Tannoy 800 user manual System Description and Philosophy, Drive unit, Cabinet

SYSTEM DESCRIPTION AND PHILOSOPHY

A loudspeaker design naturally divides into various parts: cabinet, drive unit(s) and crossover. The design of these parts cannot take place in isolation as they are all interdependent.

Drive unit.

The drive unit used in the System 800 monitors is part of the latest generation of Dual Concentric units designed by Tannoy. Among many others features, this range of drivers incorporate a dual magnet assembly, ‘tulip’ HF waveguide and injection moulded polypropylene LF cone.

The design of the HF waveguide has been arrived at by making extensive use of CAD (computer aided design). It matches the acoustic source impedance at the HF diaphragm into the acoustic environment, shaping the wavefront as it travels down from the diaphragm ensuring equal path lengths to achieve a spherical wavefront. Wavefront shaping begins at the diaphragm surface and, because the compression ratio can be kept relatively low with this design, the distortions due to air non- linearities are minimised. A hyperbolic flare has been chosen for optimum low frequency performance at the crossover point.

The HF diaphragm is made from aluminium and magnesium alloy, with optimised shape and thickness providing rigid piston behaviour up to 25 kHz. The diaphragm assembly is suspended by a precision moulded, inert nitrile rubber surround. Its very narrow roll eliminates resonances below 25 kHz and provides a very stable and consistent mounting. The roll form ensures high excursions can take place if necessary yet provides a fatigue-indestructible assembly.

The HF voice coil assembly incorporates a high temperature copper wire chemically bonded onto a kapton former fitting onto the outside of the HF diaphragm skirt. The thermal power handling of the voice coil is greatly increased thanks to its ferrofluid filled magnetic gap.

Physically, the whole HF assembly self centre mounts onto the back of the low frequency assembly using three screws carrying with it the self-centring HF diaphragm. Production and field service is therefore virtually foolproof and extremely consistent.

The LF unit uses a CNC precision injection moulded polypropylene cone, terminated by a nitrile rubber, high-compliance surround. The characteristic cone termination impedance is matched by the surround material independently of the required suspension compliance. The unit system compliance is provided by the rear suspension where the best degree of mechanical control can be provided.

The shape of the LF cone has been calculated to match the HF hyperbolic waveguide ensuring the wavefront remains spherical and perpendicular to the cone surface throughout the propagation.

Purpose-designed trim rings are used to blend the HF wavefront into the cabinet. This feature has been shown in our research to be the biggest single factor in providing smooth HF radiation in Dual Concentrics.

The heart of the LF unit is the motor system comprising the magnet and voice coil. The choice of magnet operating point parameters, air gap flux strength, voice coil details (number of turns, resistance, winding length, diameter etc.), moving mass, dynamic compliance and drive unit radiating area presents a very complex mathematical problem where the solutions can take many different forms. Reaching the correct answers is much easier if computers can be called on to assist with solving the equations, as Tannoy do for its drivers.

Cabinet.

Aside the drive units, cabinet design plays a major role in the acoustic performance of a speaker system. Among the problems which can contribute to the degradation of