Dumb cars shield against crashes. Smart cars avoid them. Shrewd cars prepare you for them.
This year, Mercedes-Benz is introducing what surely is one of the shrewdest precrash features yet: a burst of sound that causes a muscle inside the ear to contract, bracing the eardrum against the potentially deafening noise of the crash itself.
Tip o’ the hat goes to IEEE Spectrum’s auto maven, Lawrence Ulrich, who pointed out to us the importance of “Pre-Safe Sound,” standard in the 2017 E class. (Ulrich will describe this and other marvels in “Top Ten Tech Cars,” in our April issue.)
The idea of pre-crash safety has been out there for a while. For instance, a car can instantly tighten the seatbelt to minimize movement and prevent the body from “submarining” forward, under the belt. Or it can inflate a tiny airbag to nudge the driver toward the center, protecting against side impact. Or it can close the sunroof, adding to the rigidity of the cabin.
But Pre-Safe Sound goes to ear-popping lengths. When the car’s sensors sense an impending crash, the cabin is filled with a burst of “pink” noise, a broad spectrum of frequencies in which the power is inversely proportional to the frequency. That triggers the so-called acoustic reflex, in which the stapedius muscle—the smallest muscle in the body (remember that for Trivial Pursuit)—contracts, bracing it, the bones of the inner ear, and the eardrum.
The pink noise is around 80 decibels, about equal to that of a dishwasher and completely safe. A crashing car puts out around 145 dB, high enough to damage hearing, at least some of the time. Worse still—and this part is not emphasized by Mercedes-Benz or any other carmaker—is the noise created by the near-instantaneous deployment of the airbag: around 165 dB. It’s estimated that 17 percent of the people who are exposed to airbag deployment suffer some degree of permanent hearing loss.
That’s why its important to put the various safety features into action in the right order. First comes totally safe features, like emergency braking, seltbelt pretensioning and Pre-Safe Sound. The airbags come only when necessary, during the crash itself.
Pre-safe noise took a long time to go from a gleam in an inventor’s eye to a commercial offering. A U.S. patent for the idea was filed in 1997 by one Armin Kausch, an employee of a subsidiary of TRW Automotive. That patent application cited earlier work going back as far as 1960.
It takes time to get the bugs out. Remember that whenever you read that cars without steering wheels will be plying our roads before your kids are old enough to get a driver’s licence.
Philip E. Ross is a senior editor at IEEE Spectrum. His interests include transportation, energy storage, AI, and the economic aspects of technology. He has a master's degree in international affairs from Columbia University and another, in journalism, from the University of Michigan.