It was quiet. Maybe too quiet.
After exploring about 100 sounds that ranged from chimes to motorlike to futuristic, the company settled on a soft whine that fluctuates in intensity with the car's speed. When backing up, the car makes a clanging sound.
Nissan says it worked with advocates for the blind, a Hollywood sound design company, and acoustic psychologists in creating its system of audible alerts.
"While silence is golden, it does present practical challenges," a Nissan statement said. The Leaf is set to go on sale in part of the United States in December.
Nissan added the noise as lawmakers and regulators study whether to require automakers to install warning sounds in their vehicles to alert pedestrians.
With more than 1.6 million hybrid vehicles on the road, and the number of electric cars expected to rise with the introduction of vehicles like the Leaf, a number of safety advocates have warned of the dangers to pedestrians.
Last year, a National Highway Traffic Safety Administration study said hybrid vehicles were twice as likely as conventional cars to be involved in a pedestrian crash in some low-speed situations.
Others say that adding sounds sets back decades of automakers efforts to make cars run quietly. Some electric-car firms complained that silence is one of the battery-run cars' main virtues.
Nissan's sound system is the first from a major manufacturer. Nissan says a computer and synthesizer in the dash panel control the system. The sound emanates from a speaker in the engine compartment. A switch inside the vehicle can turn off the sounds, but the system automatically resets to "on" at the next ignition cycle.
At speeds greater than 20 mph, any car makes noise as the tires interact with the pavement, engineers say. The noises for the Nissan operate only at lower speeds.