For Stealthy Electric Cars, Auditory Illusions Could Save Lives

By Mark Changizi | September 13, 2012 9:30 am

Mark Changizi is an evolutionary neurobiologist and director of human cognition at 2AI Labs. He is the author of The Brain from 25000 FeetThe Vision Revolution, and his newest book, Harnessed: How Language and Music Mimicked Nature and Transformed Ape to Man.”

The silent purr of an electric car is a selling point over the vroom of a gasoline engine, but it comes with an undesirable side effect: An electric car can pounce on unsuspecting passerbys like a puma on prey. In fact, the NHTSA found that hybrid electric cars are disproportionately dangerous to pedestrians. To deal with this problem, it has been proposed that sound be added to hybrid and electric vehicles, whether it be bird-songs or recordings of someone making “vroom vroom” sounds.

In this light, I wondered whether it might be possible to add “smart sound” to these dangerously quiet cars destined to rule the road in the near future. The solution, I realized, might come from faster-than-light-speed objects on the moon. I’ll get to this crazy-sounding part in a bit.

The Melody of Movement

In setting out to solve this problem, I reasoned that when electric cars are moving very fast they make enough sound to be heard due to the rumblings of the car parts. It’s when they’re moving at lower speeds that they’re most perilous, because at these speeds they’re most silent. Therefore, if electric cars are to be fitted with some sound, it should be designed to work even at lower speeds—or, especially at lower speeds.

Next question was, What sort of sound do we want on slowish, stealthy electric cars? To answer this, it helps to grasp the sorts of cues your auditory system uses for detecting the movement of objects in your midst.

The most obvious auditory cue is that nearer objects are louder, and so when you hear a moving object rising in loudness, you know it’s getting closer.

But that’s not the most important auditory cue. To illustrate why, imagine walking along a curb with traffic approaching and passing you from behind. The important observation here is that when this happens you aren’t in the least worried. Even without seeing the car, you know it’s merely passing you despite the massive crescendo in its sound. Why?

Doppler shift illustration
The Doppler shift changes the observed pitch of the siren as the car moves.

You know the car isn’t going to hit you because of its pitch. Due to the Doppler shift, this car has a falling pitch, and this falling pitch contour tells your brain unambiguously that, although the car is going to get arm-reachably close, it is going to pass you rather than collide with you. If it were going to collide with you, its pitch would be high and constant—that’s the signature of a looming collision.

The most important cue about the trajectories of objects moving around us comes from pitch, and how pitch varies over time—the melody of movement. So, I reasoned, that’s what needs to be added to electric cars. It would be easy enough to add sound to the cars, making them noisier (e.g., the bird songs or “vroom vrooms” mentioned earlier). By doing so, one’s natural capability at recognizing the trajectories of moving objects would kick in.

But recall it is relatively slow moving electric cars that are the problem, and although we can sense the (Doppler) pitch changes for movers at speeds as low as one-meter-per-second slow walks, we could sense movers’ trajectory more ably if their pitches varied over wider ranges.

In this light I asked, Might there be a way to give slow-moving electric cars the Doppler shifts of fast-moving, easier-on-the-auditory-system, objects? Answering this question requires a detour into things that move faster than light…

Faster-Than-Light Objects on the Moon

If you shine a laser at the moon and flick your wrist, the spot of light will travel across the moon’s surface very quickly. In fact, it is quite possible that you can make it go faster than the speed of light!

Don’t believe me? Let’s do the numbers. The circumference of the moon is about 10 million meters, so your laser dot might travel about half that, or 5-ish million meters when it sweeps across. And you can rotate a laser quickly—I can do 180 degrees in about half a second. Given that the moon fills only about a half degree of the sky, the laser dot might do its 5-million-meter dash in about a thousandth of a second, which comes out to roughly 3 billion meters per second, or 10 times the speed of light.

But isn’t that impossible? Isn’t that the sort of thing that causes neutrino dust-ups?

No. Whereas it’s impossible to go faster than light, it’s not impossible to make something seem to go faster than light. A spot of light on the moon isn’t a thing at all. When a dog goes gaga chasing a laser, it’s because he thinks the erratic dot is a real thing, maybe an insect or mouse. But the spot isn’t an object at all—it’s only an illusory object. His and your brain perceive it as an object because the sequence of lit-up spots looks like the path of a moving object—that’s the same reason that videos show us moving objects even though nothing on the screen is actually moving.

So, the visual illusion of going faster than light is possible, thanks to our visual system’s eagerness to see objects. A great place to experience this for yourself is in the auditory domain.

Consider cutting with scissors. As you close the scissors, the point where the two blades touch moves from the joint outward toward the tips. That’s moving sound. And it might be moving very, very fast, even though the scissor blades may be closing fairly slowly. A simple scissor-close could lead to a sound that moves at hundreds of miles per hour or more! For a distinct example, if you drop a meter stick onto the ground, and one end hits the ground a hundredth of a second before the other, the sound-producing collision with the ground will travel along the meter stick in a hundredth of a second, or at 100 meters per second—that’s more than two hundred miles per hour.

Of course, it’s not moving at two hundred miles per hour because there is no it that’s moving in the first place. The “moving sound” in these scissor-like examples is just a sequence of distinct sounds that happen to occur one after another. It’s only your senses that think there’s something moving (and your sense’s limits don’t have to respect physics’ limits).

The crucial point is this, something I discuss in my book, Harnessed: Even though these fast movers are illusory, they still undergo a Doppler shift as if they are real and moving fast.

Driving With Scissors

We’re now most of the way to my idea for adding smart sound to quiet electric cars. Earlier we saw that the principal cue to a moving object’s trajectory is the “melodically” varying pitch from the Doppler shift, and reasoned that it would be great if slower-moving electric cars could have an amplified Doppler shift that is thereby easier to perceive.

And then, just above, I described how amplified, illusory, Doppler shifts are, in fact, possible, via mechanisms as simple as scissors. To amplify Doppler shifts there’s no need to actually place anything on the car that’s moving quickly.

All that’s needed are scissor-like, illusory-moving-sound mechanisms on cars.

Of course, I’m not actually suggesting placing (iteratively audibly closing) scissors on cars. Rather, the point is that by affixing to an electric car any mechanism creating a fast-moving illusory mover headed in the same direction as the car, electric cars can not only announce their presence with sound, but present a much amplified perception of their trajectory.

And that could be music to a pedestrian’s ears.

Image: Jeremykemp via Wikipedia


CATEGORIZED UNDER: Mind & Brain, Technology, Top Posts
  • simplicio

    Wouldn’t people then think the car was moving much faster then it actually was, and end up diving to get out of the way of a car that was slowly rolling up on them? Seems like it could cause more of a hazard then just having the car make a constant tone.

    Other possible objection: I think we use a combination of increasing loudness and changes in pitch to figure the trajectory of the car. With your scissor-device, the pitch would tell us its approaching quickly, but the volume would tell us its approaching slowly. I’m not sure exactly what peoples brains would end up telling them in that case.

    In any case, seems like an interesting and useful area for further experiment.

  • Mark Changizi

    Valid, and valid, worries. My thought is that once the market begins using some characteristic “illusory scissor” sound, pedestrians will begin to recognize that particular sound, and re-calibrate speed for that.

  • Dave Guilbeault

    A spot of light may not be a “thing”, but it’s created by things: photons. If you move your wrist on earth, those photons already in flight will not be affected; a laser beam is not a rigid rod.

    Imagine you are illuminating a spot on the moon, M1. You then re-aim your laser at another spot on the moon, M2, and emit a photon. The time it takes to traverse the earth moon distance is the minimum time between illumination of points M1 and M2. The distance between M1 and M2 is less than the Earth-Moon distance, so the apparent effect as observed on the moon is not faster than light.

    Agree on the danger of silent electric cars though.

  • Tom Saxton

    I’ve been driving electric vehicles since 2008, logging over 50,000 miles, and have never had an experience where my vehicle’s lack of engine noise created an unsafe situation.

    I have had a pedestrian walk backwards through the traffic lane in a parking lot while carrying on a conversation with someone across the lot. She didn’t notice my vehicle, but I was watching where I was driving and going slow enough to react to her carelessness, waiting for her to see me. She of course made a rude comment blaming me for the unsafe situation she caused.

    I’ve also had many times when driving through a parking garage where pedestrians are walking up the middle of the traffic lane, totally oblivious to my presence. However I find that happens with about the same frequency it did when I was driving gas cars, which I attribute to the echoing in concrete garages making it hard to hear slow-moving vehicles from behind, even when they are close.

    But the situation is more complex that just my personal experience.

    In May of 2011, I participated in a meeting of the United Nations working group that is developing a proposal for an international standard for quiet vehicles. There I learned a great deal about the subject and was able to share my insights as an experienced electric vehicle driver.

    The predominant sound made by cars moving above 15 to 20 miles per hour is tire noise. At slower speeds, it’s engine idling, fans, and so forth. As described in this article, it’s those lower speeds that are of concern.

    However, hybrid and electric vehicles aren’t the only quiet vehicles on the road. Many modern sedans are also virtually silent at low speeds where tire noise is not significant. Therefore, the UN is taking a broader approach to this problem than the US Pedestrian Safety of 2010.

    The sound made by gas cars is actually quite poor for alerting pedestrians to nearby vehicle traffic. Most of the sound made by internal combustion vehicles are low-frequency sounds that humans have difficulty locating, and carry for long distances, adding to ambient noise levels that can mask out nearby vehicles.

    For an artificial car sound to be effective, it has to be localizable and distinguishable from other natural sounds. So having an EV chirp like a bird is a terrible idea. Studies presented at the UN workgroup meeting show that the best sounds are complex, broad spectrum sounds without low frequency content.

    While the scissor Doppler effect is an interesting idea, I don’t see how it can be sustained: it has to cycle. It also seems pretty unnecessary since our ears and brains are very effective at localizing the source of sound, especially for properly designed sounds.

    The issue is even more complex for blink pedestrians. The idling sounds made by stationary vehicles are useful not only for detecting the presence of nearby cars, but also for using them as positional markers. Consider walking across a wide, busy street and trying to stay in the crosswalk without looking. The sound of the cars nearest the crosswalk idling act as navigational beacons, keeping blind pedestrians from from drifting into the perpendicular traffic. For this reason, it’s important to be able to tell a car is not moving.

    It’s also important to be able to judge the size of vehicles by their sound. Drivers behind a large, stopped vehicle can get impatient at the hold-up and decide to blast around the unwanted obstruction, only to find that there was a good reason for the large vehicle to be stopped: pedestrians in a crosswalk. For this reason, blind pedestrians may choose to avoid this risk by choosing not to cross when they hear a large vehicle at the head of the line.

    Regulating the sounds for quiet vehicles, whether power by gas or electricity, is a complex and subtle issue which unfortunately is burdened by emotion, bias, and ignorance. It demands careful thought and consideration to many complex issues.

  • Bubba Nicholson

    Cars need to be quieter, not louder. We need to fit prostheses to blind people to detect electric cars and forget about them. What America needs is less noise, not more! Our wheel wells need to be covered and sound-insulated to prevent tire noise escaping as well as to improve vehicle aerodynamics. Why? Ever live near a highway? If we can stop all the noise pollution, those homes will become valuable again. It would be nothing to identify people in the way of a car (cell-phone sensors or infra-red) that automatically sound a horn if they’re in any danger and automatically hit the brakes. These devices are already available and becoming less scarce all the time.

  • Rick Prescott

    I live on a very busy street, and the most wonderful thing I can imagine is cars (and buses) which make absolutely no sound.

    The most horrible thing I can imagine is cars (and buses) which would not NEED to make any sound, and yet are designed intentionally to make some sort of artificial sound just to announce their presence.

    Why would we do this?

    The best thing which could happen for our sound-polluted environment is that these new cars remain quiet and pedestrians stop relying on auditory cues rather than much more effective visual cues like crosswalk signals (and technological equivalents for the vision-impaired).

    By forcing pedestrians into greater awareness, silent cars could ultimately turn stories of pedestrian mishaps into something that will cause future generations to shake their heads in disbelief at how backward we were.

  • Mephane

    The problem is that there won’t be all “illusory scissors” sounding cars over night, but for a, long time (or possibly even forever) you would have both on the streets. You would need a set of sound completely different from combustion engine cars in order to be able to distinguish, and then learn different estimation of speed, distance and direction for both sets of sounds.

    That’s why I also think the best solution is to simply outfit those cars with sounds similar to a car with a combustion engine, and then only tune the artificial sounds down as the speed increases and the sheer sound of the tyres etc. provides enough acoustic cues.

  • Mark Changizi

    Unless… One outfits even combustion engine cars with the same sorts of devices, amplifying our sensitivity to *all* moving vehicle trajectories. And also for bikes, for the safety of pedestrians *and* the bicyclist.

  • Mark Changizi

    More on Mephane: My bet is that once one chooses some peculiar sound or timbre for this, we’ll learn to recalibrate the speed for just *that* timbre. But I haven’t tried the experiment.

  • Adriaan

    Regarding your worries about pedestrian re-calibration — to be honest I think this is as distinct non-issue. Correct me if I’m wrong, but isn’t the whole purpose of the simulated-fast sound NOT to make pedestrians think that the cars are approaching faster than they actually are, but *rather* to make it easier for them to distinguish between an impeding collision and a safely-passing non-collision? Pedestrian re-calibration will certainly doom the former, but I think that the latter should survive just fine.

    That is to say, given a situation where a car will miss a pedestrian, an accelerated sound means that its apparent frequency to the pedestrian will drop faster (and thus the drop be more perceptible) than a non-accelerated sound. I don’t *think* that people’s frequency-change-detection capabilities will decrease with exposure… :-)

    In fact, we can even use pedestrian recalibration to answer Simplico’s concerns. They’re certainly valid points, but I think that people would adapt? At least if the sound acceleration isn’t too significant?

  • tina juarez

    WhenI was a little girl, I was taught to STOP, LOOK & Listen before stepping off the curb. I teach this to any young child I am in the company of. Maybe a change in curricula is in order. Will Pedestrians wearing & playing loud music in their ear pieces hear any thing? Also, I have noticed that a creeping crawl through the crosswalk is replacing a full stop for gassers as well as EVs.. Driver manners would also help make the world safer for pedestrians.

  • Jason M

    I think the answer to this problem is the same answer to the problem of texting or other driver distractions causing accidents. Take control of the car away from the human. Google and others have proven self-driving cars are possible with today’s tech. It is only a matter of time for manufacturers to start releasing these types of vehicles, and my guess is that electric vehicles will be the first with self-driving capabilities. So for a slow moving EV that registers a nearby pedestrian, a short notification sound can be made (beep-beep) to alert the pedestrian and the car should be able to stop itself prior to any collision. We cannot make human pedestrians smarter to avoid walking in front of vehicles. We can make vehicles smarter to avoid the humans, and do so without adding a dumb feature like constant or variable sounds that play at low speeds.

  • Fred

    This is a total non-issue.

    These are the observations of a bicycle commuter in Sacramento where there are a large number of EV and Hybrid vehicles – even more than average as I go past the CalEPA and CARB offices every day where all of the newest models are tested and shown.

    * Hybrid and electric cars, while quite inside, are not silent from the outside.

    * At low speed there is significant noise from the transmission, tires and controller/motor.

    * At higher speed (20 – 25mph) the wind noise is added to the tire noise and an electric or hybrid sounds the same as an internal combustion car coasting.

    * Slowing down or stopping a hybrid is louder than an IC car due to the regen loads on the drivetrain.

    * If the background noise is high there is no difference in sound between a Prius and a Crown Victoria.

    I think this issue was raised as an anti hybrid EV talking point and should just be forgotten.

  • Brian Too

    I already know the sound these cars should make. Movies taught it to me.

    Many movies have portrayed electric vehicles. It may be a real sound captured on set, or one added in post-production, but such vehicles make noises very much like those of a large electric motor. If such noise needs to be added to electric cars it should be cheap, easy and non-disruptive to do so.

    I’m not joking. The reach of the movie industry is vast and has already taught billions of us what to expect. This could work.

  • Matt B.

    My best idea is whistles, molded into the front corners of the car. I’m sure it’s possible to design the whistle to be louder at slower speeds, although there’s a limit to how slow. (Then again, there’s a limit to how dangerous a car is at slower speeds.) But using a natural sound like that would prevent having to devote power usage to it, or having to design the sound to have the right psychological effect. Wind will change the sound, but people are used to that with other things, so I don’t think it’s anything to worry about.


The Crux

A collection of bright and big ideas about timely and important science from a community of experts.

About Mark Changizi

Mark Changizi is the director of human cognition at 2AI Labs and the author of several books, including Harnessed: How Language and Music Mimicked Nature and The Vision Revolution.


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