The world’s first mass-marketed electric car, the Nissan Leaf, boasts all of the safety features of a gasoline-powered model: air bags, anti-lock brakes, an impact-absorbing frame. There’s one high-tech extra: a synthesizer that emits noise to alert pedestrians to the vehicle’s approach.
But it’s not just any noise. Nissan Motor Co. spent years developing the Leaf’s unique sound, which some listeners have described as a gentler version of an airplane taking off or the approach of a spaceship in a sci-fi movie. When backing up, the car pings like a sonar.
What’s clear is that the Leaf, which is just rolling into U.S. showrooms, sounds nothing like conventional cars. And that’s no accident, said Tsuyoshi Kanuma, who led the Leaf’s sound team in developing the distinctive spectrum of high and low notes.
“It’s not loud but it’s distinct enough that you can hear it sooner than you would a gasoline-powered engine,” said Kanuma, manager of the company’s noise and vibration engineering group.
Electric cars and hybrids offer the promise of a cleaner alternative to gasoline-only cars. They’ve also raised new concerns. At slow speeds, electric cars and hybrids make almost no noise. According to a recent study by the National Highway Traffic Safety Administration, pedestrians and bicyclists are hit twice as often by hybrids as by conventional cars at intersections, parking lots and other spots where cars normally slow down.
The risk of accidents led U.S. lawmakers to draw up legislation aimed at protecting pedestrians. Congress recently approved a bill that would require transport safety regulators to set minimum sound levels for hybrid and electric vehicles. Known as the Pedestrian Safety Enhancement Act, it awaits President Obama’s signature.
Advocates for the blind applauded the measure. “The legislation that has passed the U.S. House and Senate is legislation that we fully support,” said Chris Danielsen, a spokesman for the National Federation of the Blind.
Nissan isn’t the only carmaker to add sounds to alternative-fuel vehicles. Toyota offers optional sound for Priuses sold in Japan. General Motors Co.'s Volt has driver-activated chirps, and Lotus and Harman International Industries Inc. sell sound kits for other companies’ hybrids and electric cars. But the Leaf is the first to automatically broadcast a sound when the car slows to less than 20 mph.
Developing the Leaf’s sound was a huge challenge for Kanuma’s team at Nissan’s labs in Atsugi, southwest of Tokyo. Make it too soft and people wouldn’t know that a car was approaching. Too loud and it might annoy the person behind the wheel or stir complaints from neighbors. Too strange and children and old people wouldn’t recognize it as a car.
“It got more complex as we looked into it,” Kanuma said.
For help, Kanuma turned to music theorists, movie sound-effects specialists, acoustics psychologists and transport officials.
Kanuma’s early experiments were with music. He met with a pipe organ expert in Germany and a music professor at Tokyo University of the Arts. He had his team compose songs but later dropped the idea.
“Colleagues in the U.S. told us it sounded like an ice cream truck,” he said.
Kanuma’s breakthrough came while analyzing traffic sound. Ambient noise can vary among cities. But Kanuma stumbled upon the common link: Noise generated by friction between a car’s tires and the road peaks at around 1 kilohertz.
Working with a Hollywood sound studio, Kanuma’s team mixed tones above and below 1 kilohertz. Tones at the low end of the spectrum, around 600 hertz, were easy for elderly pedestrians to pick out. Adding in higher tones of 2 kilohertz to 4 kilohertz would help the sound carry without it being too loud. That was something Kanuma heard about when studying how opera singers make themselves heard above an orchestra. “We called our sound ‘twin peaks,’ ” he said.
In late 2009, Kanuma sent a dozen sounds for testing to Vanderbilt University in Nashville. In an anti-echo chamber fitted with a ring of speakers, researchers asked subjects whether they could tell where the sounds were approaching from amid background noise. “It represents the complex listening task that blind pedestrians perform at a street corner,” said Daniel Ashmead, a professor who led the study.
At Western Michigan University, tests showed that the sounds were more audible than the engine of a gasoline-powered car. Kanuma’s team also polled experts at the Detroit Institute of Ophthalmology and met with child behavior specialists at the University of Idaho.
In early 2010, Kanuma flew to Detroit with a Leaf prototype and tested the sound at crosswalks and in parking lots. Officials from the National Federation of the Blind recommended changes. “The car’s sound started too abruptly,” Kanuma said. “They suggested giving the car an idling sound.”
Nissan engineers used some feedback. But they ignored the federation’s request for an industrywide standard and they resisted the group’s demands to get rid of a button that lets Leaf drivers temporarily disable the sound. New legislation will eventually require Nissan to do away with its sound-disable button.
Internally, the Leaf’s sound got the green light from Nissan Chief Executive Carlos Ghosn in March, after a 30-minute demonstration at the automaker’s headquarters. In late October, Nissan installed the sound in a second model, the Fuga luxury hybrid sedan, in Japan.
“I think it could one day become the Nissan sound,” Kanuma said.
Hall is a special correspondent.