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
the emitted sound field, diffractions and reflections caused by the cabinet boundaries
are too often overlooked although they are the cause of most of the irregularities
heard and measured in the higher frequency areas.
In that respect conventional rectangular, sharp corner boxes perform especially
poorly. On the other hand, the shape of the System 1200 has been designed with
careful attention paid regarding the sculptured front panel to provide smooth,
rounded edges which minimise side diffractions.
Another problem involved in cabinet design is to ensure that the box will effectively
behave as neutrally as possible, ideally without interfering at all with the sound field
emitted by the drive unit and the port. There are two main ways the box can get into
vibration. First there can be a mechanical transfer of energy between the drive unit
and the cabinet front panel. Preventing this requires the use of a rigid and stiff front
baffle, which is achieved on the System 1200 by a very thick and compact MDF
panel. The second way the cabinet can get into vibration is by transmission from
acoustical to mechanical energy. Since high acoustic pressures are present inside
the cabinet this is quite likely to occur if no attention is paid in order to minimise it.
Here the use of rigid panels is also helpful but, since their stiffness cannot be infinite
and therefore their resonances only shifted towards higher frequencies, enough
damping has to be provided in the cabinet assembly, including panels and joints.
Due to its octagonal shape and its cabinet construction, the System 1200 perform
very well in that respect. Its shape tends to decrease the largest dimensions of each
side panel, which reduce low frequency resonances, while the doubled number of
side provides additional damping.
In addition to the cabinet construction the volume and port tuning have been
carefully calculated to give the best set of parameters for monitoring loudspeakers.
There is a fundamental relationship in loudspeakers between efficiency, cabinet
volume and low frequency performance given that minimal amplitude variations can
be tolerated (as in monitoring situations). The set of parameters that are arrived at
as a solution are inevitably a compromise and the skill of Tannoy has always been
shown to be getting these particular parameters correct for the application.
