The rumble of a car's internal combustion engine is so pervasive few people even notice it anymore. Now consider what cities might sound like in the not-too-distant future when they teem with electric vehicles. The streets will be quiet—too quiet. And that presents a problem.

“Sound helps people be aware of cars,” says Kota Kobayashi, a designer at the studio Ustwo. “There’s a safety element to it.” Electric cars emit only a faint hum, a problem Kobayashi and his colleagues spent two months pondering. They worked with the audio branding agency Man Made Music to develop a range of sounds electric vehicles can emit to warn pedestrians. The project, which is more of an experiment than a practical application of the technology, presents a smart hypothesis for how automakers might make their silent fleet of vehicles safer for drivers and pedestrians alike.

Car companies already give this a lot of thought. Last fall, the National Highway Traffic Safety Administration announced a federal safety standard requiring that new electric vehicles emit an audible sound when traveling at less than 19 mph. Cars zipping along faster than that, regulators say, produce enough tire and wind noise to let people know they're coming. The subtext of the rule is that silence is dangerous. One NHTSA study found that pedestrian accidents are 37 percent more likely in electric vehicles than conventional cars. Still, the law offers little concrete guidance with regard to what kind of noise EVs should make.

Most hybrid and electric vehicles emit a low hiss as they glide down the street. Some feature warning sounds to alert pedestrians to their presence. The Chevrolet Volt, for example, sounds like a rusty, creaking door when it gets too close to an object or person. The Toyota Rav4 emits an incessant digital beep, and the Prius produces a harmonic low hum that increases as it nears a pedestrian.

These warning sounds are built around reactions, but Don Norman, director of the Design Lab at UC San Diego and author of The Design of Everyday Things, believes electric vehicle sounds should function similar to brake lights and turn signals, conveying the car's intentionality well before an issue occurs. "Don’t try to tailor your signal to the urgency of a situation, because that only works when there’s one car and one person,” he says. “When you have a complex situation, the best thing a vehicle can do is say here I am and here is what I’m doing.”

Ustwo's solution sits somewhere between a warning alert and a turn signal. The designers created something they call the risk scale, which assesses risk on a scale of Low to Extreme, depending upon the likelihood of an impact, and emits a corresponding sound. The designers offer a variety of suggestions for doing this. In one example, the car emits a sound that gets progressively louder and more frequent as it nears a pedestrian. In another, the type and volume of the sound changes with each increasing risk zone. It might start with a gentle clicking, then become a louder whomp, and then an annoying beep. In those cases with the greatest risk of a collision—someone stepping off the curb in front of an oncoming car, for example, the car might emit an ear-piercing alarm.

"It came down to understanding what do we, as pedestrians, want to know?" says designer Cesar Corral-Castilla. The scale doesn't assume that every car barreling along in the general direction of a pedestrian presents a threat. “'How can sound indicate risk to a pedestrian at varying levels?' is a very different ask than 'How can sound let you know a car is by you or approaching?'” says Dan Venne, creative director at Man Made Music. Making this distinction reduces pedestrian panic and noise pollution.

Today’s cars feature all manner of safety equipment and sensors that can help with this. The same technology that, say, tells you when someone is in your blinds pot or helps you avoid backing into a pole can tell your car there's a pedestrian standing on the corner. As cars gain increasing autonomy, this technology can help them understand more complex and potentially dangerous scenarios involving pedestrians and other cars.

The trick, says Man Made founder Joel Beckerman, lies in creating a sound distinctive for people to recognize but not ignore. (This is the problem with car alarms—no one even pays attention to them.) The sound can't be too melodic, either, because that does not convey urgency. "We want a sound that elicits the quickest reaction time," Beckerman says.

Any practical application of these ideas also must consider things like the impact of ambient noise on these sounds, what happens when several cars emit alarms simultaneously, and cultural norms—what works in New York may not work in Shanghai. "There were a million different things we could think about with cars in a city ecosystem," says Corral-Castilla. "This is just one hypothesis."

One hypothesis, yes, but a fascinating one for automakers and regulators pondering the nuances of electric vehicles and how they'll change the aural landscape of cities.