Deafening impact

时间:2019-03-07 10:04:04166网络整理admin

By Paul Marks ALTHOUGH airbags save lives in car accidents, they can also permanently damage people’s hearing when they inflate. But a new simulation of how the ear behaves could help car designers cut the risk of these injuries. During a crash, airbags are filled by an explosive propellant quickly enough to cushion the vehicle’s occupants. But when researchers at the Michigan Ear Institute studied car accident reports, they found that this rapid inflation creates a wave of high pressure that can damage the inner ear. To discover exactly what causes such damage, the US Army—which plans to put airbags in helicopters as well as road vehicles—has developed software that mimics the response of the ear to pressure waves. To help ensure its accuracy, Richard Price and his colleagues at the Army Research Laboratory in Aberdeen, Maryland started by modelling the ears of cats. Although the animals are sensitive to higher frequencies than people, their response to noise is similar. So the team anaesthetised cats and exposed them to airbag deployments while they were unconscious. On average, Price reports, the cats’ hearing thresholds were permanently reduced by 37 decibels at 4 kilohertz—just as the software predicted. “So a parallel version of the model for the human ear has been developed,” he says. Called the Auditory Hazard Assessment Algorithm, the software models how the membrane that bears the hair-like hearing cells moves up and down at 23 locations in the inner ear. “It’s the flexing of that membrane that does the damage,” says Price. From this, the software calculates the risk of hearing loss (see Figure). The model also takes account of a curious capability of the human ear: if people see an accident coming, a facial reflex contracts middle ear muscles, blocking the flow of energy to the inner ear and reducing the risk of damage. Surprisingly, the simulation predicts that airbags cause least damage when a car’s windows are closed. Price says that the higher peak pressure jams the bones of the middle ear, which protects the inner ear. “It blocks the flow of energy during that period, preventing the membrane flexing and getting damaged,” he explains. The model also shows that the way a wave rises and falls, rather than its peak pressure, is crucial in determining the extent of any damage. The airbag’s percussive waveform can be changed by simple factors such as the way the airbag has been folded, says Price, as well as the configuration of the car interior. He has recently presented his work to the research chiefs of several motor companies. Ian Flindell, a researcher at the Institute of Sound and Vibration Research at Southampton University, says that the approach is very interesting. But he points out that the precise mechanism behind hearing loss is poorly understood. “So their model could have large inaccuracies,