On the CREST of a WAVE01 May 2012
A car audio that outperforms domestic systems is on the horizon, as Keith Howard reports
It is traditional for audiophiles – people who seek the very highest quality in reproduced sound – to look down their noses at in-car audio. After all, car cabins are acoustically imperfect: engine, wind and road noise drown out the lower amplitudes of signals with wide dynamic range; and myriad on-board electronic systems provide a less than friendly electromagnetic environment.
But a technical development that puts in-car audio ahead of the domestic equivalent could realign the status quo – and that eventuality is now a distinct possibility. The technology that could achieve it already exists and has been proven in a concept car. It's called wave field synthesis (WFS).
Everyone who has used a conventional stereo or surround sound system knows of the hot seat problem. There is just one small listening area in which the sonic image is as it should be; outside that area, the image collapses towards the nearer speaker or speakers and the illusion of space is lost. For a group of listeners, like the driver and passengers in a car, it inevitably compromises the listening experience.
The effect occurs because even surround sound systems, as conventionally realised, do not recreate the soundfield you'd experience live in Symphony Hall, Birmingham, or the Marquee Club in London's west end. In the 1970s, in the aftermath of quadraphonics – the first, abortive attempt to introduce surround sound – a properly conceived, hierarchical multichannel system called Ambisonics was developed that could recreate the original soundfield, but only over a small listening area. To expand the listening space, the concept needed to be extended to higher orders and that involved many more channels than the three necessary for first-order encoding of the soundfield in the horizontal plane, or the four channels required for full periphony (which includes the height dimension). Ambisonics has never gone away – but its resilient supporters have never succeeded in having it widely adopted.
Wave field synthesis is a different approach to achieving the same end. It takes as its starting point the Huygens Principle that in school physics lessons is more normally applied to light waves, but is equally valid for sound. What this says is that each point on a wavefront can be considered to be a tiny source (of light or sound), radiating spherical waves. Constructive and destructive interference of the output from all these tiny sources results in the observed wavefront. It follows from this that, if a sufficient number of closely spaced loudspeakers is fed suitable signals, then any desired wavefront or combination of wavefronts can be created, thereby reassembling an original soundfield.
Nothing about wave field synthesis prevents it being used in a domestic hi-fi system, other than its inherent complexity and the large number of loudspeakers and amplifiers required. In-car installation is complex, too; but, as the best of current ICE systems already deploy a large number of speakers, and the ability to hide them in trim panels is a distinct advantage, in-car audio offers a more conducive environment. So, might WFS come to car audio first?
The Audi array
That possibility was lent credibility in 2010 when Audi unveiled its WFS-enabled Audi Sound Concept Q7, fitted with no fewer than 62 speakers: one subwoofer, four woofers, five tweeters and 52 midrange units, arrayed all around the cabin within the door trim, roof pillars and instrument panel beneath the windscreen. Audi declines to provide important details about the system, such as how much weight it added to the car and how much a production version might cost, but clearly some simplification is desirable to make in-car WFS a more realistic proposition.
Various academic groups are working on this now, as is Sonic Emotion, a company that has already produced vastly simplified WFS-based sound systems for home use and is currently working with an unidentified car maker. Nobody in WFS with automotive industry connections seems prepared to go on record about this ongoing R&D effort, suggesting that some car makers, at least, see a competitive advantage accruing here.
Assuming these simplification efforts are successful, the cost and complexity of WFS may soon be sufficiently reduced for the technology to become practicable for prestige production cars. At that point, a remaining hurdle would usually be the chicken-and-egg problem that confronts any new sound format: will customers buy a WFS audio system when there are no commercial WFS recordings to play on it?
But here WFS has an ace up its sleeve: it can be used to improve the sound of conventional stereo and surround sound programmes for every occupant in a car – and by an obvious margin. So be prepared to sit in your car for the ultimate concert experience.
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