Christoph Meier, a sound engineer at Mercedes-Benz parent Daimler, typically spends time making engine noise less jarring. For the carmaker's new electric models, he's had to do the opposite — create sound.
For Daimler's e-Smart city car, Meier and his team invented a "sonorous purring" that was pitched higher than conventional vehicles, while Mercedes's 416,500-euro (NZ$684,600) SLS AMG Coupe Electric Drive gets huskier tones to reflect its power.
"People expect some exterior noise from a vehicle, because we all grew up with the 'vroom vroom' of combustion engines," said Meier, who oversees 250 people as head of powertrain acoustics at the Stuttgart, Germany-based company.
Daimler isn't alone in adding noise to electric cars. Renault offers a choice of car tones — pure, glam and sport — on the Zoe hatchback, while Nissan's Leaf, the best-selling electric car, also comes with artificial sound. The issue has become more critical to carmakers as regulators look to require warning noises as soon as next year, while the rollout of more and more models requires manufacturers to seek ways to stand out.
Synthetic motor noise, like the jangly, high-pitched whir of Renault's glam track, could save lives and at the same time protect investments in electric cars. The vehicles emit almost no sound at low speeds, making them a potential silent threat for cyclists and pedestrians used to reacting to the rumble of engines. With electric cars already struggling to gain popularity, a spate of accidents could further damp demand.
"If a silent electric vehicle knocks over an elderly person or a child, it's not worth the risk," said Neil King, an analyst with Euromonitor in London. "It happens often enough in urban areas that people are stepping into the road without looking. You can't get around that."
Although no data yet exists on injuries caused by electric vehicles, the European Union takes the threat seriously enough to propose legislation making acoustic warning sounds mandatory, and worldwide guidelines are expected in early 2014, according to German auto association VDA.
Blind and visually impaired people, who rely on acoustic cues to navigate through city streets, could be most at risk.
Without noise, "we could step right in front of a vehicle and the driver would have no chance to brake in time," said Gerhard Renzel, who's blind and a traffic expert for the German association for the visually impaired DBSV. "What is important for us is that we don't get killed in traffic."
Electric vehicles are mainly silent at speeds less than 30 kilometres per hour. Then tyre and wind noise kicks in. While adding motor sounds at slow speeds may help avoid accidents, it also undercuts one of the unique selling points of electric vehicles.
"One of the big competitive advantages of electric vehicles is their soundlessness," said Stefan Bratzel, director of the Centre of Automotive Management at the University of Applied Sciences in Bergisch Gladbach, Germany. "It's a justified goal to have quieter cities."
Because of this, some carmakers are seeking to keep the din of electric vehicles to a minimum. BMW will add artificial sound to the i3 city car only where authorities demand it. Volkswagen also isn't planning to add sound to its e-Up! model unless required.
Elon Musk, chief executive officer of Tesla Motors, said in June that electric cars should direct "a pleasant-sounding noise" as a gentle warning to nearby people rather than be required to emit sound all the time.
Daimler, by contrast, sees sound as a safety feature. On the electric Smart, the engine tone is standard in the US and Japan and an option in Europe. Unlike Renault, which equips its Zoe, Kangoo and Twizy electric models with sound, the German company doesn't allow customers to shut off the noise manually.
Smart's sound mimics the noise of a combustion engine by getting louder as the driver presses down on the pedal and higher as the car accelerates. The German automaker will equip electric Mercedes models with a similar system. The real dilemma is designing an appropriate noise.
"Simply imitating the sound of a combustion engine was not an option," said Ralf Kunkel, head of acoustics at Audi, who developed a tone for the A3 E-tron plug-in hybrid, which debuts next year. "We discarded ideas of giving electric vehicles sounds such as birds twittering or leaves rustling."