Our mobility is changing, and that also applies to its sound. When talking to acoustics experts at the Audi sound laboratory with the Audi e-tron quattro it soon becomes clear: the evolution of sound is already well underway.
With this in mind, electromobility in particular has already become a focal point: the areas of performance, range and charging possibilities are among the most discussed aspects. But there is another area that throws up an interesting question: acoustics. What does a plug-in hybrid in purely electric mode or a fully electric vehicle actually sound like? Experts from the total vehicle acoustics department at Audi examine the answer to this question. Their aim is to complement the driving experience with an ideal audio experience. This is all about the quiet sounds, which need to be looked at from a new perspective in view of the developments in the coming years.
The Audi e-tron quattro concept was presented to a specialist audience as long ago as 2015 at the IAA in Frankfurt. This concept vehicle is the precursor to the first purely electric series-produced vehicle, which Audi will bring onto the market next year. In the concept car, three electric motors provide a combined top performance of 320 kW, or when boosting as much as 370 kW for short periods, and torque of over 800 Nm. Flexible management makes electric quattro drive and electric torque distribution possible, for high levels of dynamism and stability. The large lithium-ion battery stores 95 kWh of energy and is installed in the ideal position of the centre of gravity beneath the passenger cabin. The Audi etron quattro concept accelerates from 0 to 100 km/h in just 4.6 seconds – that is approximately on the level as a sports car.
An electric vehicle or plug-in hybrid vehicle in purely electric operation moves almost silently from A to B, even though these models can make use of their full power at low speeds from the first revolution. Despite this power, an electric vehicle is comparatively quiet. The noise from the engine is minimal: a major advantage that ensures the noise level inside the vehicle and in the surrounding area is reduced. However, in some situations this can also represent a certain safety risk for other road users, such as pedestrians and cyclists, because essentially only the sound of the tyres rolling – itself minimal at low speeds – remains. Our behaviour in traffic in terms of reacting to what we see and hear is still oriented to the sound of combustion engines. ‘In future, our listening habits with regard to electromobility will change. Higher-frequency, finer sounds will become much more common. It is also a matter of sound complexity. The more complex it is, the more information hearing can provide. Low frequency has in the past conveyed power and dynamism – for example the ambient sound of a sports car. That is a hearing experience that has been learned and stored by the brain. It is my belief that in the future, the distant future, that will change and become a lot less common. The sound of tomorrow will be brighter, finer and have a more technically demanding tone. You can use sci-fi films as a visionary template to give you a rough idea of the noise environments in futuristic cities. The coming years will reveal how the sound of electric vehicles develops in real life. It is all part of a process that is just getting going,’ explains sound designer Rudolf Halbmeir.
AVAS stands for Acoustic Vehicle Alerting System. This system ensures that purely electric-drive vehicles and plug-in hybrid vehicles driving in purely electric mode generate a noise that makes other road users aware of the vehicle. The requirements this noise must fulfil are defined by law. This primarily relates to the frequency and minimum noise level that must be adhered to. The regulations may vary slightly from country to country – in Europe, China or the USA – as can the point in time at which it becomes compulsory to install the system in new plug-in hybrid and electric vehicles. In China this will be the case as of 2018, in the USA and some EU countries as of 2019. According to the current status, the acoustic signal generated by the system should be activated at speeds up to around 30 km/h.
Naturally, the development of such a sound is not completed in a day. It is a creative process. ‘It is an intuitive procedure. You don’t make a sound that fits at the first attempt, you gradually get the feel for it,’ explains Florian Bock. The sound experts’ work begins at a desk, or rather a computer. Different sound files from the database are layered and mixed with the software. Over and over, until the composition is harmonious. It is above all the sensitive hearing of the experts that decides when this point has been reached, and then testing with the vehicle begins. The sound is transferred from the laptop to the vehicle via the control unit, or ‘flashed’, as it is called in the sound lab. Then testing can get underway. Initially outdoors – on asphalt, without a dyno. Simply listening. The experts can rely on their sensitive, trained ears and experience in this practical test. Thereafter, the analysis devices are brought into play in the sound lab. Louder, quieter. Higher frequency, lower frequency. ‘On the dyno, we firstly have to measure how everything sounds in the interior in order to ensure the required standards of comfort for the customer. Secondly, we can also measure whether we are fulfilling legal requirements, i.e. the specified minimum level,’ explains Gsell.
‘In doing so, we set our standard to not only make
the vehicle audible, but also to make the sound pleasant and representative of the brand with the four rings.’
The installed components play an important role in this. At Audi, among others the development engineer Markus Nußbaumer is responsible for their development, the correct networking of the control device with the vehicle and the installation of the elements. ‘Up to now, infotainment has been restricted to the interior of the vehicle. With the new legal regulations for purely electric vehicles, all that is being taken to the exterior, too. The optimal positioning of the speaker on the front of the vehicle is very important, as it influences the efficiency of the sound emissions in proximity to any pedestrians. A position as close as possible to the outer contours is ideal,’ explains Markus Nußbaumer. The chassis is also significant in the soundcheck, acting as a sort of soundboard. The sound waves emitted by the speaker are released outwards and additionally transported by the chassis – to different degrees depending on the material properties. This makes the sound audible outside and also to a lesser degree in the interior. An omnipresent sound. It is there, but is integrated as a harmonious element of the ambient sound of the vehicle.
‘AVAS is primarily about warning pedestrians. Every child knows what a car sounds like, so the current legislative proposal intends for the sound to be oriented to what people are accustomed to. The challenge is to also make a premium experience possible for the customer. The sound should have a high-quality feel to it, and to achieve that, the quality of the components has to be right, of course,’ an Audi expert adds.
A prerequisite for the optimal AVAS sound is networking of the control unit with the speaker and the vehicle. This allows the components and the vehicle to communicate with each other. Before the components are installed, their functionality, sound and durability are tested, during which they are subjected to extreme temperatures (–40° to +120° Celsius). They also have to endure dust, water and strength tests such as impact with a rock. The background: for optimum sound emission, the speakers must be positioned as far to the outside as possible, and therefore are quickly confronted with external influences.
The audio characteristics of the sound change depending on parameters such as speed, acceleration and torque. These parameters can be partially tested in the biggest Audi sound lab, including exterior and interior noise, not only when stationary, but also – thanks to dynos embedded in the floor – during a journey. Largescale pyramid absorbers on the walls ensure that unwanted reflections are prevented. Microphones record the sound and help with the analysis. In this process, four static microphones at a distance of two metres and a height of 1.20 metres are positioned around the vehicle. They are used to check the legal guidelines are adhered to. A row of additional microphones is installed on the edge to recreate situations such as driving past, which would otherwise have to be measured on a test circuit.