Airbags are an incredibly important piece of automotive safety gear. They’re also terrifying—given that they’re effectively small pyrotechnic devices that are aimed directly at your face and chest. Myths have pervaded that they “kill more people than they save,” in part due a hilarious episode of The Simpsons. Despite this, they’re credited with saving tens of thousands of lives over the years by cushioning fleshy human bodies from heavy impacts and harsh decelerations.

While an airbag is generally there to help you, it can also hurt you in regular operation. The immense sound pressure generated when an airbag fires is not exactly friendly to your ears. However, engineers at Mercedes-Benz have found a neat workaround to protect your hearing from the explosive report of these safety devices. It’s a nifty hack that takes advantage of an existing feature of the human body. Let’s explore how air bags work, why they’re so darn loud, and how that can be mitigated in the event of a crash.

A Lot Of Hot Air

Once an obscure feature only found in luxury vehicles, airbags became common safety equipment in many cars and trucks by the mid-1990s. Indeed, a particular turning point was when they became mandatory in vehicles sold in the US market from late 1998 onwards, which made them near-universal equipment in many other markets worldwide. Despite their relatively recent mainstream acceptance, the concept of the airbag actually dates back a lot farther.

The basic invention of the airbag is typically credited to two English dentists—Harold Round and Arthur Parrott—who submitted a patent for the concept all the way back in 1919. The patent regarded the concept of creating an air cushion to protect occupants in aircraft during serious impacts. Specific attention was given to the fact that the air cushion should “yield readily without developing the power to rebound,” which could cause further injury. This was achieved by giving the device air outlet passages that would vent as a person impacted the device, which would allow the cushion to absorb the hit gently while reducing the chance of injury.

The concept only later became applicable to automobiles when Walter Linderer filed for a German patent in 1951, and John W. Hetrick filed for a US patent in 1952. Both engineers devised airbags that were based on the release of compressed air, triggered either by human intervention or automated mechanical means. These concepts proved ultimately infeasible, as compressed air could not be feasibly be released to inflate an airbag quickly enough to be protective in an automobile crash.

It would only be later in the 1960s that workable versions using explosive or pyrotechnic inflation came to the fore. The concept was simple—use a chemical reaction to generate a great deal of gas near-instantaneously, inflating the airbag fractions of a second before vehicle occupants come into contact with the device. The airbags are fitted with vents that only allow the gas to escape slowly. This means that as a person hits the airbag, they are gently decelerated as their impact pushes the gas out of the restrictive vents. This helps reduce injuries that would typically be incurred if the occupants instead hit interior parts of the car without any protection at all.

The Big Bang

The use of pyrotechnic gas generators to inflate airbags was the leap forward that made airbags practical and effective for use in automobiles. However, as you might imagine, releasing a massive burst of gas in under 50 milliseconds does create a rather large pressure wave—which we experience as an incredibly loud sound. If you ever seen airbags detonated outside of a vehicle, you’ve probably noticed they sound rather akin to fireworks or a gun going off. Indeed, the sound of an airbag can exceed 160 decibels (dB)—more than enough to cause instant damage to the ear. Noise generated in a vehicle impact is often incredibly loud, too, or course. Ultimately, this isn’t great for the occupants of the vehicle, particularly their hearing. Ultimately, an airbag deployment is a carefully considered trade-off—the general consensus is that impact protection in a serious crash is preferable, even if your ears are worse for wear afterwards.

However, there is a technique that can mitigate this problem. In particular, Mercedes-Benz developed a system to protect the hearing of vehicle occupants in the event that the airbags are fired. The trick is in using the body’s own reactions to sound to reduce damage to the ear from excessive sound pressure levels.

The stapedius reflex (also known as the acoustic reflex) is one of the body’s involuntary, instantaneous movements in response to an external stimulus—in this case, certain sound levels. When a given sound stimulus occurs to either ear, muscles inside both ears contract, most specifically the stapedius muscle in humans. When the muscle contracts, it has a stiffening effect on the ossicular chain—the three tiny bones that connect the ear drum to the cochlea in the inner ear. Under this condition, less vibrational energy is transferred, reducing damage to the cochlea from excessive sound levels.

The threshold at which the reflex is triggered is usually 10 to 20 dB lower than the point at which the individual feels discomfort; typical levels are from around 70 to 100 dB. When triggered by particularly loud sounds of 20 dB above the trigger threshold, the muscle contraction is enough to reduce the sound level at the cochlea by a full 15 dB. Notably, the reflex is also triggered by vocalization—reducing transmission through to the inner ear when one begins to speak.

Mercedes-Benz engineers realized that the stapedius reflex could be pre-emptively triggered ahead of firing the airbags, in order to provide a protective effect for the ears. To this end, the company developed the PRE-SAFE Sound system. When the vehicle’s airbag control unit detects a collision, it triggers the vehicle’s sound system to play a short-duration pink noise signal at a level of 80 dB. This is intended to be loud enough to trigger the stapedius reflex without in itself doing damage to the ears. Typically, it takes higher sound levels closer to 100 dB  to reliably trigger the reflex in a wide range of people, but Mercedes-Benz engineers realized that the wide-spread frequency content of pink noise enable the reflex to be switched on at a much lower, and safer, sound level. With the reflex turned on, when the airbags do fire a fraction of a second later, less energy from the intense pressure spike will be transferred to the inner ear, protecting the delicate structures that provide the sense of hearing.

Mercedes-Benz first released the technology in production models almost a decade ago.

The stapedius reflex does have some limitations. It can be triggered with a latency of just 10 milliseconds, however, it can take up to 100 milliseconds for the muscle in the ear to reach full tension, conferring the full protective effect. This limits the ability of the reflex to protect against short, intense noises. However, given the Mercedes-Benz system triggers the sound before airbag inflation where possible, this helps the muscles engage prior to the peak sound level being reached. The protective effect of the stapedius reflex also only lasts for a few seconds, with the muscle contraction unable to be maintained beyond this point. However, in a vehicle impact scenario, the airbags typically all fire very quickly, usually well within a second, negating this issue.

Mercedes-Benz was working on the technology from at least the early 2010s, having run human trials to trigger the stapedius reflex with pink noise in 2011. It deployed the technology on its production vehicles almost a decade ago, first offering PRE-SAFE Sound on E-Class  models for the 2017 model year. Despite the simple nature of the technology, few to no other automakers have publicly reported implementing the technique.

Car crashes are, thankfully, rather rare. Few of us are actually in an automobile accident in any given year, even less in ones serious enough to cause an airbag deployment. However, if you are unlucky enough to be in a severe collision, and you’re riding in a modern Mercedes-Benz, your ears will likely thank you for the added protection, just as your body will be grateful for the cushioning of the airbags themselves.