Bowers & Wilkins PV1 User manual

Development of the PV1

The Problem
The holy grail of subwoofer design is deep and clean bass
from a near-invisible box. The immediate problem that
arises is that reduced box size both curtails bass
extension and increases pressure on the enclosure wall
and driver diaphragm.
Many small subwoofers are unable to reproduce
sufficiently extended bass and suffer from considerable
cabinet coloration, often resulting in bass that is uneven
and dissociated from the midrange and treble.
Some existing designs have tried to address this problem
by using complex, rugged drive units and very large
amplifiers in small boxes, but have neglected to give
sufficient consideration to the design of the enclosure.
Typically, the huge backpressures and mechanical
reaction forces generated by the heavy driver tend to
deform the cabinet walls, which then resonate and distort
the sound, particularly in relation to pitch definition.
Bracing the cabinet extensively certainly helps but to
satisfactorily solve the problem, you have to change the
design approach.
Pressure Vessel Concept
It is often the case that effective solutions to problems in
one scientific discipline may be derived from work done in
another. An example of that within B&W is the derivation
of our dimpled Flowport design from the aerodynamics of
a golf ball.
The Pressure Vessel takes its inspiration from deep-sea
diving bells. Their characteristic curved form resists the
pressure differences on each side of the walls to a much
greater degree than flatter panels, however well braced
they may be.
The trick is to change the way forces are distributed in the
structure, from bending (i.e. at right angles to the panel) to
being in the plane of the panel. After all, it's much harder
to squash or stretch a panel along its length or width than
to bend it.

To illustrate this, there are two analogies that we can
apply. The first is the soap bubble. An undisturbed bubble
assumes a spherical shape because the pressure
difference inside and out is perfectly balanced by the only
force the bubble can sustain - surface tension that is
entirely in the curved plane of the skin.
The second important analogy is found in the architecture
of Antonio Gaudi. For our subwoofer, not only do we have
to consider pressure changes in the enclosed volume of
air, we must also deal with mechanical reaction forces
generated in the magnet system of the driver and
transmitted to the enclosure walls via the driver's chassis.
The delicate and almost skeletal structure of the temple of
the Sagrada Familia in Barcelona was derived from a
novel simulation of the structure using string and weights.
Tension in the string of the inverted structure is analogous
to compression in the arches and pillars. The string
naturally assumes a shape where the only force is tension
along its length. It cannot support any other. There are
therefore no bending forces. The resulting arches and
pillars built following these natural forms suffer only
compression along their length, so they are very strong yet
slender compared to traditional designs.
B&W Pressure Vessel prototype
The principle of restricting forces to compression or
tension in the plane of a structure was applied to a
subwoofer enclosure in order to minimise sound radiation
caused by flexing of its walls.
Early prototypes used a single drive unit mounted in a
large spherical enclosure. This worked very well at ultra
low frequencies, as the conditions approached those of
the static analogies discussed above. However, as the
frequency increased, it was found that internal pressure
changes due to movement of the driver diaphragm did not
transmit instantaneously throughout the whole of the
internal volume. Neither were the mechanical reaction
forces from the driver chassis transmitted instantaneously
throughout the structure of the enclosure.
Gaudi's inverse force model.
Support pillars in the temple of the Sagrada Familia,
Barcelona

These effects are related to the ratio of the sound
wavelength to the dimensions of the enclosure. The
resulting pressure and force gradients allowed the
reintroduction of bending forces on the walls of the
sphere, which raised the level of radiated sound and
coloration. Going back to our soap bubble analogy it
could be likened to the way a bubble flexes as it moves
through the air.
The obvious course to ameliorate this effect was to add a
second driver to power the enclosure from the opposite
end. Because of symmetry, this effectively halved the
volume loading each driver and doubled the frequency at
which these two effects began to cause a problem. The
second prototype, shown here, used two 380mm (15 in)
diameter drivers, one mounted at each end of a steel
marine floatation device. This prototype was capable of
producing prodigious levels of clean, articulate bass, but
was hardly a viable commercial proposition. Furthermore,
the resonances were still only just outside the effective
bandwidth of the device, due to its large dimensions.
However, the stage had been reached where a more
compact and visually appealing product could be
developed.
PV1 - Concept
The PV1, launched in June 2004, was developed directly
from this prototype. From a sphere that comfortably
passes through a basketball hoop, it creates truly
extended bass that defies its diminutive and gorgeous
exterior in both quality and quantity.
Although the Pressure Vessel concept was proven in very
large enclosure, it was obvious that its prime technical
merit of reducing cabinet stress was always going to find
a particular relevance with a smaller product. Indeed, the
PV1 represents a joyous meeting of minds for both
engineer and industrial designer.
The problems inherent to small subwoofers are often
compounded further by the industrial designer’s desire to
shave off centimetres here and there. The beauty of the
PV1 is precisely that its non-bending cabinet design
permits the use of a relatively thin aluminium shell,
maximising the internal volume and therefore minimising
the amount of equalisation required from the amplifier.
Early pressure vessel prototype

A secondary consequence of the use of twin opposing
drivers is the inherently balanced kinetic nature of the
device. It is well known that the reproduction of bass
with correct dynamics requires a firm, solid foundation –
sometimes referred to as a mechanical ground. It is the
reason that speakers stand on spikes. A practical
downside is that some vibrational energy is transmitted
to the floor, with the potential of radiating sound into a
room below the listening room. This can be a particular
problem in apartments. With the PV1, there is no rocking
motion generated and the device is inherently stable.
The subwoofer therefore rests on a circular rubber base
that mechanically isolates it from the floor, yet it retains
an effective foundation.
A simple, yet effective demonstration of the inertness of
the enclosure is to touch it while the subwoofer is playing
loudly. There is virtually no discernable movement.
PV1 – Technical information
The listening experience reveals a subwoofer with
incredible transient response and pitch definition.
However, the enclosure design is not the only reason for
this. The driver diaphragms themselves also have to be
stiff enough to resist any tendency to deform when
subjected to the pressures generated inside the
enclosure and the forces from the voice coil. A very rigid
sandwich construction is therefore employed, with a
concave aluminium outer layer, a mica cone at the back
and an expanded polystyrene core.
Any compact subwoofer will require substantial
equalisation in order to have an extended low-frequency
response and this implies a high-power amplifier. The
PV1 utilises a 500W ICEpower® module, which
allows considerable output levels to be generated
without audible distortion. The aluminium enclosure acts
as the heat sink.
PV1 section
Naturally, the driver must be capable of handling this
power. The large magnet structure aids control of the
motion of the diaphragm and forms an effective heat
sink for the voice coil, which, at 30mm long, allows
sufficient linear movement. A double rear suspension
ensures that the coil remains in line throughout the large
excursion.

The vibration levels of the PV1 (red) are
compared with those of a conventional
subwoofer (blue) of similar bandwidth,
output capability and drive unit radiating
area, but using a single 250mm driver in a
heavily braced wooden cabinet.
Measurements were made by scanning
the enclosure surfaces with a laser beam
and summing over the total area in each
case.
EQ = i
EQ = ii
EQ = iii
low-pass filter 80Hz
Bandpass output level 90dB at 1m
PV1 - frequency response
PV1 - total harmonic distortion
Sound output due to cabinet vibration
100
10
1
0.1
0.01
-20
-30
-40
-50
-60
-70
-80
-90
dB
dB
%

Technical features
Description
Drive unit
Frequency range
Frequency response
Amplifier
Low-pass filter
Dimensions
Net weight
Cabinet finish
Pressure vessel enclosure
500W ICEpower® amplifier
Switch mode power supply
Active closed-box subwoofer system
2x 200mm (8 in) dia mica / aluminium diaphragm long-throw
-6dB at 18Hz and 40/140Hz adjustable (EQ at i)
±3dB 21Hz – 31/110Hz adjustable (EQ at i)
Power output: 500W
Rated power
consumption: 110W (105W - 100V version)
Input impedance: 100kohms
Signal / noise: >90dB
Functions: Volume level
Low-pass filter frequency
Bass roll-off alignment
Auto sense on/standby
Phase switch
Inputs: Line In (RCA Phono)
Speaker Level In (RJ 4/4)
Outputs: Link Out (RCA Phono)
Active 2nd -order, variable cut-off frequency
(applies only to speaker level input)
Height: 335.5mm (13.2 in)
Width: 289mm (11.4 in)
Depth: 347mm (13.7 in)
20.5kg (45 lb)
Silver with grey trim
Black with anthracite trim

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B&W Group Ltd
reserves the right to amend
details of the specifications
without notice in line
with technical developments.
Copyright © B&W Group Ltd.
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trademark of Bang & Olufsen
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