People often ask about the origins of the Fixed Suspension Transducer (FST), the Bowers & Wilkins drive unit technology that features on many of our best-regarded loudspeakers. The truth is, it was half inspiration, half insulation.

Bowers & Wilikins FST drive unit

Bowers & Wilikins FST drive unit

Loudspeaker engineers have long cursed the drive unit’s roll surround’s behaviour. To allow enough compliance to work at low frequencies it needs to be floppy but this means that it doesn’t support its own mass at high frequencies, so you get a resonance. So it would be great to somehow remove it.
Gary Geaves, now head of research at Bowers & Wilkins, was playing around with some Finite Element models, which hinted that it might be possible to remove the surround, and instead just curve the edge of the cone and attach it directly to the chassis. We prototyped this ’surroundless’ cone and, in all honesty, it was hopeless. There was a massive mechanical reflection – a standing wave – at the edge of the cone. But it got us thinking.

In organ pipes you get a different but related set of standing wave reflections formed if the end of the tube is open or closed. In the surroundless driver we noticed that the reflection at the fixed edge of the cone bore a similar relation to the reflection at the free edge of a normal cone, so we postulated that there would be an edge stiffness condition somewhere between fixed and free that would give us minimal reflection. The hunt was on for the perfect compliance.

We experimented with all sorts of things. This started with tiny rubber half-rolls but these always had there own resonance so perhaps the next big leap forward was that we tried ’solid’ materials under the edge of the cone instead. This began with O-rings of different hardness, then silicon rubber, foamed rubber, getting softer and softer. Next up was foamed gasket material; we started to see some interesting behaviour but there was still too much stiffness and a lack of damping, so we tried draught excluder.

The very first grade we tried was promising so it was off to the home insulation section of Steyning’s hardware store to get every brand and type we could lay our hands on. Luckily, we found something that worked really, really well, the frequency response was almost totally devoid of ripples, sensitivity was up and the distortion had dropped to hitherto unknown levels.

It turned out our favourite grade of draught excluder was from a rogue batch, getting a similar grade in production quantities was a rather more painful challenge but if you own a pair of our speakers with an FSP driver, you’re fortunate to hear that we got there in the end!
To read more about FST and other Bowers & Wilkins’ technologies click here.