A 2012 blog on air bags at Hearing International ended with the statement, “Sensorineural hearing loss due to air bag deployment is rare, and there have been only a few reports in the English literature. Audiologists know, however, that 140 dB of intensity is the threshold of pain and these intensities can cause hearing loss under some conditions. At Hearing International we suspect that a minimal hearing loss from these peak intensities of air bag deployment are a small price to pay for a life saving device. “

Recently, there have been strides in the use of the acoustic reflex to minimize the possible hearing impairment that may result from the intensity of the sound of a collision and the resulting air bag deployment.

Really?

Most audiologists know of the stapedius muscle and the acoustic reflex. We have used this reflex each day as we assess the auditory mechanism since the early 1970s – but what does that have to do with Muscle Cars? Some enthusiasts trace the history of muscle cars way back to the 1949 Oldsmobile Rocket 88. In the mid 1960s a whole generation of young people began buying cars, but they did not want the old boring cars of their parents, they wanted a sports car. Alas, with the burden of school, no money, no profession, most could not afford the real sports cars such as Ferrari, Aston Martin, and other famous brands, so the American car manufacturers gave them one. While the Ford Mustang and the Chevrolet’s Camaro sold well and were prominent entries into the market, some do not consider them true muscle cars, they are often dubbed as “pony cars”.

The Stapedius of all muscle cars was the Pontiac GTO. While a whole generation knows and even aspired to this car, the meaning of its name was almost a “baby boomer” mystery. According to one of the GTO designers, John DeLorean (1925-2005), the name was inspired by the Ferrari 250 GTO, a successful early 1960s race car. The GTO component of its name was an Italian abbreviation for Gran Turismo Omologato, (grand tourer homologated“) which means it was officially certified for racing in the grand tourer class. US auto manufacturers engineered muscle cars for straight-line speed, inspiring more than an occasional Saturday night drag race between traffic lights. While the true muscle cars, such as the GTO did not sell in huge numbers, sales managers at the time felt that they were the bait that lured buyers into showrooms to purchase more mundane models. These were the days of exciting cars and a culture that you just wanted to be part of – the muscle cars were definitely cool! Songs were written about many of them by famous bands such as the Beach Boys; check out the toe tapping song by Ronnie and the Daytonas. Of course these songs made it to the top of the charts, inspiring even more interest and sales of these now classic automobiles. The heyday of this genre ran from about 1965 to 1970 before collapsing under the weight of higher gas prices, more stringent exhaust emissions regulations and soaring insurance costs. With these cars the only connection with the Stapedius muscle was its contraction to the sound of very loud pipes as they raced down the street!

Stapedius and the Muscle Car

The scientific coupling of the stapedius muscle with high performance, high technology cars seems to be a far fetched audiological fantasy but it is a new century. The GTOs are now historic classics, as are Ronnie and the Daytonas. In this new century there’s a lot of high tech in the automobile industry that is getting a lot of attention. No longer is it bigger engines and better times at the drag strip that motivates sales, but cars can now talk to each other, talk to street signs, park and brake by themselves, even drive themselves — or they’re getting there, anyway. Much of this new technology is in the service of safety with the thought eliminating human error centered around the driving experience. While all this accident reduction technology is great, there are still drivers that crash out there. In addition to the obvious other injuries, the sound intensity of automobile crashes are deafening and may create not only hearing loss but hyperacusis, and tinnitus. It has long been known that air bag deployment along with the other sounds involved in car crashes are extremely loud. Studies by the Acoustical Society of America (1999) and a number of other professional associations indicate that, a single air bag deployment can reach a 140 dB impulse but the sound of multiple air bags going off and the intensity of the crash itself can reach as high as 170 dB! Enter the new Stapedius of muscle cars…the Mercedes E350!

At Mercedes-Benz there is a program headed by Rodolfo Schoneburg called PRE-SAFE® technology that has been around the company since 2002. The question asked by Schoneburg’s group was that it takes just a split second for a car accident to completely change a person’s life, what can be done in those split seconds to minimize the damage to the occupants? Their answer was that once the vehicle’s sensors have detected that a collision is unavoidable, a comprehensive system of safety features should be activated before the impact occurs. While the group created many safety innovations occurring in the first 150 milliseconds of a crash that substantially reduces injury, among the newest of the use of the Acoustic Reflex. Mercedes-Benz is now extending its PRE-SAFE® technology with something they call, PRE-SAFE® Sound beginning with the 2016 E-Class, model series 213.

How Does it Work?

The PRE-SAFE® Sound System is the first to harness the acoustic reflex to condition the ear – when a collision is imminent – for the loud noise that is anticipated from the impact of the vehicle. According to Mercedes, if an impending collision is detected that would be expected to produce a loud crash, the vehicle’s sound system plays a short interference signal. This causes the stapedius muscle in the ears to contract, which for a split second, changes the link between the eardrum and the inner ear and protects it against high acoustic pressures. Most importantly, the reflex reduces the damage to hearing. The system uses a special chip installed into the multi-media sound system of vehicle that generates pink noise. When the PRE-SAFE® system detects that an accident is imminent, a pink noise signal is played through the sound system at a volume of around 80 dB. The ear is then primed for the high level of sound that typically accompanies a collision by the stimulation of the Acoustic Reflex making it less sensitive to the loud sound that is coming a few milliseconds later.