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Concept Blade: The leading edge of acoustic technology

News International-French

4 Feb 2011

In loudspeaker engineering as in everything else, true innovation sometimes requires rethinking a problem from first principles. So when KEF set out to create the Concept Blade, its acoustic research department had complete freedom to explore radical new options. No preconceptions or aesthetic restrictions, and no reliance on existing components.

(Published on February-March 2010 – JEC Magazine #55)

Concept Blade is as result of 3 years of work. Its exceptional acoustic integrity derives from the fact that every element of the system has been conceived to perform as a single coherent unit with all parts working in flawless harmony. While the technologies incorporated into the design are often very complex, the focus has always been on simplicity. The drivers are specifically designed to behave with zero break-up or resonance over their frequency range, and the distinctive cabinet is carefully engineered not to interfere with the purity of their output.


KEF Concept Blade

At first sight, the task of a loudspeaker seems very simple: the input voltage needs to be translated faithfully into sound pressure. However, the voltage is only a onedimensional signal varying with time whereas pressure has three spatial dimensions as well as varying with time. The Concept Blade loudspeaker system is designed to execute this function while avoiding the complex physical artefacts found in conventional loudspeaker systems. The aim of the project was to produce a source that behaves in the simplest possible manner.


The enclosure

The ideal point source produces a coherent sound wave rather than the diffuse sound which can result from cone breakup and enclosure radiation. To complete the concept, the drivers of the Concept Blade all behave as rigid pistons until well outside their working range. This is a key part of the concept since drivers that break-up differently produce a sonic signature that allows the brain to recognize that there are multiple sound sources.


The delayed resonance from the enclosure may colour the sound, adding unwanted complication to the simple behaviour of the ideal source. Work by KEF engineers has focused on the three main areas causing this type of colouration: (1) vibration transfer directly from the drivers exciting the enclosure walls to radiate sound; (2) sound pressure in the enclosure causing the walls to vibrate and radiate sound, and (3) variation of the enclosure’s acoustic impedance causing the driver motion to radiate after the original signal has stopped.



In the Concept Blade cabinet, vibrations due to forces from the drivers have been completely eliminated since the back-toback side-mounted bass drivers are mechanically linked to achieve force cancellation, while the Uni-Q is decoupled from the cabinet using vibration isolation methods. Introduced at KEF in the 1970's, driver vibration isolation by decoupling has been refined with FEM analysis and laser vibrometry. The sound radiation due to the acoustic effects was modelled using FEM. An acoustic absorbent material was chosen for the cabinet interior. The material was positioned to reduce the effects of cavity modes and to force the enclosure to behave, to all intents and purposes, as a pure acoustic compliance.


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The company was founded in 1961 by Raymond Cooke and was initially headquartered in a Nissan Hut on the premises of Kent Engineering & Foundry (KEF). Cooke, an ex-BBC Electrical Engineer, was keen to experiment with new materials and technologies in order to create products with superior acoustic quality that could reproduce recordings as natural as the original performance.


The acoustical transmission of the sound from the rear of the driver through the enclosure walls proved an interesting problem since it is strongly affected by unsuppressed cavity modes. Once these were eliminated, with absorbent material, it was possible to observe the areas of the cabinet responsible for radiating most energy and to add suitable mechanical bracing to the computer cabinet models, adjusting their design so that the enclosure radiation could be minimized. Armed with this detailed understanding of the enclosure's vibroacoustic behaviour, an informed choice of construction materials could be made: the balsa wood/carbon fibre composite was chosen for its extremely high stiffness and excellent damping properties. Balsa’s lowdensity, high-strength and coarse open grain has excellent damping properties, and is used end-on to maximize stiffness. The parabolic curvature of the cabinet walls increases rigidity further still. As a result, the only sound you hear is that radiating from the drivers and not from other parts of the speaker.


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