noise.
The law doesn't say what kind of noise automakers must use. Toyota, therefore had free reign to choose anything from realistic sounding gasoline engine noise to calliope music. Fortunately, they chose a sensible solution and designed the Prius V's audible proximity warning to be both reminiscent of traditional automobiles and futuristic at the same time.
You can hear the sound they've chosen and Toyota Human Factors Engineer Daniel Smith's explanation of the proximity notification system at this YouTube link. Smith says the car's warning system operates at speeds of 15 miles per hour or less, and includes a Doppler-like pitch adjustment that lets nearby pedestrians know if the Prius V is accelerating or decelerating.