B&W electronics 70 User Instruction Handbook - page 15
-15-
With the 701 and an experimental bass radiator it was
immediately obvious that the stability of the stereo image was
improved beyond any other system we had tested. There was an
excellent absence of 'source* in so far as the loudspeakers
were concerned. If reference is made to the Hass effect it
will be noted that one of the prime criteria laid down for
stability of stereo image is that an in-phase equal amplitude
signal be received from a wide frontal arc in a horizontal
plane with particular reference to the mid frequency region.
Considerable work was carried out on various degrees of
curvature before design was finalised, but as much of this
work concerns production techniques it will not be discussed
in detail.
Hunt ("Electroacoustics" Harvard University Press) has
been the starting point for most workers on the Electrostatic
transducers and this is an opportune point in the development
story to mention the acoustical load. At the end of this
detailed treatment Hunt proved mathmatically that the ambient
air provided 85% of the load on the diaphragm and this fact
was confirmed empherically throughout the development. We
instance the original XP prototype which showed a drop in
sensitivity of 3 db. when mounted on a small Jig testing baffle.
On the current 701 assembly an appreciable change in white
noise can be detected as soon as it is lowered to top of the
cabinet. This effect is somewhat frequency discriminating
due, presumably, to another factor contained in the original
work by Hunt. The fact that at frequencies above say 10 kHz
diaphragm mass tends to become a dominant factor.
To prove the latter point one only has to add
supplementary diaphragms. In the case of the 701 E.L.S. one
supplementary cover reduced the output above 9 kHz appreciably.
A colleague who was working with the writer at this stage
of the development remarked, "It would seem that the ideal
mounting for an electrostatic unit is suspension in space."
This would appear to be a fair submission of fact but one which
in practice has to be somewhat modified. It will be noted,
however, that the mounting for the 701 assembly is free-standing
with as little surrounding enclosure as possible.