Segmented wire stator ESL's

Everything else is two dixie cups and a string 😎

Greetings all from the Jazzman,

It still amazes me that an average Joe with practically no electronics experience can build a speaker at home that rivals the high end commercial offerings and bests most of them.  And building the actual driver from scratch takes cool to a whole new level.    

My latest speaker shown above would not have been possible without ESL gurus Bolserst and Golfnut sharing their extensive knowledge on the DIYAudio Forum.  Golfnut’s white paper [1] and segmented speaker provided the inspiration and Bolserst’s gift for explanation and Segmented ESL Calculator made it easy to derive the segmentation scheme and resistor values.  These guys are the best!  

I am most happy to share with you my DIY electrostatic loudspeaker projects.
Charlie Mimbs
Savannah, GA

[1] Wide-Range Electrostatic Loudspeaker with a Zero-Free Polar Response, D. R. White, JAES Volume 57 Issue 10 pp. 822-831, Oct. 2009

How do they work?

If your hair has ever stood on end while unloading a clothes dryer, then you've felt the same force that drives an electrostatic loudspeaker (ESL).

An ESL is a push/pull sonic motor consisting of a thin plastic diaphragm suspended between two conductive screens called stators.  
A separate power supply puts a fixed DC biasing voltage on the driven diaphragm, and the output from an audio amplifier, routed thru a step-up transformer, puts the driving AC voltages on the stators.  And the ultra-light diaphragm responds with instant precision to reproduce the music with exquisite fidelity.

All of my ESL projects to date are bi-amplified hybrid designs using conventional woofers for bass with an active digital crossover upstream of the power amps.  

My first ESL’s used flat perf-metal stators, which gave great slam and imaging but beamed like crazy.  The next generation stators used segmented welding rod conductors on plastic light diffuser grids.  These had switch-selectable wide and narrow dispersion modes and a nice balanced sound but were visually just butt ugly.  Finally, my newest panels are more finely segmented for optimal dispersion, they are practically immune to arcing, and they have lovely oak lattice supported copper wire stators that both look and sound like fine musical instruments. 

Flat perf-metal panels are by far the easiest to build and they sound fantastic in the sweet spot.  They are also directional to the extreme, prone to arcing if the stator coatings aren’t applied to perfection, and they require a very stable amplifier to drive their all-capacitive load.  

Welding rod panels can be segmented for wide dispersion and balanced response and when segmented with a resistor network their part resistive/part capacitive load is easier on the amplifier.  The downside is they are time consuming to build and need to be coated for arc resistance.  
Insulated wire stator ESL’s like the one shown on this page are now my preferred choice.  They are highly arc resistant, and can be electrically segmented for wide dispersion, balanced response and easier load.  However, they are time-consuming to build; requiring a support lattice and a stout jig to stretch the wires.  

More to come on electrical segmentation but first; the basics of my newest speaker design:  

Dimensions:     66.5”H x 15”W x 19.75D
Bass cab:         4ft3, 9ft transmission line, V-section beam splitter    
Woofer:            10” Aurum Cantus AC-250MKII
Stators:             Segmented wire conductors on oak lattice, 10.5” x 46.5” active area 
Wires:               20 AWG solid copper, .010 XLPVC, 11/inch spacing, 43% open   
Diaphragm:       6-micron Mylar C with Licron Crystal ESD coating
Bias supply:      2.7kVDC from 115V/230V TFMR into a diode/capacitor cascade  
Transformers:   (2) 50VA 230V/2x6V toroidal, 76:1 ratio 
Crossover:        Behringer DCX2496, 48db/octave LR filter @ 228Hz 

Wavelengths shorter than a speaker’s radiating width tend to beam rather than spreading out.  And flat panel ESL’s are the worst for beaming because they require a large radiating area to offset their limited excursion and dipole rolloff.  The resulting “head-in-a-vise” sweet spot is great for solo listening at the focus but not so good for entertaining guests, where wider dispersion is needed.  

The typical way to widen an ESL’s dispersion is to curve its panel, and thereby it’s radiated wave front.  Segmented ESL’s curve the wave front electrically, using discrete stator conductors driving discrete zones on the diaphragm.      

My segmented panel uses symmetrically arrayed, discrete vertical wire groups sequentially driven by stepped-frequency/stepped-phase signals and functions as a line source projecting a cylindrical wave front.                  

Each stator has 90 insulated copper wire conductors arrayed in 15 groups of six wires.  These 15 wire groups are apportioned into eight discretely powered electrical sections; consisting of one wire group in the center of the stator comprising section one, and seven left/right paired wire groups arrayed symmetrically on either side comprising sections 2-8 (see Schematic).  

The section one wire groups connect directly to the amplifier interface and receive the full audio bandwidth above the crossover frequency.  The left/right paired wire groups in sections 2-8 are powered thru an RC transmission line that progressively steps down frequencies toward the panel edges.  The RC line consists of resistors inserted between the wire groups which couple with the wires capacitances to form a series of low pass filters.   

As driven by the sectioned stators, the diaphragm radiates the highest frequencies from only a narrow vertical zone at its center, and the left/right paired zones on either side radiate progressively lower frequencies toward the edges.  In this way, the width of the radiating zones is always less than the radiated wavelengths; resulting in all frequencies spreading out rather than beaming.  

·       Ideal (practical minimum) diaphragm-to-stator gap (d/s) is .062” (+.015/-.000)  
·       Wire diameter (insulation included) should not exceed the d/s 
·       Gap between wires (insulation-to-insulation) should not exceed the d/s
·       Ideal (max output) open area is 42%
·       Span between diaphragm supports: 70-100 x d/s 
·       Max span between wire supports (wire gauge/inches): 22/2, 20/3, 18/4
·       More/narrower wire groups = wider/smoother dispersion but no advantage < 12mm
·       Bias voltage: 2.5-5kHz
·       Transformer power/ratio: 80-120VA/50-100:1

Once the panel design is set, the next step is building a jig to stretch the stator wires. 

Stretching the copper wires to plastic deformation renders them perfectly straight and re-aligns their metallic structure such that they remain straight when relaxed.  Experiments showed that stretching to 1% elongation is sufficient, so my wire loops were stretched from 47.5” to their 48” final length.   

Stretching ninety 20 gauge wires at once requires a strong jig and about 4,500 pounds of force.  My jig is a ¾ MDF platform mounted in a stout frame cut from yellow pine 4x4’s.  The wire loops wrap over .063 diameter x .250 length steel pins inserted half depth into drilled 3/16” aluminum plates.  The pins are angled 4 degrees from vertical to hook the wires.  One pin plate is stationary and the opposite moveable pin plate bolts to a 3/16 steel sub plate that’s welded to two 3/4 x 12 all-thread jack rods.  Turning the coupling nuts on the jack rods pulls the movable pin plate to stretch the wires. 

Note:  For my stators' wire diameter, correct spacing required using 1mm pins.  However 1mm pins proved too weak and bent over when stretching the wires so I had to increase the pin diameter to .0625", which messed up the wire spacing.  To compensate I used 5/8-11 TPI all-thread rods as comb guides to hold correct wire spacing during glue-up.  

Below: Stator wires on the stretching jig

The wires are supported by an interlocking oak lattice which is assembled and glued down over the wires, in the stretching jig.  Before stringing the wires, the jig platform was covered with wax paper to prevent gluing the wires to the jig.  And before gluing, most wire tension was relaxed to prevent preloading and warping the stator.  Yellow wood glue was used for the wood-to-wood bonds and E6000 glue for the wood-to-wire bonds.

The vertical lattice rails were laid down into the jig first, and then the interlocking horizontal slats were glued down one-at-a-time, over the wires.  During this process, lengths of 5/8-11 TPI all-thread rods were placed over the wires as comb-guides to maintain correct wire spacing. 

CAD drawings for the cabinet, stator lattice, and stretching jig are available upon request.  Just email

Below:  All thread rod comb guides hold wire spacing during glue up

Below:  Oak support lattice assembled over wires

Below:  Completed stator 

The spacers bond the diaphragm to the lattice rails and set the diaphragm-to-stator gap (d/s) at .063”.  The vertical lattice rails are flush with the wires and its spacers are (1) layer of .063 x .075 wide 3M double-sided urethane foam tape.  The horizontal end rails run under the wires and anchor their end loops with glue bonds, and its spacers consist of (1) .047 x .075 polycarbonate shim bonded onto the wires with E6000 glue, plus (1) layer of .015 x .075 wide 3M UHB double sided foam tape over the shim (0.062 total).  The double-sided foam tape spacers bond the diaphragm to the stator instantly with minimal fuss. 

The 6-micron Mylar C diaphragm is vertically sectioned into equal thirds for stability and tensioned to 1.25% elongation using a pneumatic bike-tube jig.  The jig is an MDF platform sized two inches longer and wider than the stator and 2 inches high with all edges rounded over to .50” radius, sanded smooth and dusted with baby powder to prevent snagging the delicate diaphragm film.  A 700mm x 35mm Schrader valve type bike tube is stretched around it’s perimeter.  

The Mylar film is wrapped over the jig and secured on the back side with double sided tape.  Inflating the bike tube with a hand pump tensions the diaphragm.  Tension is gaged by first marking reference points on the diaphragm exactly 12 inches apart using a fine tip felt pen.  As the tube is inflated, the target elongation is reached when the distance between the reference marks reaches 12 and 5/32 inches.  

For my panels' span between supports, 1.25% elongation provided enough tension to prevent driving the diaphragm into the stators at high volume, yet kept the drum head resonance low enough to set the crossover frequency below the ear-sensitive midrange region.  

Since the amount of elongation/tension required to stabilize the diaphragm is dependent on the span between supports, the elongation would vary for different spans-- 1.25% would not work for all panels.

The stator is then pressed into place over the diaphragm to affect the bond.   

The diaphragm must be made conductive enough to hold a biasing voltage yet resistive enough to prevent rapid charge migration across the surface.  

Next, the periphery edges of the diaphragm were masked off with painters tape and the Licron Crystal ESD conductive ecoating was spray applied in one “just wet” coat and allowed to dry for eight hours before assembling the panels.  The coating dries to a pale blue-gray, almost clear coating about 2-microns thick.   

Below: Bonding stator to diaphragm on bike tube jig

Below:  Bonded diaphragm ready for conductive coating

The charge ring is ¼ inch wide copper foil tape applied to the periphery of the rear stator, centered on the foam tape spacers.  The wire lead from the DC biasing power supply is soldered to it and when the front and rear stators are mated together the charge ring contacts and conducts the biasing voltage onto the diaphragm. 

Below:  Rear stator with spacers & charge ring

Below:  Completed front & rear stators ready for assembly

The segmentation scheme and resistor values were derived using Bolserst's Segmented ESLCalculator spreadsheet.  From the spreadsheet options I chose Symmetric Config 2, both stators segmented, in eight electrical sections, and low-cutoff of 200Hz.  For my panel the spreadsheet calculated 120kΩ for R, with R/9 resistance on the section one wire groups, 0.75R on section two wire groups, and R on sections 3-8 wire groups. 

It’s common practice to move/reflect the section one resistances to the primary side of the transformer to protect against core saturation (see Schematic, damping resistor R1).  Reflecting section one's R/9 resistances across an ideal transformer would divide the sum by the turns ratio squared (4.6Ω in this case).  However, placing this much resistance on the primary side of a real transformer, interacting with its winding resistance, leakage inductance and winding capacitance, would result in significant rolloff of high frequencies down into the audio band.

The transformer's winding capacitance adds to the load capacitance and its leakage inductance combines with the load capacitance to generate an ultrasonic resonance peak in the frequency response and rapid rolloff above it.  Coincident with this response peak is an impedance minimum which can be a difficult load for the amplifier.  When series resistance is added on the primary side it dampens this resonance peak.  However, as previously noted, too much resistance over-damps the resonance, rolling off the audible highs. 

The spreadsheet assumes an ideal transformer is used, so it doesn’t calculate the effect of resistance on the primary side of a real transformer.  
The general guidance is to omit the section one resistors on the secondary side, add a 1Ω series resistor on the primary side and give it a whirl.  My panels sounded really good with this initial setup. 

From there the only tuning, if any, is adjusting the series resistance on the primary and/or the first two stator sections to dial in the treble response.  Less resistance increases treble and visa versa.  My old ears don’t hear the highs so well but I didn’t want less than 1Ω on the primary side and the section one resistors were already omitted, so I reduced the section two resistors from 0.75R (90kΩ) to 60kΩ to brighten up the treble, and that works for me.  

The schematic and parts list show the spreadsheet values except with section one resistors omitted and reflected as 1Ω on the primary.  I think this would be optimal for most listeners. 

All remaining resistors on the secondary side are 2W, 500V in series.  Wattage/voltage are highest across the first resistors and decrease down the line.  Multiple resistors are ganged to spread the load, as follows: 

Section 1:          none  (reflected as 1Ω on TFMR primary)
Section 2:          (3)  30kΩ
Sections 3, 4:    (3)  40kΩ
Sections 5-7:     (1)  100kΩ + (1) 20kΩ
Section 8:          (1)  120kΩ

Below:  RC network resistors



Parts list for two speakers:

Each stat panel has an interface to its amplifier; consisting of a high voltage DC power supply to bias the diaphragm and one or more step up transformers to convert the amplifier’s output into the higher voltage AC required to charge the stators.  

Each interface uses (2) 50VA 230V/2x6V toroidal transformers wired in tandem with the 6V windings in parallel as the primary and 230V windings in series as the secondary; giving a 76:1 winding ratio. The DC biasing supply uses a floating ground that’s center tapped between the transformers 230V windings.

The DC biasing supply is a simple half-wave rectifier and voltage multiplier outputting 2.7kV.  It’s powered by 115VAC mains current into a 115V/230V transformer and diode/capacitor ladder with a 20MΩ charging resistor at the output.  The charging resistor helps stabilize the charge on the diaphragm and limits the potential current that might otherwise sustain any arcing to the stators.     

Below: Amp/panel interface 

The woofer box is a single-fold, tapered transmission line stuffed with 0.5lbs/Ft3 of polyfil.  The line’s sectional area is 125% of the woofer’s cone area at the front, tapering to 100% of same at the terminus.  The cabinet is ¾” MDF sheathed in 5mm red oak plywood and the panel frame is solid red oak.  

To minimize the woofer box’s profile and footprint, its volume extends upward, behind the stat panel, and its frontal surfaces are angled to form a V-shaped “beam splitter” which deflects the panel’s rearward sound out the open sides of the speaker rather than back to the diaphragm.   

When designing the bass section I followed Roger Sanders’ lead and opted for a transmission line enclosure and a woofer with low moving mass, low QTS, and low inductance coupled with a very strong motor magnet.  The ideal matching woofer doesn’t exist of course but low inductance takes priority and the Aurum Cantus AC250 MKII I chose works pretty well.  

The speaker pair is vertically bi-amplified using a Behringer DCX2496 digital crossover feeding a pair of vintage Carver TFM-25, 225 watts/channel stereo amplifiers.

The general guidance is to set the crossover frequency is least two octaves above the diaphragm’s drum head resonance with a 24 db/octave filter slope or at least one octave above resonance with a 48db/octave filter slope.  My diaphragms resonate at about 90 Hz and the crossover is set at 228 Hz using the Behringer’s 48db/ocatve Linkwitz-Riley filter.  

Below: Beam splitter transmission line bass section


Below:  Bob Carver loves my new speakers!
               Cellphone Video from Carverfest 2016

Cellphone Video on Youtube (perf-metal panels)

Links to other pages on this blog: 

Building Perf-Metal Stat Panels

Exploded view of ESL panel (click to enlarge):


The photo above shows a metal pinwheel charged to a high voltage, and the resulting corona streaming off the sharp points.  Likewise; any sharp points on the edges of a charged stator would exhibit the same corona effects, which would quickly burn through the paint coating and create an arc path to the mating stator-- causing the panel to short-out and fail. This is why we must use great care to remove all sharp points along the trimmed edges of our metal stators!             

Choosing perforated metal for the stators:
I prefer steel over aluminum because the electrical leads can be soldered to it, it's extra weight makes it less prone to ringing and it costs less. When choosing hole size and open area, consider how the stators will be coated: A spray-painted stator with 12 mils of paint on its faces will likely have only about 3 mils on the inside hole surfaces because a paint gun can't be oriented perpendicular to the inside-hole surfaces. Powder coating uses electrostatic attraction to place the coating, so there would likely be more coating in the holes compared to painted stators. For that reason, perf with higher open area (50% minimum) is needed for powder coated stators.

Stator coatings:
Powder coating is generally considered the best option but painted coatings (laquers, polyurethanes, epoxies, i.e. Krylon, Rustoleum, etc...) have been used with success. I think most any non-metallic paint would work if applied in sufficient thickness.

I opted to spray coat my stators with automotive polyurethane because it chemically sets and I had equipment and experience to apply it myself.  I saved myself some time and hassle but automotive polyurethane isn't cheap ($125+ with cleaning solvents), so I probably didn't save any money over the cost of powder coating.

If spray coating the stators, the electrical leads can be soldered on before painting, as I did.

If powder coating the stators, the 400-degree baking step would melt a solder connection. A metal tab connector could be brazed on before powder coating or a tab could be mehanically attached [before or after powder coating], as shown in this thread: mechanical connection

The instructions below are for spray-coating steel perf stators with pre-soldered leads.

Guidelines:  Steel Perf stators should be 16-20 gauge (.063-.036) thick with 40%-60% open area and hole diameters at least 2X the metal thickness but not larger than 3/16".

Stators with smaller holes are more efficient because they produce a more uniform and denser electric field. However, smaller holes require thinner stators (1/2 hole diameter, max); otherwise the sound sees the holes as cylinders, and the air within the cylinders has an impedance that acts to roll off the treble response. So, ultimately, the stator thickness sets the lower limit for hole size. 

Stators also vibrate when playing (especially flat stators), and I suspect thin stators may exhibit audible ringing and possible distortion at their resonant frequency. So, for flat stators, I prefer to go no thinner than 20 gauge (.036") steel. 18 or 20 gauge is my preference.

There is also much debate over how much open area is best. I have only used 40% and 51% open stators and I don't hear a lot of difference-- the 51% panels seemed slightly brighter and the 40% panels were perhaps a bit softer (it could be that less open area constrains the air flow enough to dampen the diaphragm's fundamental resonance and harmonics but I can't say for sure). Some of my stator photos show 51% open perf but I currently prefer and use 18 gauge 40% open steel perf with .125 diameter holes. Again, for powder coated stators, 51% minimum open area is best because the powder coat will make the holes significantly smaller.

Links to perforated metal sources:

(best prices and my preferred source).
036 x 12 x 48 / .125 holes / 40% open: $20 each + shipping.

Trimming stators to size and working them flat:
Most suppliers will trim the metal to size; otherwise a band saw works well. The panels will need to be as flat as possible and some suppliers can roll them flat if you specify that flatness is a requirement.  None of the perf I've ordered arrived perfectly flat. The perf from McMaster Carr was the flattest I've gotten and even those pieces were slightly bowed.

After match-trimming and edge-smoothing the stators, you will likely lose about 1/8" of width so you will either need to order the metal 1/8" oversize in width, or make allowance for it in your speaker frames.  If you order other than 48" length, the ends will also have open-hole edges and require smoothing.   I advise building the stat panels first, then cutting the speaker frames to fit the panels.

The hole perforations are punched thru the metal (not drilled), and that process produces smooth hole edges on one face of the panel and sharp hole edges on the opposite face. The panel face with the smooth hole edges must face the diaphragm to minimize the chances of coronal arcing.

As already shown, any sharp points on the stator edges are prone to coronal arcing and should be ground off smooth.  

Place the mating stators together with their smooth hole edges facing the inside and their holes exactly aligned, then clamp them together and grind smooth all sharp points on the edges.

After edge finishing, inspect each panel for flatness. It's very likely that you will need to do some final straightening. I straighten my stators by hand on a flat work table covered with a 1/8" thick soft rubber "router-mat"purchased from Harbor Freight tools. The rubber mat has enough give to allow straightening the metal without over-bending it.

CAUTION: If you try to flatten out a dent in a stator with a hammer on a hard surface, chances are you will expand the metal; causing it flop in and out ("oil canning") and it would then be ruined.

Right: After trimming to size, the front and rear stators are aligned and clamped together, then sharp points along the edges are ground down and rounded-over to prevent coronal arcing.

Left: Straightening a bent stator by hand using a soft rubber mat under the stator to allow it to "give" without over-bending. For straightening sharp creases in the metal, a rubber or wooden mallet can be used. The rubber mat is a router mat purchased from Harbor Freight tools.

Cleaning and coating the stators:

For safety reasons alone, I wouldn't use uncoated stators. Even with coated stators, I would use speaker grills if there are children or pets in the home.
Before painting the stators, the wire leads must be soldered on. Mine have the power leads soldered to an outside corner. There are other coating options but my stators are spray-coated with a 2-part automotive polyurethane. First,the stator panels were thoroughly cleaned to remove the protective oil coating, then sprayed with Martin Senour Crossfire paint system with 1-part base-coat/2-part clear-coat from NAPA auto parts. I used 1-pint of thinned black base coat and a about 1.5 quarts (mixed volume) of the 2-part polyurethane clearcoat. If using a HVLP paint gun, you might get by with a single 1-quart kit of the 2-part clearcoat. I also used the fast dry catalyst and sprayed the clearcoat "just wet" so that the coats would set quickly and not thin-out over the sharp edges of the perforations. Here is the procedure:
  • Hang panels and attach weights to keep them steady while spraying.
  • Thoroughly spray-rinse panels with Naptha or laquer thinner to clean off the oil coating. Allow to dry.
  • Spray on the 1-partcolor coat as needed for coverage. Allow to dry at least 1 hour.
  • Spray on 10-14 mils of 2-part polyurethane clearcoat. Be sure to coat panel edges as well as faces and spray from different angles to cover edges of perforations. The first couple of coats should be "misted" on withat least 10 minutes tack time between coats. Spray remaining coats"just wet"; allowing a few minutes tack-time between coats.

Left: Stators hanging ready for solvent spray cleaning and coating.

Left: Closeup of a completed stator with polyurethane coating applied. Note that edges are rounded over to prevent coronal arcing.

Diaphragm/Stator Spacing & Span Between Support Spacers: 
Minimal diaphragm-to-stator spacing (d/s) is desirable for highest efficiency because the electrical field strength decreases with the square of the distance. Thus, doubling the d/s spacing would require four times the power input to produce the same volume. For hybrid speakers, as little as 1/16" (about 1.5 mm) d/s spacing can be used but 1/8" (3 mm) or more spacing is needed for full range panels to accommodate their longer bass excursion.

The Cookbook guidelines recommend placing diaphragm supports at spans equivalent to 70-100 times the d/s spacing. I prefer to go conservative and set my limit at 80x d/s for added protection against bass driving the diaphragms into the stators. With 1/16" d/s and 80x max spacing, the span between support strips should not exceed (.062 x 80)= 4.96". Since my 12" wide panels have a span between the edge spacers of 10.5" and I'm adding two 3/8" wide vertical support spacers, I end up with three sections, each 3.25" wide, which is well within the limits, so there is no problem.

Note:  The photos show my original panels, which used 3/4" wide urethane foam tape spacers.  I now recommend 1" wide tape only, with 1/4" of the tape wrapped over the stator edges as shown here: 

Many builders use plastic spacers and glue but I prefer to use 3-M brand .063"x 1" double-coated urethane foam mounting tape because it's fast and easy. The tape secures the diaphragm to the stators instantly with minimal fuss and sets the diaphragm/stator spacing at 1/16"; which is ideal for hybrid panels. The 3-M tape isn't cheap but I would not use any other brand. Also, you want to buy the tape from a source that sells enough of it to keep their stock rotated and fresh. 3-M advises using the tape within 2 years of manufacture in order to retain its full tack and adhesive strength. 3M has said that the foam tape will yellow over time but should last for about 25 years indoors (Really?... I don't know... just reporting what 3M said).

Apply 1" wide foam tape along the periphery edges of both stators; allowing the tape to overhang the edges 1/4".  The 1/4" overhang will be wrapped over the stator edges after bonding the diaphragm. 

Also apply 3/8" wide tape support strips at the intervals needed to support the diaphragm. Note in the the photos that my 12" wide panels use two 3/8" wide diaphragm support-strips. The tape sets the diaphragm-to-stator spacing (ds) at 1/16", which is the preferred spacing for hybrid speakers. The 1" x .063 double sided foam tape is McMaster Carr part number 7626A115


Left: Diaphragm shown being tensioned on a pneumatic bike-tube tensioning jig.

Diaphragm material:
I used 6-micron Hostephan polyester purchased from The Audio Circuit (TAC) in the Netherlands. TAC ships immediately but it can take several weeks to clear Customs.
Later I found 6-micron Mylar on Ebay for a great price and it arrives in about 10 days. I recommend the 6-micron film but some builders use the heavier 12 micron film for bass panels because it can be tensioned much higher. If using a mechanical tensioning jig, you will need film wide enough to wrap around the jig. Several sources and sizes are listed below.

Links to sources for polyester film diaphragm material:

Current best deal:

Other sources:
McMaster-Carr #8567K104 - 12-micron 27" x 25' roll $12.25 + shipping
Diaphragm Tensioning:

Tensioning the diaphragm is a compromise between conflicting requirements: If you use a high polarizing voltage on the diaphragm for highest efficiency you will need to tension the diaphragm very high to prevent electrical forces from pulling the diaphragm into a stator. Also, higher tension reduces the chance of bass driving the diaphragm into a stator (the pressure waves from any woofers in the room will couple to and move the diaphragms). On the flip side, lower tension gives a lower diaphragm resonant frequency; which is also desirable, as that would allow a lower crossover point. The diaphragms resonance depends on it's tension and the span between diaphragm supports. Typically, the diaphragm resonance will fall somewhere between 50-120 hz. With fairly close support spacing, as used in my panels, the resonance is probably somewhere near 100 hz; although I have not measured it.

We don't want to play the panel at frequencies low enough to excite the diaphragm resonance, as output is savagely distorted at the resonance. The rule of thumb is to set the crossover at least two octaves above resonance if using a 24db slope or at least one octave above resonance if using a 48db slope.

The amount of tensioning needed and the methods of achieving it are subjects of heated debate among builders. Some recommend heat shrinking and others prefer mechanically stretching the film. I even read somewhere that Quad at one time used mechanical stretching followed by a proprietary heat treat process to "stabilize" the tension.

I use a pneumatic "bike-tube" tensioning jig made of 3/4 MDF, sized 1" larger than the stator on all sides and the edges are 2" thick. (see the photo) There is a hole drilled on one end for the bike tube's valve stem. The bike tube is a size 27 x 1.25 (700 x 32 metric) and has a Schrader type valve (don't use a Presta-valve tube because it's valve isn't spring loaded and will leak down). The general consensus for mechanical tensioning is to stretch the film to 1%-2% elongation, depending on the span between the support strips. For my panels, I use 6-micron diaphragms exclusively and I tension the film to 1.5% elongation.

With 12 micron film, much less elongation would be needed to reach the same tension as 6-micron film stretched to 1.5% elongation. Conversely, a thinner film would need more elongation to reach the same tension. I can only vouch for 1.5% elongation if using 6-micron film.

Mechanical pre-tensioning procedure for 6-Micron polyester film:
Before tensioning the film on the jig, have the stators ready (3M foam tape spacers and center support strips in place).

1) Prepare the jig: Apply double backed tape on the underside edges to hold the film in place.
Also adjust the top edge of the bike tube flush with the top surface of the jig.
2) Cover your work surface with a bath towel or cloth to protect the fragile film.
3) Cut a piece of film large enough to cover and wrap around the jig with a couple of inches to spare on all sides. Use very sharp scissors to cut the film cleanly, as any ragged edges are prone to tearing when handling the film.
4) Layout the film onto the prepared work surface and place the jig, face down, onto the film.
5) Wrap the film over the jig edges and secure it on the backside with double backed tape. Be
very careful here-- the film is quite strong in uniform tension but tears quite easily from the
edges. If you do happen to tear the film along an edge and the tear does not extend onto the face of the jig, you can still save it by patching the tear with tape.
6) After the film is secured to the back edges of the jig, you want to pull out any slack at the four corners of the jig-- use scotch tape but don't extend the tape over the bike tube.
7) Carefully turn the jig right side up. The end of the jig that has the tube valve can hang off the edge of the worktable as needed to attach the air pump.
8) Using a fine tip felt pen, place reference marks on the film exactly 6.000" apart, in the width direction. (If the film is wide enough, you could place marks 12" apart)

In case you were getting complacent -- Hold your breath and pray during the next step because you will be stretching the film almost to its breaking point.

9) Slowly inflate the bike-tube to tension the film. When the distance between the reference
marks reaches 6.090", the film is tensioned to 1.5% elongation. (If you used 12" marks, stretch the film until the marks are 12 3/16" apart for 1.5% elongation). You can breathe again now.
10) Immediately bond the stator to diaphragm while the diaphragm is under tension (see
Bonding the stator to the tensioned diaphragm:
1) While the film is under tension, place the stator panel over the film, adhesive side down, and
press along the edges and center support strips to secure the film to the stator.
2) Release pressure and trim the excess film net to the edges of the foam tape.

As previously noted, the edgesof the stators are prone to coronal arcing, which could short-out the panel and even damage the transformers and power amp. Enlarge the photo below and note that the foam tape overhangs the edge of the stator 1/16" on all sides. Do this on both stators because it provides an extra measure of insulation that virtually guarantees that no arcing will occur between the stators.

Left: With the diaphragm fully pre-tensioned on the pneumatic jig, the stator is placed over the diaphragm and pressed down. 1/16" thick 3M double-sided foam tape secures the diaphragm to the stator.

Left: After bonding the diaphragm to the stator, the excess is trimmed away and the diaphragm is now ready for the conductive coating.

After bonding the diaphragm to the stator, I recommend
tape insulating the stator edges as shown in the link below:

Techspray conductive coating applied to the diaphragms (still wet):
In order to prevent the bias charge from leaking off the diaphragm, we want to leave the outermost edges of the diaphragm uncoated. The coating only needs to cover the inside areas and extend far enough to touch the copper foil charge ring on the opposite stator (see diagram at top of page). So, before coating the diaphragm, we want to mask off the periphery edges, about 1/8 to 1/4 inch, as shown in the photo below (blue masking tape).

Popular conductive coating options:
One light coat sprayed just wet enough to form a continuous film will dry to a clear permanent coating 1.5-2.0 microns thick with E7-E9 conductivity. This coating is tried and true and I use it exclusively.

2) Any liquid hand-dishwashing detergent containing sodium laurel sulfate: Apply a light coating, undiluted, in circular fashion, using a cotton ball or soft cloth. Some builders recommend this coating because it's easy and cheap and has ideal resistance reported to be E9-E11 range but I have no means to measure it. I did try it out on a test panel and it does work perfectly but I don't know how long it would last. Tape and adhesives won't bond to this coating so before applying it you would need to mask off the areas on the diaphragms that bond to the center support strips/ spacers.

3) Nylon in solvent (Elvamide); More on Hostaphan and Elvamide supplied by The Audio Circuit: Said to be the best coating around but it looks to be a pain in the ass to mix and apply.

4) DIY coatings posted on the DIY Audio Forum-- I haven't tried these:
Copper charge ring:
Copper foil tape applied to the periphery of the mating stator makes the electrical connection from the high-voltage DC bias supply to the diaphragm as shown below.

Soldering the lead to the copper foil charge ring

Charge ring installed on the mating stator

The connecting wire must be soldered to the copper foil before it's applied to the stator panel. In order to reduce the risk of arcing and shorting, it's very important to keep the solder joint as thin and flat as possible so that the insulating tape is not excessively compressed when the panels are assembled. I carefully sanded away the excess solder to flatten and thin the connection before transferring the copper strip to the stator.

Below: Stators being positioned for assembling the panel
This is a tricky operation because the foam tape makes a permanent bond on contact so the stators must be flat and exactly aligned when contact is made. Since I didn't have any help, I found it easier to position the panels vertically using a board clamped to my table and a bank-stop at one end, as shown.

For mounting the panels in the speaker frame, I use a firm foam rubber weatherstripping between the ESL panels and frames. This insulates the panels from the frames, not only electrically but also acoustically, as the stators vibrate somewhat in operation.  Click the "Insulating the stator edges" link below to see how the panel edges are insulated and mounted in the wooden frame.