Press Release

AUDI AG

Peter Blum, Infotainment Development, Head of Vehicle Projects

 

The motivation behind a high-end in-car sound system

 

In 1989, Audi became the first European car manufacturer to develop a premium sound system for luxury-class cars: the audio system developed in conjunction with Bose was launched in our V8 - the forerunner of the current A8. Today, Audi offers Bose sound systems in all car lines. Particularly since the appearance of digital signal processing, sound systems have reached a standard of quality that was inconceivable in 1989. Comfort functions such as surround sound reproduction and dynamic compensation of vehicle noise have since become standard at Audi.

 

Audi is now going one step further: in partnership with Bang&Olufsen, Audi is offering an entirely new type of high-end sound system in its flagship model, the A8. So why the need for another new sound system?

 

In the home hi-fi sector, a distinction between so-called mid-fi and high-end equipment has long been observed. The ongoing process of integration and cost reduction for digital electronic assemblies, together with very good simulation tools for the speakers, has made very good, low-priced music systems available. Yet in order to enjoy the finer points of music, it is necessary to exploit every technical option - with a corresponding impact on costs.

 

The same applies in the car. The technology used today means that no further significant increases in sound quality are possible. The disadvantages are restricted dynamics, limited spatial reproduction, the time response of the speakers and systems integration.

 

A high-end sound system for a car necessitates a radically new approach.

 

To dynamics:

Electrical output in a vehicle is limited by the battery voltage. With considerable effort, the output limit of the amplifiers has been raised from approx. 20 Watt to 100 Watt. However, when one bears in mind that a full-scale symphony orchestra has a line-up of about 60 musicians, one can imagine that this output is still not very high and that dynamics are sorely limited. It is important to distinguish between volume and dynamics here. We take dynamics to mean the scope for reproducing sound levels at differing volumes. In the case of cars, the lower limit to the effective dynamic range is dictated by the operating noise inside the car: 60 dBA is a typical figure here. In order to perceive the music clearly, a certain difference in volume between the music and the driving noise is needed: an electrical output of approx. 100 mW is used in order to achieve the necessary 80 dBA. But this only applies for a pianissimo. When the whole orchestra plays a tutti passage, the sound level rises by approx. 30 dB to a total of 110 dBA. This calls for a much higher electrical output, because an exponential rise in output is required in order to make an increase in volume/dynamics perceptible to the human ear.

 

As the music typically comes from four channels, the sound level per speaker nominally averages 25 W. However, our ear assesses music pleasure according to freedom from distortion, i.e. interference-free oscillation between the average and peak values for level control. For the typical music signal, the difference between the average and peak value of level control is at a ratio of 1:4. However, the output required for this level control is raised to the second power. As a result, the output required to reproduce the 25 W distortion-free is sixteen times higher, in other words 400 W.

A minimum output of 1000 W was set as the target for the newly developed AUDI AG high-end sound system, with a new amplifier concept.

 

To spatial reproduction:

One of the biggest problems facing the reproduction of music in a vehicle is the asymmetrical and non-central seated position in relation to the speakers. With the aid of a centre speaker and appropriate energising, the stage, which is normally located in the centre between the speakers, can be displaced such that it is located in front of the driver. A second "stage" is generated electronically in front of the front passenger. This makes it possible to shift the orchestra to the correct point. It is nevertheless still difficult to register the positions of the individual musicians in a vehicle. This is due to the many early reflections caused by the installed position of the speakers. The challenge is to build a new speaker that avoids these early reflections.

 

To the time response of the speakers:

The reproduction quality of a speaker depends considerably on where it is installed, as well as on the design of the speaker itself. In addition to the frequency response, the time response - in other words how well the speaker as a vibrating system follows the electrical system - is dictated to a high degree by the housing. On most sound systems in a car, the speakers are installed freely in the doors. This means that the doors act as speaker housings. As a result, the acoustic properties are not genuinely determinable. In the new Audi high-end sound system, all the speakers are therefore installed encapsulated in enclosed casings. The quality of the transient response and reverberation is thus precisely calculable.

 

To systems integration:

In addition to the challenge of implementing an attractive design and integrating it into the vehicle, the entire reproduction chain of two amplifiers, five speaker housings and a total of 14 speakers must be perfectly matched in order to produce optimum reproduction. This is why every speaker has a separate power amplifier. The energising of each channel, too, is processed separately with the result that each parameter can be adjusted separately within the overall transmission chain. The integration of the front tweeters and centre speaker into the instrument panel results in an optimum audio experience for both front and rear occupants.

 

The Audi Advanced Sound System from B&O: 1100 Watt on 14 channels

 

The Audi Advanced Sound System from B&O has 14 speakers. Each of the front doors incorporates a two-way speaker unit with a 90 mm mid-range speaker in a single housing with a volume of one litre, and a 140 mm woofer with a volume of 2.5 litres. The treble is reproduced by Acoustic Lens speakers that are extended out of the instrument panel. A 70 mm cone speaker serves as the centre speaker. There are two-way speakers built into the rear doors. The 130 mm woofer has a volume of two litres and is supported by a 25 mm tweeter. The low bass tones are reproduced by a 200 millimetre subwoofer installed in a 10 litre housing.  Together with two 70 millimetre wide-band loudspeakers for surround reproduction, it is installed beneath the rear shelf.

 

Avoiding reflections with Acoustic Lens Technology

 

The front tweeters are among the most important elements of the new sound system. They use Acoustic Lens Technology. A dome with a high-attenuation, coated silk baffle emits sound into an ellipsoidal volume. The waves reflected by the wall of an ellipse all pass through the same focal point - hence the name "Acoustic Lens". The advantage is that an acoustically ideal punctiform sound source is realised.

The way the sound is guided at the top helps to reduce the angle of vertical sound projection. This avoids early reflections by the windscreen.

 

Sound quality through speaker technology

 

The cleanness and transparency of reproduction are very important criteria for sound quality. It is important to avoid the various component signals being superimposed on one another during speaker decay. Every speaker has a tendency to vibrate because it is a spring-mass system. However, it is held in check by the amplifier because the voltage generated during speaker decay is regulated by reverse coupling of the amplifier.

 

In order to make this regulating process as precise as possible, the attenuation factor, in other words the ratio between the speaker's internal resistance and the output resistance of the amplifier, must be as high as possible.

 

The 8 Ohm coil resistance (impedance) of the subwoofer realises very high values that result in an outstanding transient characteristic.

 

One technical measure of cleanness is the degree of distortion. The magnet systems of speakers are fundamentally non-linear and thus generate anharmonic overtones. The subwoofer was optimised with a new simulation system. Distortion could thus be substantially reduced. So that these characteristics can be implemented in a stable manner, the speaker has a diecast aluminium basket and an extra-rigid paper baffle.

 

Special baffles that are both very rigid and very light are used, together with high-load aluminium oscillation coils.

 

The beads of the cone speaker, which are very important to the linearity of excursion and attenuation, are made from rubber.

 

The two-component amplifier

 

A total of 1100 W is made available for the Audi Advanced Sound System on demand, by two amplifiers. The high-performance amplifier has four channels of 125 W each for the four woofers and one 250 W amplifier for the subwoofer. The second amplifier (DSP) is responsible for signal processing and, with nine output stages of 35 Watt each, for energising the surround speakers and tweeters.

 

Audio signal processing is performed purely digitally in the A8. In other words, the source data of a CD is transported to the DSP amplifier without loss of quality. Signal processing with frequency-response, phase-response and delay-time correction as well as level setting, dynamics influencing and surround sound are performed separately for each of the fourteen channels. This assures minimal channel crosstalk and optimum utilisation of the dynamics range of the DSP amplifier.

 

Particular importance was attached to the calculation quality of surround sound. Today, most sound sources such as CD and DAB have only two programmed output channels (stereo); despite the use of these stereo sources, we nevertheless want our customers to be able to enjoy the benefits of surround sound, which is specifically characterised by the fact that it uses more than two output channels. In other words, a specific surround signal is calculated from the two channels for each speaker by means of a special energising program.

 

A high-performance amplifier encounters unfavourable conditions in a motor vehicle, because the installation space is severely limited, usually concealed behind trims, and the ambient temperature is often higher than 70 °C. The amplifier's high output necessitated an entirely new design. The ICEpower amplifiers of the development partner B&O make the most of the advantages of both worlds: high efficiency, low frequency-independent and load-independent distortion and a 115 dB dynamics range.

 

Tuning for clarity and resolution

 

Frequency response and stage effect are the components of tuning. A linear frequency response is generally assumed to represent the optimum outcome. Small deviations in the frequency response are often tolerated in return for maximum music resolution. In a car, however, the engine and driving noise mean that there is a noise level of approx. 60 dB at low frequencies, rendering modification of the frequency response necessary. Inside the vehicle, the overall signal comprising the music, passengers' conversation and interference noise is furthermore is picked up, the interference filtered out and the music modified accordingly. This dynamic modification of the frequency response enhances the sound quality quite appreciably.

 

A second important point is avoiding resonances. This is first because peaks in the frequency response are perceived as highly interfering, and second because the system is attenuation equalized for a resonance, leaving a hum note during decay. This results in an unclean sound.

 

The spatial representation of the music, or stage effect, is also very important. The task here is to compensate for various drawbacks of the vehicle interior: the listeners are seated asymmetrically in relation to the speakers, the listening space is very confined and a balanced surround audio experience is to be possible for both front and rear passengers.

 

The effect that music has also depends on the size of the performance area, for example the sound of an organ in a cathedral. To recreate this effect, a sufficiently large amount of early, directional reflections and an amount of late, diffuse reflections are added to the music.

 

Surround sound in the vehicle is the new challenge: Audi does not regard the entire vehicle as a single listening zone. After all, in order to achieve an adequate surround level for the front speakers, the surround speakers would have to be set relatively loud. However, the rear passengers would then hear the music coming at them from behind, and who likes to sit with their back to the orchestra? Audi therefore operates with two surround listening zones: one for the front passengers and one for the rear passengers. This necessitates special allocation of the channels for the speakers and a suitable delay between the signals with the right low-pass filtering and level adjustment of the surround signal.

 

The partnership with B&O: expertise and innovation

 

Audi has mastered these diverse, very special requirements of high-end sound systems for cars in an entirely new and unprecedented way thanks to its partnership with Bang&Olufsen. In hindsight, it is possible to declare that each has found its ideal partner. Audi and B&O share the same values: innovative design, the use of cutting-edge technologies, intuitive operating systems, high standards of material character and the quality of the finish, and expertise in aluminium. Then there are B&O's new developments, the Acoustic Lenses and the ICEpower amplifier, with which it has been possible to solve important conceptual problems for a high-end system for in-vehicle use.

 

A new quality of music reproduction has been achieved with the new technologies. Ample output for reproducing even the most complex of signals without distortion, in a controlled manner and with neat spatial progression. This extends Audi's ambition of "Vorsprung durch Technik" to include music reproduction.

Audi Customers and electronics (Part 1)

Audi Electronics Centre as a process-chain building (Part 2)

Audi quality as the basis for success (Part 3)

Audi Electronics Venture GmbH (Part 4)

Audi Electronics Research Laboratory - Research in Silicon Valley (Part 5)

Audi Connectivity: integration and interfaces for multimedia (Part 6)

Audi transformation of the car into a mobile concert hall (Part 7)

Audi A8 Bang & Olufsen Advanced Sound System (Part 8)

Audi innovative solutions and future trends at the driver's workplace (Part 9)

Audi Driver Assistance (Part 10)