The Reference 207

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Cabinet

The main wooden cabinet uses the latest woodworking technology to produce curved sections: these are inherently stiff and are also comprehensively braced, leading to very low colouration. The result is also aesthetically pleasing and stylish.

It is essential that the cabinet is inert, in order that no colouration is introduced into the soundfield. Ideally the panels should remain motionless, although some movement is inevitable, and extensive research has gone into making the 207 cabinet as inert as possible.

Firstly we must minimize excitation of the cabinet. This excitation takes two forms: direct transmission of mechanical vibrations from the drivers and excitation by acoustic radiation within the cabinet and, to a lesser extent, from and around the room.

The mechanical vibrations are minimized by the long-standing KEF technique of decoupling and internal acoustic excitation is reduced by acoustic damping materials. The second line of defence is to ensure that the cabinet panels have high stiffness in order that any remanent driving forces are rendered harmless. Curvature gives us an immediate advantage, and extensive internal bracing and thick panels complete the solution.

Independent Driver Loading (IDL) has been used to remove the normal problem with tall, columnar bass enclosures. These typically suffer from vertical standing waves at very low frequencies which are very difficult to control using damping materials. IDL sub-divides the enclosure into 2 separate (half-height) bass sections with greatly improved internal acoustics. Because less damping is required we also gain higher sensitivity and clearer, more open bass quality.

Low Frequency Drivers

The low-frequency units use reinforced paper diaphragms, the material properties of which are ideal for bass drivers: light, stiff and consistent.

The Ultra Low Distortion Motor System, with T-Pole and twin balanced Faraday Rings above and below the coil ensure (1) perfect symmetry of the magnetic flux profile; (2) constant coil inductance with position, and (3) reduced flux modulation, leading to dramatic reductions in harmonic distortion products.

An ‘overhung’ or long-coil / short-gap design provides large excursion capability but normally would introduce problems with heat dissipation at the top of the coil. However, the (aluminium) upper Faraday Ring, a large thermal mass situated on top of the pole, eliminates any such problems in this design. The open chassis and vented cones minimize noise due to trapped air, and subsequent distortion products.

Lower Midrange Driver

The R207 lower midrange driver is derived from the 207 LF unit, but uses a lightweight neodymium ‘pot’ magnet. Here the excursion requirements are significantly reduced, allowing the use of a short coil/long gap motor system which has an inherently linear flux profile without the need for a balanced Faraday Ring system. Nevertheless, a single Faraday Ring around the pole is retained to reduce distortion due to flux modulation.

The significant reduction in mass of the unit afforded by the use of a neodymium-based motor allows for effective decoupling from the cabinet, preventing excitation of panels in a frequency band where cabinet resonance could otherwise occur.

With stiffness requirements relaxed a little for a lower midrange unit, the diaphragm is thinner and therefore lighter. The reduced mass results in higher sensitivity, allowing the use of a phase plug instead of a dustcap, which is not required here to augment low-frequency output. The shape of the phase plug is optimized for smooth acoustic response while at the same time improving heat dissipation.

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