B&W electronics 70 User Instruction Handbook - page 20
-20-
Expanded polystyrene seemed the obvious choice for cone
material as medium and high freguencies were not involved and
indeed a number of early prototypes were made with this material.
Extended listening tests proved, however, that the polystyrene
cone material showed 'read out' colouration within the 200 - 400
Hz region. Experiments were then made with an exactly similar
unit minus the polystyrene slab and girder assembly and it was
determined by relating frequency response curves to the two
units that energy generated primarily at the apex of the cone
was largely being lost, presumably due to kinetic conversion
within the polystyrene, and in turn the polystyrene was
partially breaking up and providing this small but objectionable
colouration.
The final cone assembly does, in fact, consist of a
laminated structure in order to provide the required stiffness
and is further braced and mass adjusted by small rectangular
sections of synthetic rubber affixed at critical positions on
the front face of the cone.
One of the relatively untouched aspects of bass radiator
design has been the effective 'Q' at resonance. A simple
bridge method was evolved for this measurement and considerable
work carried out in terms of suspension compliance and diaphragm
mass to reduce this figure well below unity. Until ambient
room measurements were taken it was not fully realised how 'room
gain' could play an important part in magnifying a relatively
small rise in the response as shown on an anechoic plot to an
objectionable one note thump.
LOUDSPEAKER MEASUREMENTS
The writer is a firm believer in measurement at all stages
of design and production. Although designers in the past have
suggested that the measurement of loudspeaker response is of
secondary consideration, we feel that this view is taken because
the art of interpretating loudspeaker measurements is a subject
on which there is considerable scope for development. For
instance many manufacturers and designers proudly publish a
beautiful axial plot of their system and whilst a straight line
response is a good starting point this measurement is almost