The American rock band Linkin Park is helping Mercedes-AMG come up with just the right sound for its electric performance car.

That's not a joke. At the Geneva Motor Show earlier this month, Mercedes executives said the music group would develop the sound the normally quiet battery-powered vehicles would make when driving to alert pedestrians.

That's because electric vehicles are pretty much silent, and dangerously so, regulators say. Think about your cellphone or other electronic device — you don't hear much noise coming from it when it's on and running. Now think about your gas-powered car and all the vrooms, hissing, and revs it makes when the engine is on. An EV is more cellphone-like than you'd think.

As greater numbers of electric vehicles make it onto the road, worries about the potential danger of silent vehicles to pedestrians, cyclists, and blind people grow. The European Union has a new directive requiring all new electric and hybrid vehicles to give a sound warning to pedestrians by 2021. When traveling at speeds below 12 mph, the cars have to emit the sound to give pedestrians a heads up that there's a vehicle even if they can't hear it. When going faster than that, the tires against the road, wind resistance against the windshield, and more make the car more audible.

The U.S. set up a similar requirement for hybrid and all-electric vehicles, though it was delayed and took a while to hash out the details. The National Highway Traffic Safety Administration will require all electric vehicles (and hybrids) to emit a sound while moving up to 18.6 mph by September 2020. (The U.S. settled on a faster speed limit based on arguments that the cars are harder to hear until they're really moving.) The noise requirement will be phased in to give car makers time to modify the vehicles, with 50 percent of cars required to emit a sound by this September.

These new requirements mean companies like Mercedes-AMG and beyond need to come up with artificial sounds their electric vehicles will play while moving at slower speeds. Ahead of the deadline, many car companies already have sounds in development and are putting them in cars. Waterproof external speakers will emit the sounds loudly and clearly.

French car maker Citroen introduced an electric compact car at the Geneva Motor Show this year. Although only a concept vehicle, it included a "sound signature" that would play to comply with EU regulations. The more playful tune that oscillates based on the speed of the car shows how much range the car companies have to follow the requirements.

Here in the U.S., more automakers are going electric. About 1 percent of car sales (and growing!) are EVs, which means more of these quieter cars are sneaking up — if not intentionally — on people in the crosswalk or while pulling out of a parking space. To comply with the upcoming requirements, every car maker must have a sound plan if it has or will have an electric vehicle in its lineup.

The Nissan Leaf is one of the more mainstream and affordably priced EVs available. In the U.S., more than 14,700 of the cars were sold last year. It will emit these sounds while going forward and reversing, kind of like the beeping sounds you hear when a truck backs up — but with more musicality.

The GM's Chevy Bolt is right behind Tesla in electric vehicle sales. It reached 200,000 sales at the end of 2018, a few months after Tesla hit that marker. Since Chevy's hybrid Volt is no longer in production, the company is focusing its electric goals on the all-electric Bolt. The company developed its own AVAS sound, which stands for Acoustic Vehicle Alerting System.

Motorcycles are also part of this new regulation, so Harley-Davidson's new LiveWire electric motorcycle is revving up with an original sound emulating a "real" hog.

We reached out to Tesla but didn't hear back about what its pedestrian warning noise will sound like. We recently heard that with Sentry Mode its cars can play loud music as a theft deterrent, but that uses the car's audio system, not an external speaker.

Silence is golden, unless you're in an electric vehicle. Then it's time to make some noise.