Over the past several years, electric vehicles have garnered something of a reputation for their unusual sounds on the road. Otherworldly EV warning sounds have been compared to “a celestial choir,” a “flying saucer hum,” and, in one TikTok with 23.5 million views, the song that might play just before ascending to heaven. But the angelic warble that’s come to characterize EV acoustics might have a few drawbacks for pedestrians.
A new study conducted by researchers at Chalmers University of Technology in Sweden and published in March examined how well the average person could locate three common types of warning sounds from hybrid and electric vehicles at low speeds. It found that all three of the sounds were significantly harder for pedestrians to locate than the sound of a standard internal combustion engine.
Given that they have no combustion engine, EVs are naturally almost silent. That can be a benefit when it comes to urban noise pollution, but it’s not ideal for pedestrian safety. For the past six years, all EVs in the U.S. have been legally required to emit some kind of low-level noise—a prompt that automakers have chosen to interpret in a range of creative ways. But it might be time for some automakers to take another crack at their proprietary EV acoustics.
What do Hanz Zimmer, a didgeridoo, and fighter jets have in common?
Starting in 2019, the National Highway Traffic Safety Administration ruled that all hybrid and electric cars have to be fitted with an external speaker that must “make audible noise when traveling in reverse or forward at speeds up to 30 kilometers per hour (about 19 miles per hour).” While the law sets expectations for when these noises need to play, it largely leaves the contents of the noise itself up to automakers.
That’s resulted in a variety of EV sounds on the road, from a Cadillac alert made using a didgeridoo to the Hyundai Ioniq 5 N’s fighter-jet-inspired sound and BMW’s portfolio of i4 electric sedan noises by composer Hans Zimmer.
This unusual symphony hitting the roads has inspired quippy commentary on social media. Under a TikTok sharing the BMX iX 50’s reverse sound, one user wrote, “is this ribs by lorde?” And in a video poking fun at Tesla’s reverse audio, another commenter joked, “Every time our neighbour pulls onto the drive with their electric car my husband says ‘the spaceship has landed.’” Beyond sounding a bit silly, though, there are a few key shortcomings to the sounds that many automakers are selecting for their EVs.
Why are EVs so hard (and annoying) to hear?
Chalmers researchers examined three of the main categories of EV sounds, also known as acoustic vehicle alerting systems (AVAS): two-tone, multitone, and narrowband noise (a noise concentrated within a small band of audible frequencies, often perceived as a hissing sound).
To compare these sounds to that of an internal combustion engine, researchers studied the reactions of 52 test subjects inside a soundproof chamber. Each subject was surrounded by 24 loudspeakers and given a laser pointer fashioned out of a toy gun. When one of the speakers played a simulated vehicle sound designed to mimic the noise of an EV at a low speed, the subjects were to point the laser toward the sound as quickly as possible.
The tests demonstrated that all the AVAS categories were harder for subjects to locate than the sound of an internal combustion engine. And, according to Leon Müller, a PhD student at Chalmers and one of the paper’s authors, one of the sounds was more problematic than the others.
“[The two-tone AVAS] is significantly harder to localize than other types of warning sounds, as well as combustion noise,” Müller says, noting that in a situation with just one vehicle present, these localization errors are relatively small and not particularly concerning for traffic safety. When there are two or three EVs present, though, the situation can get a bit stickier. “In that case, the participants had much more [difficulty] localizing the cars, up to a point where most participants failed to even detect all presented EVs within an appropriate time,” Müller says.
There are a few reasons why pedestrians might have trouble locating EV sounds. First, Müller explains, combustion noise is a very broadband signal—meaning it contains a lot of frequencies, “and hence more information for our hearing system to work with.” Second, humans have had substantially more time to acclimate to combustion sounds than artificial EV sounds.
“We humans have learned over the last 100 years or so that cars sound in a particular way and how driving behavior, such as acceleration, is reflected in this combustion noise,” Müller says. “This potential learning effect might also contribute to differences in localization, especially when we need to ‘decode’ multiple sounds at the same time. One could expect that we would then also get used to EV sounds within a few years. The only problem is that they currently all sound different.”
A new sound
In the meantime, Müller believes there are two potential avenues to make EV sounds safer.
Currently, U.S. and EU regulations are limited to minimum sound levels in a specified number of frequency bands, which he argues “allows the warning signals to be anything between a futuristic spaceship sound or a racing car engine.” In the U.S., he adds, regulations don’t require a velocity pitch shift, meaning that a car might sound the same going 60 mph as it does at 25 mph. To address these problems, Müller says the regulations should “make more clear demands on the sound characteristics.”
On the automaker side of the equation, the Chalmers study indicates that a more broadband AVAS signal, similar to the noise radiated by tires when driving faster, is preferable to a two-tone or multitone AVAS.
“[This sound] is potentially less annoying than tonal sounds and has the advantage that we already have ‘learned’ to interpret this noise since we hear it every day,” Müller says.
In the long term, he adds that adaptive AVAS solutions—like pedestrian detection technology—could help EVs radiate a more advanced warning sound directly in the direction of the pedestrian, thus improving safety and reducing noise pollution.
“One important bottom line here is that we are not saying EVs are bad or dangerous. With the right type of warning signal, they are not,” Müller says. “On the contrary, they have the potential of reduced noise pollution since the warning sound can be controlled, while the combustion noise in [internal combustion engine vehicles] is always there.”
