As drivers increasingly ditch gas-guzzling cars for electric vehicles, a persistent problem remains for pedestrians: hearing them coming.

To address that risk, regulators in Australia, Europe, the United States, and other regions now require EVs to emit artificial warning sounds at low speeds, forcing some automakers to recall their cars for failing to meet safety standards. What might seem like a simple safety measure has become a complex web of engineering, involving acoustics, signal design, human perception, and regulatory compliance.

Automakers including BMW, Hyundai, Mercedes-Benz and others have enlisted sound designers, acoustics specialists, and even musicians to craft audio signatures that satisfy safety mandates while reinforcing brand identity. But designing those signals means navigating strict technical requirements and the messy realities of urban soundscapes. And it’s still not entirely clear how well those warning signals actually work in real-world conditions.

To learn more about the many challenges that automakers confront, IEEE Spectrum spoke with Michael Roan, an acoustics professor at Penn State University who has researched how artificial vehicle sounds are perceived, particularly by visually impaired pedestrians.

What challenges do automakers encounter when creating sounds for EVs?

Michael Roan: There’s a lot of challenges. A speaker has a certain frequency response. When you put it somewhere, you face the possibility that there’ll be dead zones. If I transmit some sound and it’s in an enclosed area, that sound doesn’t propagate straight out to the listener. It bounces off the road and inside the enclosure that it’s in, whether that’s the wheel well or under the hood.

The signals that they use are very tonal, and so that creates the possibility of destructive interference and constructive interference. That means you could have spots where it’s really loud, and then spots where you can’t hear it at all. Trying to make a source that is even around the exterior of the vehicle is really challenging.

When automakers are creating EV sounds, what are they trying to optimize for? And how do different priorities clash?

Roan: It’s really about safety, then brand. And then, of course, cost—how much is it gonna cost me to put this system in 100,000 vehicles, and have it be reliable, and not cost a fortune.

The clash is figuring out how to satisfy the safety regulations without being annoying. I’m sure you’ve heard plenty of electric vehicles creeping up on you while you’re walking on the sidewalk, and it plays that whirring spaceship sound. It’s shrill, hard-edged, and very tonal. That’s satisfying the regulations, but it’s not aesthetically pleasing.

So how can you do both? That’s where you have to mix in a little artistry to make sure that’s happening. There’s perceptible, but then there’s annoying. In Europe, people are particularly sensitive to noise pollution. If one of these cars is sitting there in an intersection, it’s okay. But what happens when there’s 50 sitting in an intersection? Is that going to destroy the acoustic environment for people living in that area? Balancing all that is a really complex issue.

Real-World EV Sound Challenges

How do EV sounds typically hold up in real world conditions with traffic, construction, and other ambient noise?

Roan: I don’t think anyone has answered that yet. I bet nobody would say that ‘if you satisfy the regulation, then in a real world situation, you’ll be X amount of safe.’ That doesn’t exist, because nobody really knows. It hasn’t been tested in a real world environment where you put out listeners in a real intersection in a city like New York City. But that’s just a gut feeling based on experiences that hasn’t been proven out.

Is there interest in standardizing EV sounds, where one sound could be a one size fits all type of solution?

Roan: I don’t think the car makers would want to do it. They really like to separate themselves, to have some sort of identity that’s different from the other car makers. That’s what makes them special.

What would it take to create a speaker that is loud enough and meets regulatory standards without being annoying?

Roan: There’s been a lot of things looked at. For instance, we played around with putting a piezoelectric actuator on the hood and using the hood as a source of sound. One [option] that would be really great is you put an array of speakers on the bumper in the front and the back and beamformed it. That way, it focuses on the sidewalk where people would only hear it as the car went by in a very narrow spatial range. But you need this array of speakers as well as signal processing to beam that sound where you need it to go. That would be ideal for lowering noise pollution, but it’s very expensive.

Do you think the current EV stand regulations meaningfully improve safety, or are they mostly compliance driven?

Roan: I think they improve safety. When we did our testing on a Chevrolet Bolt car, we did a control set where there was no added sound and tested them against different additive sounds. For the no sound case, the probability detection was significantly lower. It needs to be there. I think it’s a great start.

But there’s not a lot of research going on in EV sound safety anymore. In a sense, the [U.S.] government’s like, ‘Well, we’ve got to figure it out, have a nice day.’ But as more EVs roll out, maybe we need to go to Norway where they have a huge amount of EVs on the road to see how their safety is going. I don’t know if anybody’s even looking at that, to tell you the truth.