To notify pedestrians of these silent-running hybrids on the road, Lotus came up with a simple solution -- make the vehicle sound like all of the other cars on the road. The Safe & Sound system synthesizes external engine sound for electric and hybrid vehicles, playing an artificial engine noise from the front of the car for the benefit of people on foot or riding bicycles nearby. The sound should be instantly recognizable to anyone who's ever heard a car drive past them on the road.
To develop the artificial engine sound, Lotus measured hybrid vehicle road speed signals in order to determine the appropriate pitches and frequencies a conventional engine would make at different speeds. Lotus also made and analyzed recordings of engine sounds in order to find a common frequency or "voice" for their sound. A waterproof, 300-watt loudspeaker placed near the hybrid's radiator sends out the synthesized engine noise so pedestrians can hear the approaching vehicle. A throttle position sensor determines how fast the hybrid is going, and then transfers that information to a synthesis controller. After matching the speed with the correct frequencies measured earlier, the controller outputs sound to an amplifier and through the loudspeaker. If the car happens to be all-electric, the Safe & Sound system operates continually; if the car is a gas-electric hybrid, however, the system will produce noise only when the electric motor is running.
The Safe & Sound system is particularly beneficial to blind and partially sighted people. While crossing the road, those with limited sight rely heavily on their own hearing or the hearing from another helper such as a trained guide dog to detect nearby vehicles. With audio cues from something like Lotus' Safe & Sound system, the risk of accident or injury is significantly reduced.
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