PRE-SAFE® Sound: playing 'pink noise' in the split second before impact.
It takes just a split second for a car accident to completely change a person’s life. Mercedes-Benz PRE-SAFE® can help to minimize the damage. Once the vehicle’s sensors have detected that a collision is unavoidable, a comprehensive system of safety features are activated within the short time that remains before the impact occurs. It takes just 150 milliseconds to deploy the reversible seat belt tensioners, for example. To put that into context, the duration of a human blink is 100 milliseconds.
“PRE-SAFE® was first introduced at Mercedes-Benz in 2002,” says Rodolfo Schöneburg, Head of Vehicle Safety, Durability and Corrosion Protection at Mercedes-Benz Cars. “It kicks in before the collision occurs, preparing the vehicle and its occupants. PRE-SAFE is therefore one of our systems that is based on what actually happens in real-life accidents and helps to save people’s lives.”
Harnessing the stapedius muscle reflex in the ear
Mercedes-Benz is now extending its PRE-SAFE® technology with PRE-SAFE® Sound. It will be debuted in the new E-Class, model series 213, available from 2016. The system is the first to harness a natural reflex to condition the ear – when a collision is imminent – for the loud noise that is anticipated from the impact. 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 so better protects it against high acoustic pressures. Most importantly, the reflex reduces the damage to hearing.
Rodolfo Schöneburg, Head of Vehicle Safety, Durability and Corrosion Protection at Mercedes-Benz Cars.
That may sound simple enough, but as is so often the case with advanced technology, there is a lot more to it than meets the eye. “People have long been aware of the stapedius reflex and its protective effect,” says Wilfried Bullinger, who works in Mercedes-Benz Cars development on innovative occupant protection systems. “To make sure you trigger the reflex, however, you normally need high sound pressures, typically around 100 decibels. This would be like listening to a trumpet playing from around one metre away. But this volume would definitely be too much for use in a car.” Then the experts discovered a way of tricking the body.
‘Pink sound’ on all frequencies
The triggering of the stapedius reflex is contingent not only on how loud the noise is, but also on the spectral composition of the signal that is used. An individual tone carries the required energy only on a single frequency, meaning that this frequency has to be transmitted to the ear at a very high volume. However, if this energy is spread out across as many tones, i.e. frequencies, as possible, it can be transmitted at a much lower volume.
A frequency spectrum known in physics as pink noise is ideally suited for this purpose. It sounds like a bit like diffuse traffic noise, the breaking of waves or a waterfall. “This finding was the breakthrough,” says Bullinger. “100 decibels was too much. But around 80 decibels, as now deployed by PRE-SAFE Sound®, was perfect. This is equivalent to the level of noise at the side of a busy road.” At the end of 2011 human trials of the pink noise method proved that the concept worked – a major milestone had been reached.
A plea to the ear to protect itself
The path from here to series production was by no means straightforward, but it is comparatively easy to explain. The objective was then to integrate the concept into the vehicle. For the first time, PRE-SAFE® is accessing the car’s multimedia system, which is installed with a special soundchip to generate the pink noise. When PRE-SAFE® detects that an accident is imminent, a pink noise signal is played through the sound system at a volume of around 80 decibels. The ear is then primed for the high level of sound that typically accompanies a collision.
Wilfried Bullinger works on passenger safety, PRE-SAFE®, and post-crash safety at Mercedes-Benz.
This occurs for example through the crunching of metal and glass under huge forces and within a split second of time. “It’s a very significant level of noise,” explains Rodolfo Schöneburg. “It can lead to temporary impairment of hearing – and this is exactly what PRE-SAFE® Sound works to counteract. Although the system cannot completely prevent the damage caused by an accident, it can help to reduce it.”
PRE-SAFE® Sound is not only a purely technological breakthrough, it also represents the beginning of a new chapter in another area. For the first time, human reflexes are being made an integral part of automotive safety engineering – it is almost as if we are providing our own sensors to achieve a greater degree of safety. “For us this is a major paradigm shift,” says Schöneburg. “It will pave the way for other new systems.” The safety experts are open as to what these might be. “But it does mean that we are focusing on the people in the car – who were already important of course – more than ever before.”
' Soundchip generates pink noise '
Car with protective reflexes: Mercedes-Benz PRE-SAFE®
PRE-SAFE® is a comprehensive safety innovation system from Mercedes-Benz that made its debut in 2002 in the S-Class.
The PRE-SAFE® system is now available across the model range, from the A-Class to the S-Class. In numerous hazardous scenarios, it takes action to pre-empt and protect. The aim is to prepare the occupants and the car for the imminent collision so that the seat belts and airbags provide the maximum amount of protection.
The PRE-SAFE® mechanisms are reversible. If an accident is avoided, the pre-tensioned seat belt strap automatically relaxes, for example, and the occupants can return their seats and sliding roof to the original positions. The anticipatory occupant protection system is then immediately ready for use again.
The front-seat belt tensioning, the repositioning of the passenger's seat in an emergency situation and the closing of sliding roofs were among the first PRE-SAFE® functions.
Other PRE-SAFE® functions
· PRE-SAFE® Brake causes Mercedes-Benz cars to brake automatically if an imminent rear-end collision is detected. Launched in 2006.
· PRE-SAFE® Impulse uses the seat belts to pull the front occupants deeper into their seats, away from the direction of the collision.
· PRE-SAFE® Rear reduces the seat belt slack for the passengers sitting in the back.
· PRE-SAFE® Sound helps to protect the ears from the noise of a collision (world premiere in 2016 in the new Mercedes-Benz E-Class).
· PRE-SAFE® Impulse Side moves the passenger threatened by a side collision as far away from the acute danger zone as possible in the time before the crash occurs (world premiere in 2016 in the new Mercedes-Benz E-Class).
Connection with other systems
Examples of what can activate PRE-SAFE® include emergency or panic braking, heavy over- or understeer, evasive steering movements or an acute intervention by the adaptive brake assist. Probable accidents can be identified at an early stage because PRE-SAFE® is connected to the brake assist systems and the Electronic Stability Program (ESP®). Their sensors detect potentially hazardous road situations and within milliseconds send the relevant information to the electronic control units.