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.
