Industry firsts – Fixed Suspension Transducer

FST CM10

Bowers & Wilkins’ team of acoustic engineers are constantly pushing the limits of their understanding of audio technology. The result of this dedicated pursuit of perfection is an almost constant flow of technological advances that continue to keep the company’s products (at least) one step ahead of the competition. Here we look at the work that led to the Fixed Suspension Transducer.

The Fixed Suspension Transducer – normally shorted to FST™ – is a trademarked Bowers & Wilkins technology used on all core speakers with dedicated midrange drivers. Instead of a normal roll surround, the FST uses a narrow ring of foamed polymer to support the outer edge of the cone.

The small movements of the cone lightly compress and stretch the ring. Because the ring’s surface area is small, it radiates relatively little sound, and what little movement it does have always follows the cone edge because it is so tightly coupled.

But you can go a stage further. By choosing the mechanical properties of the ring to match those of the edge of the cone, more of that bending wave energy travelling up the cone passes through to the surround. And if the surround properties are resistive or lossy, the energy can be converted harmlessly into heat. The result is that far less energy is reflected back into the cone to cause resonances than with a similar driver having a normal surround, therefore delivering a clearer, more precise midrange.

fst-render

A sudden draught
But FST’s development took place in some rather unusual circumstances. Steve Pearce of the Steyning Research Establishment (SRE) explains: “We knew what we were gunning for; that we wanted better cone surrounds,” he says. “So Stuart Nevill [now Head of Engineering at SRE] took it upon himself to investigate other materials. Among these was some draught excluder, which he bought from a local hardware store and which worked really well. But when we contacted the manufacturer to get some more, it didn’t work.

“They analysed it, and it turned out that we had a faulty batch! In the end we found another material that had the properties we wanted and was standard. It just happened that the batch of draft excluder in the shop that day was perfect for us, even though it was faulty for the job it was made to do.”

Although FST was originally developed to improve the Kevlar cones’ performance, it is not limited to use with that material; the new 800 Series Diamond’s Continuum cones also benefit from the technology.

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