Making the Silent Motors of Electric Cars Audible

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In recent years, automotive engineers have developed ever-quieter—and, thankfully, more fuel efficient—internal combustion engines. But they’ve also grappled with the question of how to mimic the desirable thunder of gas-guzzling muscle cars, often turning to finely tuned exhaust systems and even piped-in electronic noise in the process. Electric vehicle owners, however, would prefer their ride stay silent.

Today, electric vehicles, or EVs, are enjoying a bit of a moment. Tesla now touts 445 “Supercharger” stations with more than 2,750 individual chargers across the US. Earlier this year, BMW used its Super Bowl ad slot to tout its new i3 EV. Even a rust-belt city like Indianapolis may get an EV-only rental business in the not-too-distant future.

To many owners, their popularity makes perfect sense.

“The endless stream of power an EV delivers is intoxicating,” says Chris Neff, Electric Auto Association board member and long-time EV driver. And the silent operation associated with the torque an EV motor produces, he says, is a major aspect of an EV’s true pleasure.

“A lot of us like cars, we all do, and [engine] noise is good for that type of car, but you do it all day long, it starts to wear on you,” Neff asserts. By comparison, the silence of an EV is relaxing.But the noiselessness of EVs also poses a potential hazard for pedestrians, especially the blind or vision-impaired, who may not realize an EV is rolling by until it’s too late.

Five years ago, Congress sought to remedy the problem with the Pedestrian Safety Enhancement Act of 2010. Sponsored by then-Senator John Kerry (D-MA), the bill was overwhelmingly bipartisan—it passed unanimously in the Senate, and by a margin of 379-30 in the House—and mandated that electric and hybrid vehicles automatically generate a sound that “shall allow the pedestrian to reasonably detect a nearby electric or hybrid vehicle in critical operating scenarios.”

In January 2013, the National Highway Transportation Safety Administration proposed Federal Motor Vehicle Safety Standard No. 141, which would set the standard for EV noise as required by the PSEA. (More recently, the NHTSA delayed its final ruling until the end of this year in order to allow for more study and analysis.)

In working to set the minimum standards, the NHTSA also issued a series of sample sounds “that could be proposed for use as alerting sounds,” some of which “would meet the proposed specifications; some would not.” The sample noises include existing EV alerts, synthesized alerts based on ICEs, and digitally produced synthesized alerts. Chipping or chirping sounds, according to the NHTSA’s Notice of Preliminary Rulemaking, will not meet the PSEA’s standard, which mandates the noise be recognizable as a motor vehicle. The NHTSA also “tentatively” determined that EVs and hybrids would be required to produce a sound whenever the vehicle is operating at a speed under 30 kph (18.6 mph), since studies indicated that tire noise is a sufficient pedestrian warning above that speed.

Prior to the legislation, some EV manufacturers were already hard at work producing technology designed to warn pedestrians and cyclists of an oncoming EV.

“We’ve taken a very proactive position in this idea of pedestrian safety,” says Chevrolet Volt vehicle chief engineer Andrew Farah. Chevy recognized the pedestrian safety issues EVs might pose during the development of their EV1 model in the 1990s, Farah says. The first generation Volt has a pedestrian horn, a button located on the end of the turn signal stalk that, when activated, produces a less aggressive sound than the standard horn. However, as Chevy worked to develop its pedestrian alert system further, they discovered that the National Federation of the Blind felt strongly about the importance of having a passive alert system—one that didn’t require driver activation.

As a result, Chevy developed a humming noise similar to the kind of white noise machine many people use to lull themselves to sleep. That system would automatically engage any time the car was capable of motion and moving less than 30 mph. This model—available now in the Chevy Spark and later this year in the 2016 Second Generation Volt—goes a step further, actually shifting in frequency as the car accelerates.

Nissan has also incorporated safety noise into its EV, the Leaf. The Leaf, which began production in 2010, “has a sound generator called the Vehicle Sound for Pedestrians that operates at speeds up to about 17 mph,” Nissan spokesperson Brian Brockman explained in an email. “The system generates unique sounds both in forward gear and reverse,” and has been incorporated into the vehicle since Leaf’s launch, he added. However, Brockman said that he couldn’t comment on the proposed NHTSA rule or whether Nissan anticipates that its current sound would meet the proposed standard.

Engineers have also worked to strike a balance between making no noise and mimicking a Mack truck.

“The real question is: are we making too much noise? And the answer to that is no,” Farah states. “We haven’t had any complaints from use in the field.”

Still, some EV enthusiasts remain determined in their opposition. “In my opinion, making quiet cars noisier is trying to solve the wrong problem,” EAA historian Darryl McMahon wrote in an email. McMahon pointed to situations in which simply making quiet cars noisier won’t necessarily increase their safety for pedestrians. “Should we not outlaw headphones and earbuds for pedestrians, as these affect their ability to hear approaching vehicles?” he asked. “Instead of making the urban soundscape noisier, we can develop real solutions to enable those with hearing or vision challenges to better recognize all vehicular traffic (and other obstacles) when appropriate for them.”

Neff, the EAA board member, says that many EVs already make “a nice sound similar to a turbine under hard acceleration,” though such a noise is unlikely to meet NHTSA guidelines. Neff also alluded to existing radar technology that allows EVs to detect when pedestrians are in the vicinity of the car, and engage the passive sound warning system.

Farah agreed that such technology could theoretically be implemented. “All of those things can be done,” he said, “but we’re not sure that it’s a cost-effective solution.” And as many EV sticker prices stretch to the $30,000 mark and well beyond, cost is a very valid concern.

Of course, there’s always the ultra-low-cost, old-fashioned approach. “Sometimes I’ll roll my window down, poke my head out and say something,” Neff chuckles.