Car crashes are dangerous, sometimes fatally so. Car manufacturers, therefore, are doing what they can to protect drivers and passengers alike. To date, they’ve come up with features like crumple zones, seatbelts, air bags, and more. These innovations typically focus on external physical injuries or things like concussions — injuries caused by the impact of the crash. But the crash itself often creates another type of arm, one typically overlooked. Crashes make loud noises — blasts which can cause hearing loss.
A rather recent feature in some Mercedez-Benz vehicles aims to help with that.
Mercedes introduced its current crash protection suite of features, called Pre-Safe, in 2002. Like the safety features of every other car company, Pre-Safe has been improving over the years, adding more measures over time. In 2015, they announced Pre-Safe Sound, which temporarily shuts off our ears… kind of.
To be clear, our ears don’t have an off-switch. But they do have a lot of little muscles, and one of them, called the stapedius muscle, can help block loud noises. The stapedius muscle sits between the eardrum and the inner ear. If the stapedius contracts, the sound which hits the eardrum doesn’t pass cleanly into the inner ear; instead, some of that sounds reflects harmlessly back onto the eardrum. When we’re faced with a loud noise, something called the “acoustic reflex” kicks in. It’s a combination of involuntary muscle contractions, including a contraction of the stapedius muscle, creating this muting effect.
A car crash, though, is too loud and too sudden for the acoustic reflex to help significantly. But there’s a way to fake it, typically called “pink noise.” Effectively, pink noise is a sound which fools our ears into thinking that the world has become really loud of all of a sudden. As Geek.com explains, pink noise “has more energy than a single tone because of the way it’s spread across the spectrum” and therefore “can have lower volume but still trigger the stapedius reflex.”
And that’s exactly what happens. As Wired reports, “when the car senses an imminent impact (using onboard cameras and ultrasonic sensors), the stereo plays a loud static-type noise around 85 decibels [the pink noise],” which is “not so loud that it hurts.” (That’s about as loud as a garbage disposal or a food processor.) That 85-decibel noise hits your ears just milliseconds before the crash, which is plenty of time for the stapedius to contract into action. Your ears shut down a bit, making the noise from the crash impact not nearly as harmful.
From the Archives: The Sound of Silence: Loud noises hurt our ears. But what about near-complete silence?