B&W electronics 70 User Instruction Handbook - page 10
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capable of meeting the initial design requirements set out
in paragraph three above with an enclosure size of some 2 - 3
cubic feet.
In so far as the mid and upper frequencies were concerned
the choice of units was threefold - Moving coil, Electrostatic
or Ionic. It will be noted that the Ionic unit is currently
used in our P2 monitor.
At an early stage in development new alternative Ionic
units were manufactured in prototype form with a view to
increasing power handling capacity and extending the frequency
response at least two octaves below that provided by the
existing Fane unit. Whilst certain progress was made, both
the physical dimensions of the horn loading and the R.F.
oscillator power requirements showed that, although improvement
in the Ionic unit was possible, the disadvantages were those of
physical size and adequate R.F. screening. For instance, one
prototype had a horn length of some two feet and an R.F.
oscillator of 70 watts, a totally unmanageable quality both in
terms of size and adequate screening to provide freedom from
T.V.I. For these reasons the Ionic method was dismissed and
attention focused on Electrostatic and Moving coil units.
Although considerable advance has been made in the design
and development of Moving coil units they do not, in the writer's
opinion, approach the Electrostatic transducer in so far as low
distortion, transient behaviour or flexibility of broad polar
distribution.
Evaluations carried out by Mr. Harwood on a somewhat similar
'state of the art' project and, for those interested, reference
may be made to three articles published in "Wireless World" in
1968 where it was shown that even offering an idealised Moving
coil unit to a slot radiator, the polar characteristic was not
well maintained above 3 kHz. It was decided, therefore, to
explore the possibility of making an electrostatic unit, working
from first principles with a view to comparative measurements
between the Electrostatic and Moving coil units which could be
developed.
Some six months after the design programme was originated
a series of prototype units were produced which enabled a prime