Hybrids are lovable. They’re cute, kinda awkward looking and much less demanding when it comes to meal times at the fuel pump. One good thing about hybrids is also one of the bad things about these cars: they’re silent and to a concerned group of pedestrians, that silence could be very dangerous.

The National Federation of the Blind have raised their concerns about the hazards posed by hybrids, especially problematic for children, seniors and blind pedestrians who depend on the sound emitted by cars in order to travel safely and independently. The federation is calling on the industry to design an inoffensive sound that will give pedestrians a signal that a car is coming.

Two new solutions may solve the problem of too-silent hybrids. A year ago, Stanford University researchers in electrical engineering, the biosciences and sound engineering founded Enhanced Vehicle Acoustics, which markets external sound systems for hybrids. EVA’s sound system for the Toyota Prius, which runs only on electricity at low speeds, emits sounds similar to that of a regular combustion engine, but at a fraction of the sound level. When the hybrid reaches speeds of 20 to 25 mph the sound produced by the tire noise and wind generated by the car is normally sufficient for bystanders to hear so EVA’s system shuts off.

Now Lotus, one of the premier automakers in the world known for its high-performance vehicles, is also coming up with a similar technology to help give hybrids a bigger voice on the road. It’s outfitting a Prius with a waterproof speaker under the hood of the car giving it the roaring sound of regular vehicles. Lotus says its “safe and sound hybrid technology” simulates a real engine sound making it instantly recognizable that the vehicle is in motion.

To synthesize the engine sound, a road speed signal is taken from the vehicle and the waterproof loudspeaker system emanates the sound from the front of the vehicle. Once the vehicle has passed, the sound is not heard. When the car is operating on the electric motor only, throttle and speed synthesize sounds projects a realistic engine sound. If the hybrid’s engine starts operating, at higher speeds or throttle demands or lower battery levels, the control system automatically stops the external synthesis. When the powertrain control system switches the car back to running on the electric motor only, the synthesis controller instantaneously sets the system running again.

It is all completely automatic and the driver hears almost none of the additional sound. The device kicks in automatically to produce an artificial engine noise when the hybrid car runs on its electric motor. When the car's combustion engine takes over, sensors fitted to the engine and suspension turn off the sound. The engine noise is produced by a waterproof loudspeaker positioned next to the car's radiator, making the sound seem to originate from under the bonnet. The system produces a pitch and frequency designed to help pedestrians identify the car's speed and distance. Lotus adapted the system from technology previously developed to block outside noise from the inside of its cars.

Via WIRED, Enhanced Vehicle Acoustics, Lotus

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