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EVs Need to Sound Off

Sept. 7, 2018
The NHTSA has spent over five years developing warning sound rules to protect pedestrians from slow-moving EVs, but some issues remain unresolved.

Electric vehicles (EVs) may reduce pollution, but they also create a safety hazard. A 2009 study by the U.S. National Highway Traffic Safety Administration (NHTSA) found that crashes involving pedestrians and bicyclists have higher incidence rates for EVs than internal combustion engine (ICE) vehicles in low-speed vehicle maneuvers, such as reversing or leaving a parking zone. These accidents commonly occurred in zones with low speed limits, during daytime and in clear weather.

The study revealed that an EV is two times more likely to be involved in a pedestrian crash than a conventional ICE vehicle when it’s slowing or stopping, backing up, or entering or leaving a parking space. Vehicle maneuvers were grouped in one category considering those maneuvers that might have occurred at very low speeds where the difference between the sound levels produced by the EV versus ICE vehicle is the greatest.

Also, the study found that the incidence rate of pedestrian crashes in scenarios when vehicles make a turn was significantly higher for EVs than ICE vehicles. The NHTSA study concluded that the incidence rate of bicyclist crashes involving EVs for the same kind of maneuvers was significantly higher than those with ICE vehicles.

Sound Legislation

In response to the NHTSA findings, the U.S. Congress passed the Pedestrian Safety Enhancement Act (PSEA) of 2010, which was enacted into law in January 2011. It mandates that the Department of Transportation create safety standards for car manufacturers to create a sound that alerts pedestrians to the presence of an EV moving at slow speeds.

Under the PSEA, the added sound must be "recognizable" as that of a motor vehicle in operation. This standard is projected to reduce the number of incidents in which EVs strike pedestrians. This regulation will require all new EVs to have an audible alert system if they're manufactured on or after September 1 of the calendar year—the clock on that starts three years after the date the final rule is issued.

The NHTSA found that sound produced by an EV is from its tires, the air, and sometimes the whine of its electronics. If the car was going fast enough, tire noise was usually enough to warn pedestrians and bicyclists of possible danger. When an EV moves slowly, its generated noise is barely noticeable, posing a danger to anyone nearby. To be safe, a slow-moving EV should produce a sound that indicates:

  • Its presence
  • Its approximate location
  • Whether it’s moving toward or away from the listener
  • Roughly how fast it’s moving

The generated sounds would be heard frequently even in light traffic and continually in heavy traffic, so they must not be annoying. Also, the generated sound should be different from sirens, horns, and backup signals, all of which are intended as aggressive warnings. The challenge for EVs is to make sounds that alert and orient, but not annoy.

The NHTSA published its standards in December 2016, which established new Federal Motor Vehicle Safety Standard (FMVSS) No. 141: Minimum sound for hybrid and electric vehicles. NHTSA’s sound standard is intended to ensure that blind, visually impaired, and other pedestrians are able to detect and recognize nearby EVs by their sounds. In response, the agency received submissions from petitioners requesting changes as well as technical questions. After consideration of the petitions and all supporting information, NHTSA has decided to grant some changes, deny some, and request comment on others.

Effective April 27, 2018, all hybrid and electric vehicle regulations were amended and will be applicable beginning on September 1, 2020. The initial compliance date for newly manufactured vehicles under the 50% phase-in as specified in FMVSS No. 141 is delayed by one year to September 1, 2019. Petitions for reconsideration of this final action were due by April 12, 2018.

Resolving the Issues

Satisfying the petitions received by April 2018 is more complicated than it appears. Among the perplexing issues is how to control the sound produced by the EV. One decision was to allow variations in alert sound across different vehicle types, and to reduce the number of compliance criteria.

Vehicles of the same model designation might have different powertrains and bodywork, which have the potential to influence both the emitted sound and the air-generated sound when the vehicle is in motion. Therefore, it’s not accurate in all instances to consider all vehicles of the same make/model to be the same in terms of meeting the FMVSS No. 141 requirement for the same make/model vehicles. It was left up to NHTSA to interpret how “model” should be defined for the purpose of regulating similarity of the pedestrian alert sound. NHTSA is amending its final rule so that alert sounds can vary across different vehicle trim levels in addition to varying by make, model, and model year as provided in the final rule.

NHTSA said EVs will have to meet a requirement based on sound level in either two or four one-third octave bands at the vehicle manufacturer's option, and a vehicle may alternate between meeting the 2-band and 4-band specifications depending on test speed. Vehicles complying with the 4-band option must meet minimum sound pressure levels in any four non-adjacent one-third octave bands between 315 Hz and 5000 Hz. This includes the one-third octave bands between 630 Hz and 1600 Hz (these bands were excluded in the original Notice of Proposed Rulemaking, NPRM).

Vehicles complying with the 2-band option must meet minimum sound pressure levels in two non-adjacent one-third octave bands between 315 Hz and 3150 Hz, with one band below 1000 Hz and the other band at or above 1000 Hz. The two bands used to meet the 2-band option also must meet a minimum band sum level.

In addition, the standard’s regulatory language will be changed to permit the alteration of the alert sound as originally equipped on a vehicle for repairs and recall remedies. The NHTSA is simplifying the criteria so that the digital sound file and the sound-processing algorithms will be the only specific criteria that are required to be the same from one specimen test vehicle to another.

In response to a request to allow vehicles to be manufactured with a suite of driver-selectable pedestrian alert sounds, the agency is neither granting nor denying that. Instead, NHTSA intends to issue a separate document at a later date to seek comment on the issue of driver-selectable sounds.

NHTSA denied a request on changing the crossover speed from 30 km/h to 20 km/h. At the crossover speed, the pedestrian alert sound turns off. The agency determined that the available information on lowering the crossover speed doesn’t warrant making that change. NHTSA's detection-based analysis didn’t support lowering the crossover speed to 20 km/h. Given the absence of new data, NHTSA had no basis to revise its previous conclusion about crossover speed. In addition, the National Federation of the Blind expressed its support for the 30 km/h crossover speed and disagreed with lowering it to 20 km/h.

The NHTSA will seek comments on a request to allow hybrid and electric vehicles to be equipped with multiple, driver-selectable alert sounds before granting or denying that request. Amending the requirements to allow multiple sounds per vehicle would be a substantial change to the final rule. NHTSA didn’t solicit or receive comment on the number of driver-selectable sounds that should be allowed, so it believes it’s appropriate to seek public comment before determining whether to grant this request.

EVs will be required to emit sound at minimum levels while the vehicle is stationary (although not when the vehicle is parked, i.e., when the transmission is in “park”), while in reverse, and while the vehicle is in forward motion up to 30 km/h.

The question of whether EV testing should be indoor or outdoor was raised. The agency acknowledged some advantages of indoor testing in hemi-anechoic chambers, but also pointed out several reasons why outdoor testing on an ISO-compatible test pad is preferable. It was concluded that the agency intends to conduct its own compliance tests using outdoor facilities.

The agency noted that the absence of a specific test procedure for indoor testing doesn’t mean indoor testing is prohibited. On the contrary, vehicle manufacturers, suppliers, and others have the discretion to conduct FMVSS No. 141 certification tests indoors as long as they can certify that a vehicle fully complies with the Safety Standard.


About 140,000 EVs are now registered in Britain compared with just 3,500 in 2013. By 2030, the National Grid predicts there could be as many as 9 million EVs on Britain’s roads.

Chris Hanson-Abbott, whose firm Brigade Electronics is a distributor of vehicle safety products, is an adviser to the UN working group on quiet road transport vehicles. The group came up with the industry standards now being introduced across the world as electric cars become mainstream. “The objective is to have warnings that are audible but are not environmentally disturbing,” said Hanson-Abbott, who introduced the reversing alarm to the UK in 1976.

Hanson-Abbott said battles have raged over what sort of sound the vehicles should emit and when they should emit it. The agreed standard is a mix of tonal sounds and white noise that will cut out once the vehicle gets to about 20 km/h and the sound of the tires becomes sufficiently audible.

“White sound is very pleasant. It’s the sound of falling water,” said Hanson-Abbott. “It has two unique characteristics. One is that it’s very pleasant on the ear and the second is that the source direction of that sound is instantly recognizable. The moment you hear white sound, you can point directly at where it’s coming from.”

In contrast, “tonal” sound emitted by diesel or petrol cars can bounce off hard surfaces, making it difficult to judge its source. “It’s a huge improvement on the noise emitted by petrol or diesel vehicles because its sound source is directional,” said Hanson-Abbott. “That’s a massive safety factor.”

Active Pedestrian Warning System

1. Kia’s new Niro EV has an Active Pedestrian Warning System that recognizes pedestrians and uses its front speakers to warn them of an approaching vehicle.

Electric vehicles have a quiet powertrain, so pedestrians aren’t always aware of an approaching EV. Kia’s new Niro EV (Fig. 1) has an Active Pedestrian Warning System that recognizes pedestrians and uses its front speakers to warn them of an approaching vehicle. This warning system combines the Niro EV's cameras, object-recognition technology, and the car's front speakers. As a result, it recognizes the pedestrians and warns them of its presence. Kia’s Niro EV, an all-electric, zero-emissions crossover EV, is the company’s second globally sold electric vehicle after the Soul EV, and will go on sale in Europe at the end of 2018 and North America in Q1 2019.

Jaguar is taking a different approach on generated sound. It’s even adding artificial motor sounds to its new all-electric I-Pace (Fig. 2). The British automaker says that it was trying to reduce road, wind, and motor noise as low as possible to create the best experience possible in an electric vehicle. The exception is when the car is accelerating.

2. Jaguar is adding artificial motor sounds to its all-electric I-Pace.

Iain Suffield, a sound design specialist at Jaguar, said, “I added some sounds at about somewhere between 50 and 100 km/h such that it represents when you are accelerating up with the sense of power of the car.” The sounds are coming out of the vehicle’s speakers with different tones based on the speed.

In Japan, there was a national outcry when a guide dog and its owner were killed by a reversing electric vehicle whose driver had used a pause control to deactivate its sound emitter. The new standards, which in Europe will be introduced via an EU directive, will require activation by default when the vehicle is on.

Different Strokes for Different Folks

Nissan unveiled the IMx, its newest electric concept that sings (Fig. 3). It emits a noise like a demonic string quartet tuning its instruments. Nissan calls the feature Canto—which literally means “I sing” in Italian—and it’s built to alert pedestrians that the very quiet electric vehicle is coming, even at low speeds.

3. The Nissan Imx EV sings to alert pedestrians that a very quiet electric vehicle is coming.

Nissan released its first pedestrian warning with the 2011 Leaf. The Canto concept improves on the model, adapting its tone and pitch to the car’s actions—accelerating, decelerating, or backing up. The carmaker's designers of course wanted to create noises that put pedestrians on high alert, but were careful to design sounds that “enrich the aural environment of the typical city street,” according to a statement. If a city street naturally sounds like the warm-up room at an ‘80s synth rock convention, this Nissan crossover concept should fit right in.

“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 that 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.

Ustwo's solution is somewhere between a warning alert and a turn signal. The designers created what they call the risk scale. It assesses risk on a scale of Low to Extreme, depending on the likelihood of an impact, and emits a corresponding sound. The designers offer a variety of suggestions for doing this; for instance, the car emits a sound that gets progressively louder and more frequent as it nears a pedestrian.

In another example, 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 cases with the greatest risk of a collision—say someone stepping off the curb in front of an oncoming car—the car might emit an ear-piercing alarm.

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.’”

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