The midrange is where the real musical action is invariably found, and a smooth midband is an invaluable loudspeaker quality. Enter Kevlar®. It’s been Bowers & Wilkins’s cone material of choice since 1974, and with good reason. The basic woven fabric is first impregnated with a stiffening resin that cures during the cone forming process. The cone is then further treated with a polymer coat, which seals the fibres and adds damping. The result is a semi-flexible cone, which exhibits a peculiar style of break-up behaviour, not found in more conventional materials, that maintains a more constant dispersion pattern at all frequencies in its range and transmits far fewer delayed, time-smearing sounds to the listener.
Not only does it deliver a cleaner sound, it can do so to a wider group of listeners. Kevlar is a synthetic aramid fibre, manufactured by DuPont®, and probably best known for its use in bulletproof vests. Indeed, those same mechanical properties of strength and the ability to dissipate the energy of a bullet also have benefits for speaker cones. Bowers & Wilkins first started using Kevlar as a cone material in 1976, with the introduction of the DM6 speaker. At that time, the science of speaker development was rather less developed than is the case today and it was a case of trying out promising materials, measuring the response of the driver and listening to the result. So although we knew that Kevlar could give better results than other materials around at the time, especially in the critical midrange, we didn't know in any real detail how the cones were actually behaving – in effect why they sounded better.
Our Research Director, Dr Peter Fryer, has long been a pioneer in the field of laser interferometry applied to speakers. Using this technique, we can look at how the driver diaphragm moves in response to different signals. Two of the most useful signals are a sine wave: a pure tone at a single frequency, and an impulse: a click sound that contains all frequencies at once. Looking at the behaviour at a single frequency with a sine wave readily shows standing waves or resonances in the diaphragm at that frequency. It also gives an indication of the way the sound disperses as it leaves the cone. For example, at higher frequencies, a semi-flexible diaphragm can exhibit motion where little radiation comes from the outer area and most comes from the central region.
This effective reduction in the radiating area has the benefit of widening the dispersion of the driver compared to that of a pure piston. This is exactly what happens with a Kevlar cone. Its effective radiating area gradually decreases with increasing frequency and, as a consequence, its dispersion is much more uniform with frequency than is the case with a very stiff material. The impulse response of the driver indicates how time-coherent it is.
Continued vibration of the diaphragm after the input signal has stopped can often lead to time smearing – a form of coloration – and resultant impairment of the clarity of the signal. However, not all delayed diaphragm motion necessarily leads to delayed sound being broadcast to the listener. Let's compare the impulse response of two drivers. They are identical apart from the cone material, one has a plastic cone. The plastic is homogeneous; in other words the mechanical properties are the same on all directions.
The second driver has a cone of woven Kevlar, treated with a resin to control the stiffness and a PVA compound to add damping and seal the fabric. Being woven, the Kevlar cone's mechanical properties are different depending on the angle to the direction of the fibres. Both cones are terminated at the outer edge in the usual way by a half-roll rubber surround. If we look at laser scans of the two different cones at different points in time after an impulse signal has been applied the conical shape of the diaphragm is lost in the process. At the time just after a signal has been applied, just the centre of the cone has started to move in both cases. With the plastic cone a circular bending wave starts to spread out from the centre of the cone. However, with the Kevlar cone the wave front begins to assume a square shape, imposed by the weave.
When these bending waves reach the joint between the cone and surround, some of the energy is reflected back into the cone and some passes into the surround. This is because the two materials have different mechanical properties. It's similar to the situation when you look out of a window. As well as the view from outside, you can see a reflection from inside the room. In that case it's because glass and air have different optical properties. Further reflection occurs where the surround is attached to the chassis or basket of the driver.
When these reflected waves reach the centre of the cone, they are reflected back out again and so on, until damping in the materials eventually dissipates the energy. Because the wave front in the plastic cone is circular, these repeatedly reflected waves set up a pattern of concentric rings which radiate delayed sound to the listener that adds to and colours the initial sound received. Although reflections do occur with the Kevlar, they happen at different times around the edge and the movement pattern of the cone is more random. The total area of the cone moving forward at any given time is more balanced by the total area moving backwards and far less of this delayed energy is actually radiated as sound to the listener; the air just shuffles across the surface of the cone.
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