B&W 800 D User Manual

Development of the B&W 800D

Development of the B&W 800D

Contents

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

Project brief . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

Drive units . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

Enclosures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

Crossover. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

Performance. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

Industrial Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

Appendices

I Diamond Dome Tweeter. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

II The FST Midrange Driver . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

III The use of Rohacell® in loudspeaker cones . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

IV Tapered tube theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

V Sphere/tube midrange enclosure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

VI Matrix™ cabinet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

VII Decoupling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

VIII Finite Element Analysis. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

IX Laser Interferometry. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

3

Bowers and Wilkins’ 800 Series first saw the
light of day in 1979 with the introduction of
the original Model 801. Its radical shape,
composed of separate enclosures for each
drive unit, was to remain relatively constant for
almost 20 years, proving, like so many concepts
to come from the R&D division at Steyning, that
good ideas, based on sound principles stand
the test of time.

The development of the flagship Nautilus
speaker, launched in 1993, introduced a raft of
new ideas that clearly warranted adaptation to
a broader range of products, the result of which
was the Nautilus 800 Series. That Series was to
redefine the high-end audio speaker market and
the development of the then top model in the
range – the Nautilus 801 – was covered in a
previous paper. The subsequent development of
the Signature 800, which refined and extended
some of the principles used in the Nautilus 801,
was also the subject of a paper.

This paper describes the development of a new
generation 800 Series, using the top model
800D to describe the principles and techniques
to be found in the range. There are significant
new developments, but there is much in the
new models that carries over from the old.
These existing techniques are discussed here
once more, so that this paper may be read in
isolation, without reference to the previous
publications.

Project Brief

High-end audio products are about performance.
The investigation of new ideas, materials and
processes is a continual process, sometimes
coming as small steps and sometimes as
significant leaps. In this case, our engineers
had been pursuing several projects that
promised a significant improvement in
performance and the brief was simply to
incorporate the results into products.

Overview

A loudspeaker system can be divided into three
basic constituent parts:

• The drive units

• The crossover

• The enclosures and supporting structure

In an ideal situation, the drive units, seamlessly
blended by the crossover, should transmit
a perfect audio replica of the electrical input
signal. The rest of the structure should remain
perfectly stationary and serve only to support
the drive units, absorb the unwanted radiation
radiated from the rear side of each drive unit
diaphragm and be shaped to aid even
distribution of the sound away from the
loudspeaker. How good a speaker sounds
can be measured by how close the designer
can get to this ideal. In the real world, of
course, we fall somewhat short. Drivers suffer
from distortions of all kinds and enclosures
vibrate and add their own coloration to the
sound. Crossover components add unwanted
artifacts to the electrical signal before it even
reaches the drivers.

We shall examine our design philosophy to all
these categories separately, but in fact, in the
design process, they must be treated as a
whole, because they all interact. The choices
the designer makes in one area are affected
by what he has to work with in another.
Inevitably, choices have to be made and it is
down to the skill of the design engineer to
make a balanced judgement and optimise the
whole. It is a skill that combines science with
art. The science provides understanding and
points the way forward. For as long as the
scientific understanding is incomplete, however,
an understanding of the art of music is essential.
In high-end audio, it is not sufficient simply to
achieve a pleasant sound, the designer must
strive to recreate as closely as possible the
impression of being at an event, of being able
to imagine performers in front of the listener,
of raising the goose bumps on the skin and
hair on the back of the neck. That is the target,
and virtually impossible to describe by a set
of numbers.

The listener must be the final arbiter of how
well the target has been met. All we can do
within the scope of this paper is to examine
the science. In the sections immediately
following there is a general overview of each
of the techniques used and they are covered
in greater detail in the Appendices at the end
of the paper.

Introduction

4

Perhaps the most radical of the new
technologies used in the speaker is the
diamond dome of the tweeter. The acoustic
development is covered in detail in Appendix I.

One of the surprising outcomes of the new
design when compared with the existing
aluminium dome design is that the –6dB
frequency is lower (The blue horizontal line in
figure 4 represents the -6dB level after the
tweeters are equalised flat to 90dB by the
crossover). This may at first glance seem
strange, considering that diamond is much
stiffer and has a significantly higher break-up
frequency than aluminium. The answer is
simply to be found in the ‘ideal’ response of
an infinitely stiff dome of the same shape,
which suffers a deep dip in the response
around 70kHz because of the difference in
arrival times of sound generated at different
parts of the dome (Represented by the green
shaded area in figure 1). At 70kHz, the
wavelength of sound in air is 4.9mm (0.19 in)
at 20C, which is comparable with the height
of the dome. That the aluminium dome has
the higher –6dB frequency is simply because
the response is on the way down from a high
amplitude resonance at 30kHz.

We took the view that the diamond dome
should follow this ideal as closely as possible
and we should not attempt to achieve a flatter
acoustic response through various devices that
would either cause the total radiating area to
deliberately deviate from piston-like behaviour
at a lower frequency or by engineering cavity
effects in front of the diaphragm.

Our listening experience had repeatedly and
consistently shown that the most important
criterion affecting the sound quality was how
closely the radiating surface remained piston­like in the accepted range of human hearing
below 20kHz. We were therefore not tempted
by any perceived marketing need to follow
popular (mis)conceptions of what is required
to properly convey the improvements offered
by high sampling rate digital recording formats.
We kept the acoustic response of the infinitely
stiff dome as our target. If one removes the
acoustic time delay effects by examining the
structural acceleration response of the dome,

one sees that it is flatter and more extended,
as expected (see Appendix I).

It should be remembered that deviation from
piston-like behaviour does not suddenly happen
when the break-up resonance frequency is
reached. It builds up from a much lower
frequency. It is similar to the effect of anti­aliassing filters used in digital recording. Those
used in the standard 44.1kHz CD format may
have cut-off frequencies above the accepted
limit of human hearing, but deviations in the
phase and associated group delay begin well
below 20kHz. It is the shifting of these build
up effects well above the limit of hearing that
is most important, not necessarily maintaining
a flat acoustic amplitude response to 100kHz
or whatever, although, of course, the two
are related.

That it is possible to produce a diamond dome
at all is due to relatively recent developments in
the production of industrial diamonds.

The standard technique for synthesising
diamond is to simulate the conditions that
occur in nature, ie the high pressures and
temperatures that are found inside a volcano.
The technical difficulty in achieving temperatures
as high as 2,100C (3,800ºF) and pressures
exceeding 50 kbar limits the size and shape
of the diamond components that can be
manufactured by this process.

In the 1980s, the invention of a chemical
vapour deposition (CVD) technique for
growing diamond overcame this limitation:
the deposition temperature was halved and,
more critically, growth could now be achieved
at sub-atmospheric pressures. The technique
succeeds in producing diamond under
conditions for which graphite is the thermo­dynamically stable form of carbon by creating
a carefully balanced chemical environment
that stabilises the diamond surface as it
grows; in effect, the kinetics win over the
thermodynamics. This very specific environment
is generated by exciting a gas mixture of
hydrogen with a small percentage of an alkane
(carbon source gas) and other gases (such
as argon and oxygen). The resultant plasma
contains alkyl radicals, hydrogen atoms and

5

Drive Units Tweeter

30

20

10

0

-10

-20

-30
10000 20000 100000Frequency (Hz)

Simulated acoustic Frequency response

30

20

10

0

-10

-20

-30
10000 20000 100000Frequency (Hz)

Simulated acoustic Frequency response

1 FEA simulated acoustic responses of aluminium (top) and

diamond (bottom) domes. The response of an ‘ideal’ dome
is shown shaded in each case.

6 7

high-energy electrons. A range of power
sources can be used to excite the plasma,
the most common being microwaves, heated
filaments and arc discharges. The diamond is
deposited directly onto a suitable substrate
material, for instance tungsten, molybdenum
or silicon. This substrate can be removed after
deposition to leave a freestanding diamond
layer. The layers produced can be millimetres or
microns thick with areas greater than 100 cm2.
It is also possible to replicate complex shapes
machined into the substrate. The diamond itself
is polycrystalline and of high purity and,
because the properties are selected and
controlled, diamond materials grown by the
CVD process can actually outperform natural
diamond in many applications.

In developing the tweeter dome, B&W worked
closely with one of the world’s foremost
producers of industrial diamonds, Element 6,
based in Ascot, UK. As in so many industrial
applications, although the basic process was
well established, there were practical difficulties
peculiar to this application that had to be
overcome. Depositing diamond to the profile
of the spherical section of the dome itself
was fairly straightforward, but the vertical ring
location for the voice coil (see figure 3) proved
particularly tricky. Forming and ejecting with
parallel sides and maintaining material thickness
at the sharp corner were difficult. This part of
the profile is crucial both in ensuring repeatable
accurate location of the voice coil and also in
increasing the dome's stiffness to raise the first
break-up frequency. This is the first time such
a profile has been manufactured and the design
is patented.

The dome itself does not constitute the whole
of the radiating surface. The supporting surround
plays an important role in determining the
tweeter’s response.

During the development of the Nautilus 800
Series, deficiencies in the plastic film half roll
surround used on the then standard tweeter
design were ameliorated by using a flat foam
polymer surround. Its motion remained better
phase matched to that of the aluminium
dome and gave a smoother overall response.
However, in the new systems, we wanted to
use crossover filters with more gradual roll-off

2a

Tweeter continued

3 Diamond dome profile

5 Tweeter impedance with (red) and without (blue) a silver

layer on the magnet centre pole.

rates (see the subsequent section on the
crossover) and this necessitated lowering the
tweeter’s fundamental resonance frequency.
This could only be achieved by reverting to a
half roll profile to increase compliance, but we
were able to take advantage of a new synthetic
rubber material that avoided the shortcomings
of the original plastic film. We were thus able
to achieve good phase coherence with the
dome and usefully lower the fundamental
resonance frequency.

Frequency response deviations from the ideal
are not dependent solely on the dome and
surround. Any moving coil drive unit is a current
driven device, with the force on the voice coil
represented by the formula:

F = Bli

where F = force, B = magnetic flux density,
l = length of coil in the magnetic gap and
i = current.

Yet for various reasons, mainly to do with
controlling bass response, amplifiers are
voltage sources. The high frequency response
of a drive unit is therefore affected by the
inductance of the voice coil and, in order
to maintain high frequency response, the
inductance should be minimised. To that
end, not only does the tweeter employ a single
layer ribbon wire voice coil to minimise the
number of turns, it also uses a silver plated
centre pole in the magnet structure.

Copper is more usually used for this purpose.
The electrically conducting layer acts as a
shorted turn in the secondary windings of
what is in effect a transformer and reduces
the inductance of the primary windings (the
voice coil). Accommodating a layer of non­magnetic material widens the magnetic gap,
with a resultant decrease in flux density and
hence drive unit sensitivity. Silver, having a
higher conductivity than copper, is effective
with a thinner layer and is used here to
maximise sensitivity.

Both the tweeter and bass drive unit diaphragms
of the 800D are designed following the ‘stiff is
good’ principle. However, good reproduction in
the midrange has a particular requirement that
precludes this approach if a single drive unit
is to be used to cover the whole range. With
stiff diaphragms, the dispersion progressively
narrows as the frequency increases and the
wavelength becomes similar to or smaller than
the diameter of the diaphragm. With bass units,
this factor is never a problem, because the
wavelength is always significantly greater
than the size of the drive unit. At 400Hz,
the wavelength is just under 860mm (34 in),
compared to, say, 380mm (15 in) or 250mm
(10 in) or less for the bass drive unit. At 4kHz,
the wavelength is 86mm (3.4 in) and so with
any drive unit of a size large enough to give
high output levels with low distortion at the
bass-to-midrange crossover frequency,
beaming is likely to be a problem. Off centre
listeners are going to hear a sound with a
significantly different balance from that on axis,
and image precision will suffer.

Having established that we do want to achieve
high sound levels and do not want to use more
than one drive unit, the best option is to use a
drive unit with a more flexible cone material.
That does mean that the cone is virtually certain
to be operating in its break-up region for much
of its usable range, but the usual deleterious
effect of this (delayed resonances colouring
the sound) is ameliorated greatly if the correct
material is chosen.

Woven Kevlar®has been used by B&W since

1974. For the Nautilus 800 Series, the way we
used Kevlar in midrange-only (as opposed to
bass/midrange) drive units was improved by the
use of a new design of outer cone support or
surround. Such drive units go under the name
FST, standing for Fixed Suspension Transducer.

Midrange

For the Signature 800 and Nautilus 800, the
magnet structure was improved by using a
Neodymium-Iron-Boron (NeFeB) magnet driving
a thicker top plate. The use of a short coil in a
long magnetic gap lowered harmonic distortion
and improved detail retrieval. The reduced bulk
of the magnet had a minor secondary benefit
in reducing the bulk of obstructions behind the
cone and hence the amount of sound energy
from the rear of the cone being reflected back
through the cone to add delayed coloration.
This approach is carried over to all models in
the new 800 Series. Completely new to this
Series is the chassis (basket), which provides
greater strength than before without compro­mising the open area of the original. The use
of Kevlar®in the FST drive unit is discussed
in detail in Appendix II.

2b

2c

2 Diamond dome manufacture.

a Domes awaiting removal from the forming substrate.
b Laser cutting the outside diameter.
c Checking material thickness.

4 Responses of new diamond dome tweeter (black) and

Nautilus 800 Series tweeter (red)

14

12

10

8

6

4

Impedance magnitude (Ohms)

2

0

3

10

Frequency (Hz)

4

10

8 9

The midrange enclosure is carried over from the
Nautilus 800 Series with a small change to the
exterior design where the tweeter is mounted.
The tweeter is more enveloped, but this is an
aesthetic development with no acoustic
significance, except that the tweeter is mounted
further forward (see the section Crossover).

The unique sphere/tube design overcomes the
bandwidth limitations of simple tube loading and
is described in Appendix VI.

Because tube loading results in an overdamped
high-pass alignment, it is not applicable to
passive system bass cabinets because of the
inability to add boost equalisation. Therefore,
like the Nautilus™800 Series products, the 800D
employs a Matrix™-braced vented-box enclosure
(see appendix II).

The inertness of the cabinet is further enhanced
by using 38mm thick panels, also contributing
significant mass. In addition, smoothly curving
the rear surface greatly adds to the stiffness of
the cabinet and gives an interior shape that
modifies the internal acoustic resonance modes,
since there are fewer parallel surfaces. The
combination of an internal Matrix™construction,
together with both a massive and stiff external
‘skin’, makes the combination uniquely
resistant, not only to sound transmission from
inside to outside, but also to intrinsic cabinet
structural modes.

Bending thin wood laminations under heat and
pressure is widely used in the furniture industry
for the manufacture of chairs. However the
ability to accurately match and join two such
curved panels together without a witness
groove and to maintain the accuracy required
to fit the Matrix™panels inside is beyond the
capability of many suppliers. Special storage
conditions for the raw laminations, with
controlled temperatures and humidity are
essential and sophisticated CNC 5-axis routing
machines are required to shape the edges and
cut-outs of the curved panels.

The Tweeter incorporates Nautilus™technology
through the use of a tapered tube, filled with
wadding attached to the rear of the unit and
matching the hole through the pole (See
appendix V). The exponential profile has been
designed to ensure that the cut-off frequency
of the tube is low enough to absorb all the
energy in the operational bandwidth of the
tweeter, but allowing a shorter tube than in the
Nautilus™. It also allows the absorptive wadding
to be packed loosely at the mouth of the tube
and to become gradually compressed towards
the end. This allows the sound energy radiating
from the rear of the dome to pass through the
pole piece and into the tube without being
reflected back up towards the dome. This
variation in packing density ensures that the
acoustic impedance is varied smoothly, and
that there are no sudden changes that would
cause such a reflection of energy. As the
passband of the tweeter is similar to that in
Nautilus™, the onset of cross modes in the
tube is not a problem, occurring well above
audibility in the human ear.

A secondary use for the tube is as a heat sink.
The small dimensions of the magnet assembly
result in a low thermal mass. Making the tube
of zinc alloy and ensuring a good thermal bond
to the magnet back plate significantly reduced
the operating temperature of the unit. When
fed music from a 600W amplifier run just below
clipping, the operating temperature is reduced
by around 20C. In fact the tweeter was found
to be capable of withstanding unclipped high
frequency peaks from an amplifier rated up to
1kW, without the coil burning out. The tweeter/
tube combination is housed in an outer die-cast
shell which defines the outer housing of the
unit. The tweeter diaphragm only moves a
maximum of 0.5mm. Therefore, it is crucial

Bass Unit Enclosures Tweeter Bass

restricted and one cannot make up for lost
sensitivity by adding amplifier power, as is the
case with a powered subwoofer. So, work
began on finding a material that would add
further stiffness, increase inherent damping and
act as a better sound barrier than the materials
we had used in the past.

The material chosen has a composite sandwich
construction. Sandwich construction cones are
not new. The famous Leak Sandwich speaker
of the 1960s used a bass cone having an
expanded polystyrene core bounded by thin
aluminium skins, as did the flat fronted, oval
B139 from KEF that followed shortly after. Both
these diaphragms were thick and were a better
sound barrier than the paper cones common at
the time. However, they were fairly heavy and
expanded polystyrene as a core material can
now be improved on in terms of stiffness and
internal damping to achieve higher break-up
frequencies and better-controlled resonances.

The core material chosen was Rohacell®, again
an expanded foam material and one that is
commonly used in aircraft construction, due
to its light weight and relatively high strength.
This is bounded on both sides by carbon fibre
skins in woven mat form with a high level of
resin to add stiffness. Neither Rohacell®on its
own nor a Rohacell®/carbon fibre sandwich is
a new cone material, although the introduction
of both is relatively recent. What is novel in
the 800 Series is the cone thickness that has
been achieved through improvements in the
manufacturing process. Most Rohacell®cones
are in the 1-2mm thickness range. In the 800
Series, the core thickness is 8mm, which aids
the suppression of sound transmission
considerably.

The audible result of the new cone material,
with its enhanced stiffness and reduced sound
transmission is to improve what is referred to
as bass attack or dynamic bass. Most bass
lines in music do not consist of steady tones.
The waveforms have an extended frequency
range and the reduction in coloration in the
upper bass/lower midrange cleans up the
presentation significantly.

A detailed discussion of Rohacell®/carbon fibre
sandwich cones is to be found in Appendix III.

Midrange

to isolate it from mechanical energy arising
elsewhere in the system. To this end, the
tweeter and tube are held in the housing
with rings moulded with a Shore 1A hardness
elastomer. The housing in turn is decoupled
from the midrange enclosure below by the
use of two isolator pads of high compliance
gel material.

The top isolator has been shaped to sit in the
scallop of the midrange head enclosure and
cradle the underside of the tweeter housing.
Raised ribs have been designed into this
isolator to create maximum compliance at
this interface, in order to absorb any energy
transmission between the midrange head
enclosure and tweeter body. The bottom
isolator sits between the connector and the
underside of the midrange head enclosure
to ensure that both the sections of the Molex
cable connector are isolated from the midrange
head enclosure. The tweeter is allowed to float
free and reproduce the input signal without any
external interference.

The 800D uses two 250mm (10-in) diameter
bass drive units. At B&W, we have long
promoted the use of stiff, rigid cones for bass
drivers. Bass/midrange drivers are a different
matter, because of the same bandwidth
conditions that apply to the FST midrange
driver, but for bass-only drivers in 3-way
systems, the ability to withstand deformation
when subjected to the high pressure differences
inside and outside the cabinet is the best way
of achieving that dynamic performance often
described as ‘slam’. The stiffness also pushes
the onset of break-up to higher frequencies,
extending this piston-like behaviour.

At B&W, we have commonly used two materials
for this application – aluminium and a fibre pulp
mix of kraft paper and Kevlar, further stiffened
by resins. Both materials are stiff, but metals
in particular suffer high Q resonances outside
their working range, due to their low inherent
damping. They must be well attenuated by the
time the break-up region is reached to avoid
intrusive coloration. In the Nautilus 800 Series,
the paper/Kevlar®mix was chosen over
aluminium for two reasons:

• It was difficult in practice to form aluminium
cones of large diameter that fulfilled the bass
alignment criteria. Either they split during
forming or the thickness had to be increased
such that they became too heavy.

• Paper/Kevlar®has higher internal
damping and break-up resonances were
better controlled.

However, even paper/Kevlar®is fairly dense
and results in relatively thin section cones if a
reasonable sensitivity is to be achieved. This
can allow a certain amount of sound energy
from inside the cabinet to pass through and
cause low levels of coloration. As general
driver, cabinet and crossover quality has
improved in recent times, even this very low
level of coloration deserves corrective attention
and the sandwich construction of our PV1
subwoofer driver has shown that a thick cone
construction can have benefits in this area.

Simply adding thickness, however, is not a
universal panacea. In a passive speaker, we
cannot afford to add mass at the same time.
The choice of alignments becomes too

6 Forming curved cabinet sides at B&W Denmark

10

make it turbulent, which may be heard as wind
noise, particularly because it can excite the
organ-pipe resonances of the tube.

Far more serious problems occur when laminar
airflow tries to leave the tube at high velocities.
If the curvature of the diffuser (flare) is too
sharp, the minimal momentum of the air at the
base of the laminar boundary layer is insufficient
to pass the resulting sharp, adverse pressure
gradient without stopping or stagnation. Slightly
downstream, the pressure gradient (higher
velocity with lower pressure to lower velocity
with higher pressure) causes the flow at the
base of the boundary to reverse and a turbulent
eddy is created in the form of a rotating torus
(this is how smoke rings can be blown). The
boundary layer now becomes the region that is
between the eddy and the main flow, but it has
now separated from the surface of the diffuser.
It tries to follow the pressure gradient formed by
the turbulence, but may form more eddies trying
to do so, and so on.

The turbulent wake thus created is responsible
for the ‘chuffing’ noises that even gently flared
ports can produce under some conditions.
The separation can sometimes be so extreme
that a turbulent jet can hit a listener at some
distance from a speaker. The aerodynamics
of reflex ports is actually rather complex and
somewhat unusual in that it involves alternating
flow in two different pressure regimes (at and
below port resonance), three octaves of the
frequency spectrum (different systems have
different tunings), completely indeterminate
starting conditions and well over 100dB of
level difference.

Aerodynamics research into reflex ports at B&W
is still in its infancy. Classical wind tunnel work
is very difficult because the alternating flow
makes a mockery of smoke trails. Recent work
with Computational Fluid Dynamics has shown
that ports are very difficult to model accurately.
This is partly because of the large number of
variables, and also because the flow regime is
influenced so heavily by small-scale turbulence
creation, which is less well understood than
large-scale fully-developed turbulence (more
is known about how aircraft stay in the air than
how midge flies do). Therefore, work has been
largely empirical, using comparative rather than

11

The movement of air in and out of tuning ports,
which may represent quite a considerable
physical displacement, often causes ‘chuffing’
noises as the air interacts with the discontinuities
found at the internal and external ends of the
port tube. These noises occur as turbulence is
formed at the discontinuities. Even when the
inside and outside ends of the tube are given
smoothly rounded profiles, the problem is not
totally cured, though it is mollified.

The reflex port is a well-established device to
improve the bass response of a transducer in
an otherwise sealed box of finite dimensions.
As the power handling, excursion and linearity
of bass drivers have steadily improved over the
years, the limitations of a simple tuned port
have become apparent. At low levels the
behaviour of the air in the tube can be correctly
approximated to a solid piston bouncing on
a known air volume and at a specific tuning
frequency; a readily predictable and essentially
acoustic problem. At higher levels, aerodynamic
effects become increasingly important and the
associated loss means that a given rise in bass
driver input level will yield a smaller rise in clean
port output level. This also means that the port
is not reducing the excursion of the bass driver
as effectively and the system will thus behave
increasingly like a lossy sealed box design; the
combined effect is known as ‘port compression’
and can often create an ultimate ceiling to
achievable bass levels.

Well before any ceiling is reached, the energy
losses associated with port compression cause
problems and it is the way energy is lost rather
than the amount lost that causes serious
acoustic problems. At very low velocities, and
with a perfect entry, air travelling through a real
port tube will pass smoothly along streamlines,
which do not interfere with one another. Close
to the walls of the tube is a thin boundary layer
caused by skin friction, with a relatively high
velocity gradient. It provides the transition
between the stationary walls and the moving air.
Laminae of air rub against each other causing
pressure drag through noiseless viscous losses.
These are minimal at low levels but increase at
a geometric rate in proportion to velocity. At
high enough velocities, if the tube is excessively
long and rough (or just very rough), the high
shearing energies in the boundary layer can

Flowport

7 Representation of streamlines exiting port flare.

a Laminar airflow following curvature of flare
b Higher velocity turbulent airflow separates from surface

of flare causing large scale eddy formation

c Small scale turbulence due to dimples encourages

laminar streamlines to remain attached to boundary

a

b

c

absolute benchmarks, because it is difficult to
make reliable measurements of turbulent noise.

Theoretical predictions of air velocities down
the port were checked with a new Doppler
measurement system, to establish the kind of
flow regime operating around chuffing levels in
terms of the Reynolds number (a dimensionless
indicator of turbulence levels). This showed that,
with care, it was possible to maintain laminar

flow down the port tube, but that air could
detach from the flares at fairly modest levels.
Simply making the flares more gentle would
not guarantee silence.

Anyone studying aerodynamics will soon learn
that turbulence is not always a problem. In fact,
many aerodynamicists engineer turbulence to
their advantage (indeed, some aircraft would
not stay in the air without it). If a boundary layer
is turbulent prior to the stagnation point it will
be less inclined to separate because the base
layer has increased kinetic energy. This means
that the surface flow can be swept further
downstream before pressure conditions
stagnate it and the lower pressure in the layer
that results from the higher velocities within the
eddies adheres the main flow to the surface
profile better. Thus, small-scale turbulence can
be used to delay the large-scale turbulence
caused by separation.

Artificially creating turbulence in the air moving
down the tube can delay the onset of chuffing
to higher bass unit input levels, but problem
wind noise happens far earlier, especially when
turbulent air is sucked back in to the port as the
flow alternates. In addition, the thickened bound­ary layer effectively constricts the flow, causing
pressure drag and thus airflow compression.
This constriction also alters the effective area
of the port, which in turn affects the Helmholtz
tuning. Thus it is otherwise desirable to delay
the onset of turbulent flow down the tube to as
high a level as possible. A more optimal solution
would thus be to use a smooth tube and limit
artificial turbulence creation to the problematic
stagnation area. (figure 7)

It is quite easy to produce turbulence where
it is needed; aircraft use vortex generators,
(vertical strakes) ahead of separation points.
These strakes project into the main flow and
are very effective, but when the same technique
is applied to port flares it creates too much
wind noise at lower levels.

Enter the golf ball. It can travel twice as far
as an equivalent smooth ball because of its
distinctive dimpled surface. The dimples are
very carefully shaped to produce tiny separation
points and favourable conditions for the creation
of vortices within them. The ball is thus covered

by a thin turbulent boundary layer that moves
the separation point further round the ball. This
decreases the ball’s wake and hence its drag,
and it was this technology that was used to
improve the performance of the port flares.
Because a round port flare is axisymmetric,
it was first thought that a series of rings with
the cross section of a dimple might work (and
be easier to prototype). However, the regular
vortices formed simply became the new separa­tion points and at lower levels there was audible
wind noise because they were so abrupt. So
real, pseudo random dimples were tried on
the surface of the flare. These immediately
improved the chuffing phenomenon as
predicted, but there was still wind noise caused
by deep dimples at the edge of the tube where
flow velocities were highest. These were filled
but at the expense of earlier separation levels.

A process of experimentation refined the size,
shape and distribution of the dimples to
maximise headroom and minimise wind noise.
Small, smooth dimples are thus used where
velocities are highest and larger, more abrupt
dimples are used where velocities are lower.
This greatly refines the exit flow regime and also
ensures that a minimum of turbulence is carried
back down the tube when the flow is reversed.
It was found unnecessary to make the dimples
totally random over the whole flare, but as long
as they are locally irregular, perceptible wind
noise is incoherent and unobtrusive.

In the case of the 800D, the port is down firing,
so more wind noise is acceptable and the
dimples are optimised for maximum high level
flow. In use, the dimpled ports delay the
nuisance chuffing noise to significantly higher
levels. However, and perhaps of even greater
importance, when large-scale separation does
occur the resulting turbulence is far more
incoherent and thus less apparent. A reduction
of 6dB in certain regions of the noise spectrum
was measured, particularly around the problem
organ pipe frequencies. Port compression is
also decreased and the tuning frequency is
more stable at higher levels.

Having achieved excellent cabinets for each of
the drive units independently, it is important that
vibrations and radiation from each driver do not
leak into the enclosures of others. Decoupling
has been used extensively in the 800D to isolate
drive units, apart from bass units, from their
enclosures and the individual enclosures from
one another. A discussion of the technique can
be found in Appendix VIII.

Decoupling was not used for bass or bass/
midrange units in the Series. While it has the
potential for reducing vibration in cabinet walls,
listening tests have always confirmed that
this is more than offset by a reduction in the
speaker’s ability to portray ‘slam’. A similar
effect is noticed if the bass cabinet is not firmly
anchored to the floor, for example using spikes.
It should be noted that it is only bass drive units
that are required to operate in the stiffness
region, below their fundamental resonance
frequency. All others operate entirely in the
mass controlled region.

Decoupling

8 Gel gasket used for vibration isolation between tweeter and

midrange enclosures