An Unconventional Loudspeaker system Latest change 2019-01-22
Electro Static Loudspeaker
For those who are not familiar with ESL's a few words about the
A spreadsheet is presented
for the most important calulations.
Some measurements were done to find properties of the
transformer and the modules.
loudspeakers generally consist of two acoustically open and
electrically conductive plates (e.g. perforated metal) called
the Stators, and in between an electrically conductive foil
called the Membrane. The stators are symmetrically driven by a
so called Step-Up transformer which converts the output voltage
from the amplifier to hundreds or a few thousand Volts.
The membrane is held at a high DC voltage, 2 .. 5 kV is not
unusual, symbolized by the battery in the drawing left.
The transformed signal voltages drive the membrane by
A requirement is that the electrical charge on the membrane
stays constant. If it changes with the displacement of the
membrane there will be intermodulation distorsion. To realise
this constant charge one can do one or both of two things,
connect the membrane to the high voltage source trhrough a high
resistor, many MegOhms, and/or use a membrane with a very high
area resistance. The high voltage source is permanently switched
on in most cases. For many ESL's it takes minutes to build-up
The distance between the membrane and the stator is around 1 mm,
the area of stator and membrane can be from a few to many tens
of dm2. The smaller units are used for the highest and midrange
audio frequencies, for a full-range ESL a large area is
Because the membrane is chosen very thin (usually 2..4 microns)
it has very little mass and so can easily move at high
frequencies. This is the primary reason why ESL's provide such
clear and resonance-free mid- and high frequencies, compared
with e.g. squakers and dome tweeters where substantially more
mass has to move.
Of course there are disadvantages too:
Because the area of the membrane is much larger than e.g. the
membrane of a dome tweeter, the radiation is very much in one
direction. Yes, there are ways to overcome this, but
anyhow....... In my design with the cone this problem with
directivity is absent.
ESL's are basically dipole radiators, equal amounts of sound
emit from the front side as from the back side. For the lower
frequencies this requires a large baffle, because otherwise the
radiation from front and back will cancel each other.
Reproduction of very low frequencies is nearly impossible.
On the other hand, homebuilding ESL's is very well feasable, the
materials are not very expensive and there is a large community
of DIY ESL builders. Before you start your own project, read the
literature and the experiences of others.
Picture from Fikier: "Elektrostatische Luidsprekers" (NL),
published by (at that time) Elektuur, 1993.
system I used 4 modules (2 for each unit) from a set of
old-model Solosound loudspeakers. https://www.solostatic.com/
Solosound v 4 (picture taken from
original Solosound system consists of these units and separate
bass boxes. The ESL's operate from around 300 Hz up.
in the eighties I bought a used set of these ESL's
without the bass units and combined them with transmissionline
bass/mid-units with KEF B139 and KEF B110. (SOTA, State Of The
These ESL's each have 4 modules like below, mounted on a
hard-foam body of 28 x 76 cm.
Solosound Module. Outer size 100 x 150 mm.
Each module is a sandwich of stator-PCB, spacer, membrane,
spacer, other stator-PCB.
The stator PCB's have the metal layer on the outside, covered
with a layer of some laquer for insulation. Remember we may have
over 8000 Volts between the metal and the membrane. Normally
1000 .. 2000 Volts will produce sparks in 1 mm of air.
The membrane is made of a very thin plastic foil, which has been
made electrically conductive with a material having a high
The membrane is connected to the high-voltage source, the
Stator-PCB's are symmetrically driven by the step-up
transformer. The solder joints at the top of the picture connect
to the membrane.
Fill factor, or how "open" are
The holes are 4 mm, so they have a surface area of 12.6 mm2.
We have 170 holes, so 2140 mm2.
The metallized area is 119 x 72 mm, or 8570 mm2. Fill
factor = 2140 / 8570 = 0.25. This makes me consider a better design in future.
To the left the
schematic, drawn here with 7 diodes to demonstrate the principle
of a cascade voltage multiplier.
The actual Solosound unit has 16 diodes and 16 capacitors and so
will produce around 5000 Volts.
The capacitors (47 nF) and the diodes must be capable of
withstanding at least 1000 Volts each.
The circuit is directly connected to the mains voltage, which
makes it cheap but not so safe, although it is connected to the
ESL by high value resistors.
The power consumption however is almost nothing, after the
charge has been built up.
on the ESL module.
With a caliper I measured the distance between membrane and
stators to be 1 mm, the board thickness was 0.8 mm.
The electrical capacitance was found to be 35 pF for 1 module
and 81 pF for both, including wiring capacitance. Measured with
on the Step-up transformer.
The DC resistance of the primary is 70 milliOhm, the secondaries
have 54 and 60 Ohm for each halve.
The step-up ratio was found to be 1:87:87 or 1:175 from primary
With the now known capacity of the ESL's we can calculate the
capacity to be seen on the primary side of the transformer. 81 *
175 * 175 = 2.48 uF.
Measuring with the LCR-meter on the primary side gave 4.5 uF.
The difference must be the parasitic capacitance inside the
To find the inductance of the secondary I connected a capacitor
of 47 nF, much larger than the parasitic capacity, injected a
signal through a resistor of 30 kOhm and tried to find a
resonance by sweeping the frequency. From C and Fres I could
calculate the inductance. It turned out that the inductance was
very much dependant on the signal level applied. At small signal
levels the inductance decreases dramatically.
I found 14 H at 1 Volt RMS, 38 H at 10 V RMS and 80 H at levels
of 90 V and up.
The reason for this decrease with lower signal levels is the
very non constant μ, the
relative pemeability of the iron in the core. I still do not
fully understand why this does not lead to substantial
distorsion by audio transformers in general. Yes I know that
feeding the transformer from a very low impedance makes the
inductance less important, but that low impedance is not always
The so called B-H curve gives the relation between the applied
fieldstrength H and the resulting induction B. The μ is the angle of the long axis of the
ellipses. The relation is μ
= B / H. At larger H the ellipses deform because of saturation
of the iron. At lower fieldstrengths we see the angle of μ decrease.
Square Wave Response.
Upper trace: Input signal, lower trace: Output to 1 of the
stators. To feed the transformer from a very low impedance a
resistive divider was used: from Amplifier output to transformer
4.7 Ohms, parallel to the transformer 0.1 Ohm. For this test an
analogue test amplifier was used.
The non-sloping output wave indicates that the transformer does
its job up to very low frequencies.
The ringing was further investigated and turned out to be a
resonance at 41 kHz with an amplitude of +5.5 dB. Below 22 kHz
no more than 1 dB rise in output amplitude was seen. This
resonance occurs between the load capacity and the leakage
inductance of the transformer.
for an ESL. Download the spreadsheet.
of the calculations speak for them selves.
For the sound pressure at 1 meter distance I introduced an
imaginary cylinder of 1 meter radius and 1 meter high,
centered around the double cone. This is supposed to be the
area over which the radiated energy is distributed. From that
the sound pressure at 1 meter distance is calculated.
Of course these are calculations with several assumptions and
simplifications which make the results not much more than
Two conclusions leap to the eye, line 33 tells us that the
efficiency of the loudspeaker is very low (but everyone knows
that), and line 31 suggests that you do not need much voltage
for an electrostatic headphone. I did not work it out but
maybe it is feasable with a transformerless vacuum tube