Sound-system design on the new Jaguar XJ

6 September 2010 | By David Wilson

When designers at Jaguar decided to create an automotive sound reproduction system for the firm’s XJ model, they tapped into the expertise of engineers from Bowers & Wilkins at an early stage of the car’s development.

It wasn’t the first time that Bowers & Wilkins had worked with Jaguar engineers - they had previously collaborated on a sound system for the XK model. But while the loudspeaker company had no influence on the positions of its speakers in that vehicle, the engineers were presented with a clean sheet of paper for the XJ.

That provided a number of advantages, not least that the Bowers & Wilkins engineers could work with the Jaguar designers to secure the locations for the 20 loudspeakers of the 1,200W sound system to achieve the best acoustic results, particularly in the car’s doors where their position is usually only considered after window and door mechanisms are packaged.

The collaborative approach helped to avoid compromises to speaker size, packaging and sound compensation that can stand in the way of optimal sound reproduction, while achieving a sound image that can be appreciated by all the car’s occupants, rather than just the driver and the front passenger.

Aware of the fact that a car’s occupants are more easily able to discern where higher frequencies are emanating from than low frequencies, the Bowers & Wilkins designers realised that they would need to place high-frequency tweeters as close to the ear level of the occupants as possible to create a realistic rendering of a sound stage in the vehicle.

For that reason, they located two 25mm high-frequency aluminium tweeters close to the wing mirrors of each of the front occupants’ seats. To cover the less critical midrange and bass, they positioned one 100mm Kevlar driver and one 200mm Kevlar driver in the front doors of the vehicle.

For the back of the car, they mounted a co-axial 25mm and a 100mm Kevlar driver in the top of the door to cover the high-end and mid-range frequencies, while a 165mm unit was positioned in the door to cover the bass frequencies.

Aside from optimally locating the speakers, the design team also needed to ensure that the proximity of the speakers to the passengers did not result in a less than satisfactory audio experience. To solve that issue, the high-frequency and mid-range speakers were directed to fire across the car, allowing the car’s occupants to hear both channels at an equal volume.

To create the impression that the sound stage appears to be in the road in front of the car, and balance the effect of the speakers nearest to each of the passengers, they deployed dual-drive 25mm and 100mm mid-range speakers in the dashboard of the car under the windscreen. On the rear parcel shelf of the car, two 200mm bass drivers and two dual-driver 100mm speakers were used to create
a rear surround effect for the rear passengers.

According to Stuart Nevill, a senior applications engineer at Bowers & Wilkins, the speakers in the XJ are of an infinite baffle design. That differs from the earlier XK model which, due to space restrictions, used one ported subwoofer in a sealed enclosure to enhance the bass from the system. In the XJ, Nevill said, the number of subwoofer drivers obviated the need for such a ported design, allowing the company to produce a cleaner sound.

Creation of the bespoke speakers demanded that the Bowers & Wilkins engineering team took into account the form factor of the drivers, especially when designing the speakers for the doors. A compromise had to be met to produce drivers with profiles that would not only fit into the door well at a depth that was both shallow, yet deep enough, to provide the required movement of the voice coil.

The use of co-axial speakers is an example of how the requirements of the car packaging and audio system quality were successfully balanced. Using co-axial speakers might have affected the sound, but by creating a novel mounting arrangement and redesigning the tweeter finger guard, the designers were able to enhance the quality of the sound from them instead.

Once the engineers had developed prototypes of the loudspeaker drivers, they tested the system in an acoustic environment before deploying it in a test vehicle. The system was then tuned so that the car’s occupants would experience the same sound wherever they sat.

The Bowers & Wilkins automated software optimisation process began with the use of automated software from Audyssey that captured the audio image produced by the speakers using microphones strategically placed in the car. The data collected was then used to automatically program a digital signal processor that controls the frequency response, phase, amplitude and timing for all the audio signals emanating from the loudspeakers, creating the optimum performance for every listener.

Nevill said that the biggest challenge in designing the sound system in the Jaguar XJ was ensuring that the front passengers hear the surround sound behind them, and the rear passengers hear the front channel sound in front of them.

’We first optimise the listening experience of the passengers in the front seats, and having established that as our base line, we provide as close to an ideal audio experience for the rear passengers as possible without compromising the sound quality delivered to the front. While it’s difficult to fulfil all those conflicting requirements, by having full control of all the audio signals from the speakers, we can avoid distracting the front passengers with sounds from behind, particularly vocals,’ he added. ’To do so, we use the precedence effect or psychoacoustic masking where the dominant sound arrives milliseconds ahead of the second source.’

Controlling the output from the drivers in such a fashion enabled any imperfections created by the cabin environment to be digitally corrected, and once achieved, provided a foundation for the engineers from Bowers & Wilkins to fine tune the system manually with individual audiophiles.
The engineers also deployed Audyssey’s adaptive equalisation system to compensate for imperfections created in the cabin of the car when it is on the road. By using two in-built microphones in the cabin to measure low-frequency road noise, the digital signal processor in the car can compensate for it by lifting the bass frequencies of the system.

The XJ audio system itself can be driven in three different modes by the radio, CD player or through a USB port into which MP3 players can be plugged. The entire system is controlled through an LCD touch-screen interface located in the dash of the car.