Enclosure; Drive Units; Crossover - Bowers & Wilkins Nautilus Owner's Manual

Enclosure

Exponential line loading is used for all four
drive units, primarily to achieve freedom from
resonance and reflection, but also for the
damping effect on the fundamental resonance
in the low frequencies.
The two dome units are tightly coupled
to their respective wave guides via a
hollow pole magnet which causes minimal
discontinuity to the advancing wave front.
A hollow pole magnet is also to be
found behind the lower mid diaphragm
supported by a chassis which itself forms
the first 50mm of the wave guide. The tube
to which this is attached is shaped to
smooth the transition from the chassis.
The distinctive natural appearance
of Nautilus is derived from the best
compromise of folding a tapered tube to
save space whilst maintaining the highest
curvature-to-width ratio. Traditionally,
folded pipes have involved many 180°
bends which have serious frequency
dependent transmission properties.
10mm thick Glass Reinforced Plastic (GRP)
has been used for the exterior surface which,
when coupled with the doubly curved shape,
results in an extremely stiff enclosure.
The inner turns of the spiral are bounded by
a foam filled GRP which braces the opposite
faces of the snail in a manner similar to the
Matrix type enclosure perfected by B&W.
The external form of the Nautilus has
been achieved using a blend of hand-built
forming and advanced CAD technology
borrowed from the automotive industry.
This enables B&W to maintain
mathematical accuracy of the wave guide
and to produce the final mould tool to sub-
millimetric accuracy.
The high gloss surface finish is the
result of using a two-part acrylic paint
with a deep lacquer coat for lustre and
durability. A 50kg block of polished
terrazzo type material supports the
complete mollusc.

Drive units

Each drive unit has been developed to operate
as a piston within its intended frequency band,
with two octaves between the upper roll-off
and the first sign of break-up modes. Extensive
use of aluminium in the diaphragms makes this
possible. All voice coils are wound on polyimide
formers to eliminate eddy-current losses which
are particularly serious at high frequencies.
A 9.5kg (21lbs) magnet with a 100mm
(4in) voice coil acts as the massive motor of
the 300mm (12in) bass unit. This, when used
in the exponential line enclosure, results in a
high-pass behaviour so over-damped that
the traditional second-order characteristic is
replaced by two distinct first-order slopes
and no stored energy.

Crossover

The division of the signal into the four
required bandwidths is accomplished in the
Nautilus Active Crossover via totally non-
resonant circuitry. Simple 1st Order networks
correct the driver responses. Both inputs and
outputs offer single and balanced operation,
the latter being particularly useful in noisy
electrical environments, although a slight
A 250 micron one-piece aluminium
cone/centre dome ensures coherent motion
to beyond 1.5kHz. From 220Hz to 880Hz, a
100mm flat-fronted unit is employed to prevent
the gentle cavity resonance found in conven-
tional cone units at around 2kHz interfering
with the output from the upper midrange unit.
A rare earth magnet assembly with hollow
pole is used to minimise the obstruction to the
rear radiation from the diaphragm.
Two domed units of anodised aluminium
of similar construction, 50mm and 25mm
in diameter, handle the 880Hz-3.5kHz
and 3.5kHz-25kHz ranges respectively.
All drive units are completely mounted on
silicone rubber O-rings to decouple them
from the cabinet.
subjective improvement has been observed
when using balanced signal interconnections
at every stage.
With suitable pre- and power
amplifiers, one unit is required for each
loudspeaker and should be sited close to
the power amplifiers. A power supply
connection for each crossover is required.
IMPORTANT: YOUR DEALER
MUST CHECK THAT THE SERIAL
NUMBER LOCATED AT THE REAR
OF EACH CROSSOVER UNIT
MATCHES THAT OF THE LOUD-
SPEAKER – AND THAT THE
STATED VOLTAGE IS CORRECT
FOR YOUR MAINS SUPPLY.