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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