Electric vehicles (EVs) are so quiet that their operation has created an unexpected public safety hazard. Though environmentally friendly, the vehicles’ near-silent approach at low speeds poses a real danger to unsuspecting pedestrians.
Automakers are required to fit EVs with approach-warning systems, but the exact sounds are mostly up to them. This freedom has given researchers an exciting, unique opportunity.
Graduate student Mei Suzuki and her team at the Acoustical Society of Japan took on the challenge. The researchers intended to develop a sound that wasn’t random noise but a purposeful signal to inform others of the vehicle’s presence.
“In our research, we aimed to design approach-informing sounds based on onomatopoeia that [are] evoked by the image of a ‘quiet vehicle,’”said Suzuki in the press release on December 1.
“These stimuli included sounds created based on onomatopoeic sounds and pink noise,” Suzuki added.
Search for the perfect noise
Electric vehicles are noiseless on roads because their power comes from batteries and electric motors, eliminating the need for a traditional combustion engine.
The goal was to create stimuli that naturally convey the presence and approach of an EV.
The researchers developed a library of sounds using two main methods.
First, they designed sounds based on onomatopoeia — words that imitate the sounds they represent, like hiss — specifically chosen to reflect the image or concept of a “quiet vehicle.”
Second, they incorporated pink noise, a filtered sound known for greater energy in the lower frequencies.
The team tested a library of these created sounds. Volunteers rated each one based on criteria like “The sound is easy to notice” and whether it had a “sense of urgency.”
These tests happened in two settings: a controlled studio and real road conditions, ensuring the results were practical. The sounds were subsequently ranked according to the feedback received.
Low-frequency pink noise
Clearly, the best performer was a specific version of pink noise — a sound characterized by an uneven energy distribution across its spectrum.
This low-frequency noise was less likely to be obscured by other background sounds. It means that an approaching vehicle could be heard clearly by volunteers, even amid construction or busy traffic.
“The reason this sound stimulus was rated highest was its strong low-frequency components and its similarity to automotive running noise,” said Suzuki.
The research doesn’t stop with cars. The team is already planning to introduce similar sounds for smaller, lightweight devices like electric bicycles and e-scooters.
“Starting this year, we are conducting research on the sound design of approach warning sounds specifically for micromobility devices,” said Suzuki.
“Since research on approach warning sounds for micromobility devices is largely unexplored, we believe this could contribute to reducing collisions involving pedestrians and visually impaired individuals,” the researcher added.
This next phase will be crucial for safety on crowded urban streets.
Suzuki presented her team’s efforts at the Sixth Joint Meeting of the Acoustical Society of America and Acoustical Society of Japan in Honolulu, Hawaii. The findings offer automakers a clear, data-driven direction for vehicle sound design.