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