The four-ring symphony: Sound development at Audi – the car as an orchestra
- Target implementation of sounds captures the essence of the brand
- How an Audi sounds depends on the individual model and the customer’s expectations
It’s all in the sound – this is especially true for cars from Audi. So-called “sound design” has become one more important component in vehicle development at Audi. You could call it the acoustic calling card of the brand with the four-ring logo. How an Audi should sound depends on the individual model and the customer's expectations. The latest examples of Audi’s active sound design are the Audi S4, Audi S5, Audi A8 V8 FSI, the new Audi TT TDI and TTRS, and the Audi R8 supercar.
One man who is heavily involved in this development is Dr. Ralf Kunkel, Head of Audi Acoustics. He utters some sounds you wouldn’t expect from an engineer, discussing things like the “car as an orchestra,” “the art of composing and conducting,” “good musicians” or “sour notes,” and the “four-ring symphony.” His basic philosophy: “The sound must meet the expectations of the listener. Sound evokes emotions.”
He uses an example to clarify what he’s talking about: using a photo of a beer glass, he plays the sound of water running from the tap into a glass. Listeners involuntarily furrow their brow: that doesn’t fit! “That’s right, the expectations have simply not been fulfilled here. And it’s just the same with cars.
The sound of a car must meet the expectations the customer has formed, based on its design and model positioning,” Kunkel explains. In other words, cars with sporty S engines like those in the Audi S4, S5 or the Audi TTRS also have to sound sporty and robust – and “sound” fast. In these cars, acoustical restraint would only disappoint the customers. In the A8, on the other hand, you expect a sound that conveys comfort and subtly underscores the car’s effortless elegance.
Expectations are one challenge that a sound developer has to deal with. Each person perceives sounds differently, so the sound designer must work to “objectify” subjective impressions. The goal is to ensure that the Audi sound always provokes a positive response. At the same time, a car’s sound design must comply with any legislation that applies to acoustics.
Subjective sound impressions are evaluated according to the criteria of psychoacoustics, which began developing measurement parameters in the 1950s. The evaluation of noises ranges from “stimulating” or “soothing” to “objectionable.” “You can basically say that the occupants of a car tend to perceive higher tones as more unpleasant. Sounds of that kind can get on your nerves and even provoke aggression. And that is, of course, the last thing you want when driving. Bass tones, on the other hand, are more likely to be perceived as pleasant,” says Kunkel. It’s the art of the acoustic engineer to blend the right mixture of sounds.
Kunkel explains that specific sounds must be considered early on in the development phase of a new Audi model’s construction. “It’s a demanding subject in terms of complexity,” he said. “And we in Acoustic Development are involved right from the very first design sketch – that is, in the concept evolution phase.” He adds: “A compromise always has to be found between sound design and other component and vehicle characteristics and taking into account the available space and legal requirements.” It is therefore not possible to implement every single measure that would help achieve the optimum sound design.
About 60 engineers work on the analysis and design of vehicle sound and vibrational comfort characteristics at Audi. Alongside these, there is a special team dedicated solely to sound design. In addition, there are numerous experts in individual departments whose work revolves around functional noises. The subject of sound also plays a prominent role for the developers of audio systems. And the “Squeak and Rattle” team at Audi tracks down annoying squeaking and creaking noises and their causes.
Audi’s intensive sound design operations began three car generations ago. The acoustic engineers at Audi determine what sound should be made by the engine, the air intake and exhaust systems, what acoustic feedback is generated during acceleration, how much road noise is heard in the interior, what sounds are made by the power windows or indicator, and how solid the door sounds when it shuts.
In this context, Kunkel likes to speak of the “car as an orchestra.” He differentiates between “good musicians” and “sour notes” within this orchestra. Naturally, the latter should be brought into tune. “In the past, the work of an acoustic engineer was primarily targeted at the suppression of sound. That is what we call sound cleaning,” Kunkel explained.
Added to this is the task of promoting the “good musicians” within a car. These include the body, the exhaust system, the engine, the air intake and the engine mountings. The undesired “sour notes” are created, for example, by the tires, drivetrain, transmission, ancillaries and wind noises. The repertoire is diverse: tires might sing, transmissions can rattle, body panels may buzz and whirr, prop shafts can hum, generators may howl or drone, and fans can hiss. In the end almost every noise from the engine, the tires or the suspension, is transmitted to the car body. This is only in part due to the sound that is carried through the air.
Most vehicle interior noise is caused by so-called structure-borne noise, the transmission of noise within solids or liquids such as in the steering servo’s hydraulic lines. Without constructive intervention, the body would become an uncontrollable source of noise – an orchestra whose playing is muddled and out of control. Kunkel and the other acoustic experts at Audi therefore assume the role of the orchestra’s conductor. “A car body is one big body of sound, like the case of a grand piano,” Kunkel said. “A vehicle body that is vibrated to the oscillations of a piece of music by using electrodynamic shakers creates the atmosphere of a concert hall. This is then equivalent to an oversized loudspeaker.”
To compose the four-ring symphony, Audi acoustic engineers “play” on a mixing console consisting of various design methods and means. The overriding principle is to alter the sounds at their source, which is why the acoustic experts focus much of their efforts on the engine area. The acoustic engineers set the tempo and specify whether the sound should be “piano” or “fortissimo,” from selecting the materials, the design of the engine block, the specifications for the stiffness of the crankshaft, simulation of the entire drive train, calculations for the charge cycle for the air intake, and the construction of the exhaust system.
Various features are fitted to reduce the transmission and emission of noise such as hydraulic damping elements, sandwich panels, damping foils, panel braces, acoustic baffles, seals and foam filling for cavities in posts or sills, grommets that are placed on apertures, absorbing layers lining the bonnet and bulkhead or in the floor and wheel arches, and absorbers in the interior such as the headliner, seats or carpets.
The sound designers look at sound generation, for example, in the supply of fresh air to the engine, i.e. the air intake. Kunkel describes the sound generated by fluctuating pressure during the charge cycle as “the organ-like instrument of sound design.”
Both the interior and exterior sound can be influenced considerably by making specific adjustments to the intake manifolds’ pipe geometries (length and diameter) and also by using resonators.
Apart from the air intake, the engine and exhaust system in particular are suited to composing sound. This is the reason why pneumatically switchable flaps can be found in the exhausts of both the R8 and the new TT RS. This component ensures that the exhaust manifold geometry has a small pipe cross-section at low engine speeds and a correspondingly larger cross-section at high engine speeds. Thus, an evenly sonorous richness of tone is achieved across the whole engine speed range, which always pleasantly reminds the driver of the vehicle's power.
Audi engineers also specifically work on the interference phenomenon, i.e. the overlapping of sound waves. This can be done with the specific aim of altering individual sound waves to create new noises with a completely different character. When the sound waves are identical, a noise can even disappear completely because the waves cancel each other out. “Not everything that sounds like a rough, mechanical engine noise is caused by the mechanics,” Kunkel said. “For instance, a subtle rawness was added in the Audi A6, to emphasize its sporty nature, despite the silky V10 engine. In the S6 as well, with its sporty credentials, Audi strove for a balance between comfort and performance.”
Creating as pleasant a sound as possible depends on a variety of factors. The complexity of sound design lies in coordinating all components in the vehicle that cause noises, so that they all sound good. “Every sound can sound harmonious in itself, but disharmonious in a sequence. The deciding factor is the overall acoustic concept, the interaction of sounds in the vehicle,” Kunkel said.
This also involves taking outside noise influences into consideration. The engine is only the dominant source of noise up to a speed of about 80 km/h (50 mph). Beyond this, the rolling noise of the tires is the primary noise-maker, and starting at 140 km/h (87 mph), wind noise is the main source. Integrating all of these aspects into sound development efforts is highly time-consuming.
After all, although many individual areas of the acoustics can now be simulated on a computer, vehicle acoustics as a whole cannot. Sound design therefore requires not only a lot of experience, but also countless tests on various test rigs, such as the hydropulse rig, the comfort test rig and rolling road as well as the engine and gearbox, exterior noise and component test rigs.
Achieving the melodious Audi sound also demands noise test drives on a wide variety of test tracks, tests in special booths to assess noise damping, plus the use of dummy-head microphones and the evaluation of noises by acoustic engineers in the sound laboratory.
These development efforts don’t result in a uniform Audi sound, but rather in a customized acoustic concept for each model. The essential quality features of the Audi sound are a balanced distribution of frequencies, load-sensitivity and spontaneity of the sound, a subtle degree of rawness, zero resonance and smooth sound development as engine revs increase.
The optimum sound character of every Audi in the model range will continue to be honed in the future. “The development is only just getting started,” Kunkel said. Even so, sound design is already setting the tone at Audi.