Bowers & Wilkins’ FST™ mid-range drive unit maximises the effects of Kevlar® by improving cone response times and integrity of sound transmission. Surrounds both keep the cone in line and help to absorb bending waves. As the mid-range diaphragm only moves a little, Bowers & Wilkins has improved outer edge cone termination by means of “surroundless” suspension. A foamed material ring, with resistive mechanical impedance identical to that at the edge of the cone, is placed under the cone’s edge. Bending waves travelling up the cone are almost totally absorbed by the foam ring, which also compresses sufficiently to accommodate cone movement at mid frequencies.
All our careful refinements of the Kevlar cone would count for nothing if we didn’t do all we can to minimise other sources of disturbance. Moving air behind the cone, for example. The chassis of the FST driver is designed to minimise any impedance to the flow of air from the rear of the driver. To reduce the clutter immediately behind the cone still further, we’ve adopted neodymium magnets, which deliver the same magnetic force from a smaller device.
The FST driver is a refinement in the use of Kevlar cones and you should first read the section Kevlar cones before going on. Laser Interferometry gives us detailed information about how various diaphragms operate and a much clearer insight into ways of improving their design.
With cone diaphragms, the outer surround can be a major cause of problems. Its primary purpose is to maintain an air seal and help keep the voice coil aligned in the narrow magnetic gap as the cone moves back and forth. It must be flexible enough to cope with the maximum required movement of the cone. However, this very flexibility is the reason why the surround doesn't always follow the movement of the cone in a coherent manner. Problems start to occur at the so-called surround dip frequency, where the surround begins to move in the opposite direction to the cone and partially cancels its output. There are various design tricks that enable you to smooth out this effect to some extent, but it would be nice to avoid it altogether.
There have been driver designs in the past that have done away with the surround altogether – free edge designs had some popularity in the 1950s and 1960s. However, there are a couple of drawbacks with this approach. Firstly, there is no air seal with the cabinet and the driver effectively reflexes itself with a poorly designed port. Secondly, the unterminated edge of the cone allows more severe cone break-up. Bowers & Wilkins engineers therefore did a bit of lateral thinking, based on the realisation that if you restrict yourself to mid frequencies, where the required cone movement is small, you can design a different type of suspension.
Instead of a normal rolling surround, the Fixed Suspension Transducer (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 (see the section Kevlar cones) passes through to the surround. And if those 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 than with a similar driver with a normal surround.
Compared to a regularly terminated Kevlar driver we can note two things about the FST surround. Firstly, the whole cone begins to respond much quicker in a given time. In the time domain this equates to a better transient response. In the frequency domain it indicates a more extended high-frequency response, and indeed the response of this driver is smoother to higher frequencies and allows a better-integrated crossover to the tweeter. Secondly, the final pattern of cone movement is even more random than for the regular driver, with a resultant increase in clarity for the listener. Additional features of this driver, not connected with Kevlar are the copper sheath on the magnet centre pole, a device for decreasing harmonic distortion, and the skeletal chassis (basket) design, which minimises reflections from the chassis and optimises the coupling between the driver and its enclosure.
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