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Sep 04 2014

KEF's Z-Flex Surround: Smoother HF Through Science

Every component and sub-assembly in a driver affects the overall performance of the driver. That's a pretty basic and logical statement, but when you really consider what that actually means, you realize how complicated speaker design is.

Consider the surround. At first glance it's basically the bit that holds the speaker cone in place, but the surround also moves, and with that movement come resonances and unwanted vibrations that can alter or change the sounds a speaker is trying to replicate.

In the cut-away of a bass driver (right), a typical "half-roll" surround (#3) is shown. Typically made of butyl rubber or other foam material, the half-roll surround, when properly designed, can perform quite well. However, KEF's use of Finite Element Analysis (FEA) has allowed our engineers to discover other possibilities that result in a better listening experience when listening to music through our products.

The biggest benefit from the Z-Flex surround is with the high-frequency response of our Uni-Q Driver Array.

For optimal performance the Uni-Q tweeter is placed in a perfectly smooth waveguide, which in the case of a Uni-Q would be made up from the mid-range driver and the surround. In an ideal situation, the high-frequency driver produces a smooth (hemispherical) 'point-source' radiation pattern, resulting in a tweeter with higher sensitivity than a tweeter placed in a flat baffle (the front edge of your speaker cabinet). Previous Uni-Q designs have used the conventional half-roll design, but the half-roll design, by the nature of its shape, compromises the ideal waveguide for the tweeter.

With the half-roll design there is an abrupt discontinuity in the waveguide at the transition from the cone to the surround which causes a secondary radiation. That secondary discontinuity, or diffraction, tends to muddy or smear the sound from the tweeter. Figure 1 shows the diffraction from a half-roll surround (in the area between the tweeter and the pink arc about halfway through the graph). Figure 2 shows the absence of the diffraction with KEF's Z-Flex surround. The absence of that diffraction results in smoother high-frequency response and dispersion.

Figure 2. High-frequency response with KEF's Z-Flex surround.

In order to get the true experience from a musical performance or soundtrack, there's a lot that needs to be considered when designing (or purchasing) a speaker. And one thing to consider is that the surround is way more than just the rubber bit that holds the speaker cone in place.

Jack Sharkey for KEF

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