Note: Descriptions are shown in the official language in which they were submitted.
CA 022411~6 l99X-06-22
W O 97/24013 PCT/SE96/0~675
E~ECTROSTATIC LOUDSPEAKER
The present invention concerns an electrostatic loudspeaker of
the kind described below. The loudspeaker is primarily
int~n~e~ to reproduce music at the highest quality level.
An electrostatiC loudspeaker of prior art comprises in
principal a thin conductive ...e...bLane which is positioned
between two electrodes having the shape of perforated plates
preferably entirely made from electrically conductive
material. A positively charged membrane is made to move by
means of a voltage difference between the electrodes which by
signal alternate between positive and negative charge. Within
the limits of operation the deviation is mainly proportional
to the difference of voltage between the electrodes. Another
kind of electrostatic loudspeakers is described in US-patent
no 3,980,838. The loudspeaker described therein has two
membranes positioned on either side of a stator and kept at
distance from this by means of distance rings. In order to
obtain a sound-reproduction a stepped up signal volta~e is
supplied to the membranes. In this and other known
constructions of electro-static loudspeakers the membrane and
stator are charged with different kinds of charges and the
loudspeakers operate with both attracting and repel 1;ng
forces.
With prior art electrostatic loudspeakers it is difficult to
obtain a sufficiently high soundpressure with kept up quality,
i. e. the available force is to small. The outer ~ n~ions of
the loudspeaker which shall include a wide span of low
frequencies b~_ -~ unwieldy large and electrostatic loud-
speakers of prior art can also be a severe load to the
amplifier of the sound reproduction equipment. Further as
already mentioned this prior art designs make use of both
repelling and attracting ~ields, i. e. charges of different
polarity, whereby the breakdown voltage in air will be
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WO 97/24013 PCT/SE96/01675
comparatively low. The force which it is possible to obtain in
the elements will thus be comparatively low in the order of
some ten N/m2.
i
The present invention con~-~rns electrostatic loudspeakers in
which membrane and stator have charges of the same polarity.
It is hereby made possible that yreat forces can work in the
loudspeaker without any risk for sparking between stator and
membrane. This means that a very high loudpressure can be
obtained while maint~in;~g the quality and with loudspeakers
which have much smaller outer ~ n~ions than those of prior
art designs. The invention is also for a transformer with a
signal return function for operation of loudspeakers of this
kind.
The invention will below be described more in detail with
reference to the embodiment which is shown in the enclosed
figure. This shows a cross-section of a loudspeaker and a
transformer unit which is co~Gted to the loudspeaker.
The loudspeaker shown in the figure has two membranes 19, 20,
each of which has a corresponding stator 2, 9 shaped as a grid
or the like and made from electrically conductive material.
The two membranes are parallel to each other and joined
together by means of a firm, hard or stiff material 6, which
has very low weight per volume. This material has essentially
the same cross-section ~; on~ion as the membranes, and these
are attached to either side of the material. Preferably the
membranes are joined to the material over the entire surface
or pointwise by points which are evenly distributed over the
surface. Suitable materials can be foamed polyurethane or
other polymere material haviny 95-99% air content and a weight
per volume which is below 40 kg/m3.
Membrane and stator are mounted in a frame 3, so that pairs of
stator and membrane have electrically conductive contact with
each other while the pairs of membrane-stator are electrically
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W O 97n4013 PCT/SE96/01675
insulated from each other in the carrying and uniting design.
The electrically con~ll~tive mounting in the frame 3 by
fastener 1 is ~1exible and airtight so that the loudspeaker
can be built into e. g. a box with a solid back side without
causing any problems with acoustic short-circuit and blowing
sounds caused by soundwaves which pass through the slit 25.
The outgoing signal from the amplifier is by wires from a
transformer supplied to the loudspeaker at connections 11, 12.
The pairs of membrane 19, 20 and sta~or 2, 9 always have the
same potential and repel each other with a varying force when
the potential varies. At an i~ in~ signal all voltages on
the right respectively the left side are equally changed. When
the voltage increases on the left side it is reduced on the
right side and the membranes are displaced to the right.
Thereby that adjacent membrane and stator have the same
potential and that membranes of different potential are at a
constant and comparatively large distance from each other,
h~gh potentials up to about 140 kV may be u~ A in the
membrane-stator-pair.
In a preferred embo~; -nt of the invention there is between
membrane and corresponding stator an inlay of thin, light nets
or the like 7, 8, which preferably are made from textile
material which is electrically conductive or has been made
conductive by a treatment. In the figure only two such nets
are shown on the left side of the loudspeaker. However the two
sides ought to be made in the same way and the n-~h~r of nets
may vary d~p~n~;ng upon the actual application. If for example
four nets are used on each side, the space between membrane
and stator is split up into five equal parts. The distance
between equal charges is reduced to 1/5 and the force between
membrane and stator is thus increased 25-fold.
In the figure there is also shown a transformer unit for the
loudspeaker whereby the direction of transmitted signals have
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W O 97/240t3 PCT/SE96/0167
been indicated with arrows. The signal ~o~;n~ in by the lead
16 from an amplifier in the equipment is supplied to a
transformer wi n~; n~ which on its correspon~i ng se~on~y side
has a centre connection with a voltaye of a constant
potential, so called bias 17. The ; n~_ ~ n~ signal is supplied
to the loudspeaker-connectors 11, 12 through resistors 13, 14.
By means of further transformer win~;ngs 21, 22 which are
connected in parallel with the resistors 13, 14 one obtains a
correcting signal which is the outvoltage from the secondary
w~n~lng 23, which by the lead 1~ as a r~o~n~cting signal is
returned to the amplifier preferably to a compensating
controlling unit before the final step of the amplifier. This
signal sends information about the movements of the membranes
to the amplifier, the outsignal of which is corrected. In the
figure there is shown two different possibilities 18, 24 for
~nn~ting of the secondary wi n~ i ng 23 to earth, whereby the
alternative with earth connection at the middle of the
~con~A~y win~ing is used to obtain a balance return
connection.
