Silence may be a virtue under most circumstances, but not when you're driving around a leafy, residential neighborhood in your Toyota Prius . That's why the manufacturer came out with its very own vehicle proximity notification system last year -- a "futuristic," underhood noisemaker designed to alert pedestrians and the visually impaired to the plug-in's presence. Now, Toyota has offered more details on its safety system, in a freshly released demo video starring the 2012 Prius V . The car's artificial engine noise, as the company explains, only kicks in at speeds below 15 miles per hour, allowing it to broadcast its audible heads-up across parking lots or other low-speed zones. The sound itself, meanwhile, emanates from external speakers and consists of a blend of high and low frequencies that won't be by muffled by background noise or physical obstacles. The vehicle's pitch shift technology also enables the tone to rise in pitch as the car speeds up and to fall as it slows down, giving others an idea of how fast it's approaching. Accelerate past the break to see the full video for yourself, while John Kerry nods in silent approval