Tannoy NFT and DMT Series II

Of all loudspeaker manufacturers in the world Tannoy has the greatest number of loudspeakers in use for sound production in british and European studios. An enormous number of successful recordings have been produced on Tannoy Monitoring since the first introduction of the Tannoy Dual Concentric loudspeaker in the 1950s.

The Dual Concentric design philosophy is world known for its precise stereo imagery and for the ease of finding sounds within a sound stage. The presentation of the sound image makes long production sessions much less fatiguing than with other monitoring systems because the brain does not have to work as haard correcting for acoustic anomalies in the time and frequency domains.

During the lastest years great strides have been made in the analytical understanding of loudspeakers. In parallel, the explosion of computing power available to physicists, electronics, acoustics and mechanical engineers has resulted in loudspeaker design techniques advancing at a faster rate than ever before.

Tannoy's massive experience and its highly innovative and skilled design engineering team, has placed the company in a most enviable position. This is reflected for the 1990s in what is a frankly exceptional - revolutionary rather than evolutionary - range of studio monitors.

A loudspeaker design naturally splits into various parts; lower frequency, higher frequency, crosover network and cabinet. The design of these parts cannot take place in isolation as they are all interdependent.

Traditionally, Tannoy has used a single magnet driving both low frequency and high frequency magnetic air gaps. This gives a very compact drive unit with acoustic source alignment. In the new designs of Dual Concentric units the HF unit and LF unit now have separate, dedicated magnet systems. This is because the HF waveguide design has become so sophisticated it cannot be made by processes suitable for magnetic flux carrying materials.

High Frequency Drive Unit

The HF waveguide can therefore no longer be an integral part of the LF magnet system. In splitting the magnet systems an extra degree of design freedom allows for very high precision casting and moulding processes together with accurate self centring diaphragm assemblies. Both production processing and in-field repairs can then guarantee consistent performance.

A new design of waveguide has been arrived at by making extensive use of CAD. We call it waveguide because there is a direct analogy with electromagnetic radiation in that characteristicimpedances must be carefully matched without introducing standing waves. The Tannoy HF waveguide matches the acoustic source impedance at the HF diaphragm into the listening environment.

The waveguide shapes the wavefront as it travels down from the diaphragm ensuring that path lengths are equal, that the wavefront is perpendicular to the fixed surfaces and that wavefront is spherical. Only small errors of fractions of a millimetre can upset this condition and cause phase shifts in the waveguide. Accuracy of design and production are essential in achieving the correct conditions within the waveguide.

In this way, transvers modes are minimised and high frequency dispersion maximised. 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 a new design. The waveguide requires total piston movement over the operating range since any breakup modes within the diaphragm will result in phase-shifted components at the start of the waveguide propagation. A rigid piston diaphragm operating to above 25 kHz is made from aluminium and magnesium alloy.

A special machine has been designed and built toform and extrude the diaphragm with a 2 mm skirt. This configuration gives the most rigid diaphragm and ensures reliable handling for production and field servicing.  Aluminium is notoriously difficult for adhesive working and we put the diaphragm through a special alkaline etching process followed immediately by the build process to ensure relibility.

The diaphragm assembly is suspended by a precision moulded, inert nitrile rubber surround. This has been designed and tooled using high-precision, numerically controlled machining techniques. 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 diaphragm is driven by a new design of voice coil assembly.high temperature polymide-insulated, copper-clad aluminium, rectangular ribbon conductor is chemically bonded onto a glass-fibre former fitting onto the outside of the HF diaphragm skirt. This gives a high temperature (polyimide), very low mass (aluminium wire, glass fibre), high rigidity (rectangular wire, former to outside of diaphragm skirt), high reliability (nitrile suspension, copper clad aluminium) assembly.

Leadout materials are crucial for HF units and our new design incorporates beryllium copper flat strip to eliminate fatigue breakages and prevent fusing on unsupported areas under overload conditions.

tehHF diaphragm assembly is factory mounted onto the waveguide by a newly designed high-precision productionprocess. This ensures that the spacing between diaphragm and waveguide is consistent and the whole assembly self centres under all conditions when placed on the magnet assembly. Field replacement is therefore extremely simple and no difficult soldering or centring techniques are required.

The HF magnet assembly uses an anisotropic barium ferrite magnet for maximum energy product (BHMAX), a newly developed magnetic air gap coolant for lowest viscosity and highest thermal rating, a copper flux stabilising ring around the pole piece to minimise voice coil inductance and control the highest frequency energy, and a cavity damper to control the rear cavity compliance beneath the diaphragm.

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.

Low Frequency Drive Unit

The heart of the LF unit is the motor system comprising the magnet and voice coil. Computer optimisation of the low frequency magnet gives linear flux linking to the voice coil using low carbon steel pole pieces and an anisotroipc barium ferrite magnet. A specially designed pur copper stabilising ring fits over the outer pole where it reduces eddy current lossles, lowers midrange distortion and increases thermal cooling by a massive 50%. In this way both power compression and reliability are considerably enhanced.

The choice of magnet operating point parameters, air gap flux strength, voice coil details, moving mass, dynamic compliance and drive unit radiating area presents a very complex mathematical problem where the solutions can take many different forms. The optimum solution depends on the intended use of the drive unit in particular cabinet systems and the expectations of the and user.

This is the skill or "black-art." element of loudspeaker design. Reaching the correct answers is much easier if computers can be called on to assist with solving the equations. Tannoy has an in-house software facility producing purpose-written programs to solve these equations in both numerical and graphic terms.

The LF voice coil uses polyimide insulated, chemically bonded rectangular section copper wire wound onto a high temperature aluminium former for robustness and reliability in thermal conductivity. A specially designed heat barrier wound onto the end of the former protects the adhesive bond to the LF cone from excessive temperatures.

Robust, fatigue-free leadout braid connects to a polarised, vibration-proof, high-current ternminal barrier connector.

The shape and materials from which the cone pistons are made reflect the optimisation of drive unit to cabinet size and end use. System 6, 8, 10 and 12 LF units use a CNC precision injection moulded polypropylene cone. System 15 and 215 have a traditional pulp cone with apex treatment and air-dry felting process. For cones of this size there is no better alternative when mass, rigidity, piston movement and natural upper roll-off charactereistics are considered.

All LF drivers have their cones terminated by nitrile rubber, high-compliance surrounds. 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.

In all cases 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.

Brand new pressure dic-cast chassis have been tooled for the new range drawing extensively on new thinking for LF drive units. It is important to eliminate trapped air cavities as these can provide unwanted compliances, upset the mechanical Q design requirements and cause unwanted acousyic colourations because of Helmholtz resonances and reflections from the chassis surfaces smearing the energy/time response.

The new castings have a very open construction with vented rear suspension features to eliminate low Q cavites, improve thermal cooling and prevent major reflections. Rigidity has been optimised in the axial plane to complement the cabinet philosophy while the front surface profile has been designed to prevent diffraction at the cabinet surface.

The five sizes of chassis each have purpose-designed trim rings 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 (assuming, of course, that the HF unit is well designed in the first instance).

Crossover Network

There are two philosophies in designing loudspeaker crossover networks; the minimal and the conjugate. The minimal approch requires that the drive units are inherently well behaved and that each section, LF and HF, require minimum equalisation to achieve a smooth flat amplitude response.  The conjugate approach requires that the drive units are accepted as they are but are well characterised. The crossover network is then calculated to provide inherent equalisation to ensure a smooth amplitude response.

The two approaches differ in design emphasis. The minimalist designer concentrates on the drive unit design in controlling the final performance, while the conjugate designer concentrates on complex electronic analysis of networks to achieve the same measured result.

Tannoy has always followed the minimalist philosophy as far as possible. This is because listening trials with loudspeakers always point to those with the least crossover design complexity as being more realistic, involoving and convincing in their reproduction. However, this makes the drive unit designer's task more difficult as it is much harder to control performance through the mechanical parameters than through the electrical crossover components. It also puts much greater constraints on production repeatability of processes and test methods. However, overall the result in our belief is a better loudspeaker.

In crossover networks it is vital to use the very highest quality components for series connected elements. Resin impregnated, air cored inductors,  very high grade film capacitors and DMT cpacitors are needed for best sound quality. Internal wiring has an effect and in the new Tannoy Monitor series high-purity, long-grain crystal, low-oxygen copper wiring is used.

DMT research showed when a capacitor was encapsulated in a vibration absorbing material it changed both the sound texture and dynamics. Every variable of capacitor construction was investigated and custom capacitors designed optimised for sonic performance and with high-purity copper leads.

Vibrations inside the cabinet can effect the performance of inductor coils.  Tests show that reducing the vibrations reaching the inductors can heve a marked effect on system bass end resolution. Coils vacuum impregnated with resin are chosen to reduce the effects of vibration.

Air cored inductors radiate a significantmagnetic field which affects nearby components. Similary inductors can be affected by a driver's magnetic radiation. For these reasons it was decided to produce a split crossover with the inductors mounted on the cabinet cross-brace away from the driver magnets and other crossover components. The sound quality improvements more than justify additional manufacturing costs.

The crossover networks in the new series use simple low order slopes 96 dB and 12 dB per octave0 mainly to control the power distribution and balance. The components are of very high quality with Hard-Wiring (no printed circuit boards) and mounted on the back of the terminal panel at the rear of the cabinet. There is no need to remove drive units to gain access.

Terminal Panel

The terminal panel is a new design especially tooled for the new series. The option of conventional wiring or Bi-Wiring is available by a unique high quality gold plated sliding mechanism with large diameter robust terminals. The benefits of Bi-Wiring for monitoring are easily heard where the extra pair of cables can be accomodated.

The new terminal panel on System 12, 15 and 215 also includes a sliding link which provides adjustment of the high frequencies on a shelving basis from 2 kHz to 25 kHz with plus or minus 1,5 dB adjustment. The systems are calibrated in production to be flat to within specification when set to the flat adjustment position. All terminals and contacts are gold plated to eliminate contact potentials and oxidation. The terminal panel carries the crossover mounting and can be removed from the outside of the cabinet.

Cabinet

With well designed drive units the majority of the aberrations in the loudspeaker system are due to the cabinet. Most of the irregularities heard and measured in the higher frequency areas are due to diffractions and reflections caused by the cabinet boundaries.

The amount of acoustic energy transfer that the drive units can lauch into the listener's space is dependent on rigid mounting since action and reaction are equal and opposite. When the displacements of the HF diaphragm are calculated for sound levels in the region of 80 to 100 dB sound pressure level, the movements involved are extremely small, often fractions of a thousandth of an inch. However tiny these displacements are they carry information that is required for accuracy in the resulting sound stage.

It stands to reason therefore that the drive unit must be held in space very rigidly so that the HF diaphragm displacements are not themselves modified by the LF displacements which have inherently much more energy associated with them. The obvius method of doing this is to mount the drive unit rigidly into a rigidly made cabinet. But in doing this, a new set of problems appears.

Rigid systems are characterised by high stiffness. The natural resonance of the high cabinet stiffness - achieved by, say, cross bracing and bracing the driver to the rear of the cabinet - and drive unit mass, brings the natural resonance frequency into the audio band, typically around 100 to 200 Hz. This produces an objectionable colouration which can be mitigated in aural terms by some listeners by the increase in "speed" and HF clarity provided by the rigid system. However, it is notan ideal solution.

In its new Monitors Tannoy has taken a radical approach pointed to by measured parameter research into cabinet systems coupled with listening tests. The Tannoy cabinets are stiff but with a high level of internal damping. A very complex internal bracing structure in each of the cabinets allows the drive unit to be held rigidly but also to be able to dump its resonant or reactive energy into the lossy couplings of the cabinet. The joints between the driver and the bracing structure have a special compound which is very stiff at high frequencies but will absorb energy in the critical colouration areas.

The cabinet panel are made from MDF but are laminated an each side to increase their stiffness. However, the layer of adhesive between MDF and laminate acts as a lossy energy absorbent medium.

The cabinet panels are coupled into each other through hardwood rails at the corners, the dissimilar materials providing further modification for any inherent reactive energy components in the cabinet caused by the drive unit.

The rigid crossbracing structure is floated inside the cabinet using an adhesive system which will absorb the redundant energy from the rear of the drive unit chassis and maagnet system and yet provide the stiffness needed to allow very fine HF resolutions from the HF unit diaphragm

In addition to the cabinet construction he 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.

Cabinet Finish and Grille

The cabinet is finished in a high impact resistant, texture paint. The grille is held by plastic split dowels located in the grille frame which fit into rubber lined holes in the front panel.