This loudspeaker is a professional mid-field reference monitor, able to reproduce with accuracy and at high levels frequencies as low as 40 Hz. Using an advanced 12” Dual Concentric driver, if offers the advantages of point source operation in a very cost-effective design. The attractive octagonal shape of the cabinet with its rounded edge corners and front panel contributes to the excellent acoustic performance of the cabinet.

Due to its point source operation it will perform equally well in both landscape and portait orientations.

Drive unit.

The drive unit used in the System 1200 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 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.