Stator material selection, preparation and contacts
Right now I have two choices for the stator material:
1. The aluminum I have is cut 12"x40", has perforations .1875" with an open area of 51%
2. The steel material has perforations .078" with an open area of 45%.
Aluminum's conductivity is about an order magnitude better than the steel, its lighter and can be anodized. With steel I fear its own weight will cause it to warp if not properly housed in a frame. The steel was better machined less sharp edges and the perforations have a nice rounded edge. It looks like I will be working with my already cut aluminum. I will have it anodized soon, before doing so need to discuss the way I will make my contacts with it. I was just going to pass a bolt (metal or Nylon?) trough one stator (drilled through the acrylic spacers (0.08" thick) out the other stator. The wires will have clips (u shaped, smoothed out edges) attached to the bolt, and lightly tighten a bolt on the other end. Its straight-forward, but making good lasting connections is obviously a must. For the diaphragm contact I have ¼” copper tape that will be recessed into the acrylic touching the coated side of the mylar. I was just going to solder a connection to the copper before its recessed into the acrylic so I don’t melt the acrylic with my solder gun. Does this sound reasonable?
After its anodized do I have to scrap the surface so that the contacts rest on the bare aluminum surface?
What's a good cleaning agent to clean off the aluminum's grease before its anodized black?
Conductivity is not important for this application.
You worry about gravity but you will have considerable electrical forces on the stators (think in the order of 20-40N) so proper housing and support is needed. The same goes for the spacer frame that will have to support the diaphragm (forces here: 100s of Newtons, and the bad news here is that if it slackens one milimeter you will have lost most of the diaphragm tension). Also you want to have a very consistent diaphragm to stator spacing all over the area. This, together with the choice of D/S spacing itself and diaphragm tension will be the most critical parameters of your design. Most other stuff you can change afterwards (electronics, HT).
Sharp edges are a show stopper, if your stators have sharp edges around the holes then you need to get those removed first.
Small nylon bolts can't be fastened good enough for making a proper stator contact. Best would be to have a nice solder lip spot welded to the stator. But any proper non-oxidating contact with enough pressure will do.
I don't see the necessity for recessing the copper tape. On the contrary, if you just put it along the inner edge of the spacers it will ensure good pressurised contact all over the area. This is another critical part of construction.
ESLs are high impedance devices. As such the conductivity of the stators only plays a role above values of several kOhms (think of the resistor values needed to segment a panel). Regarding conductivity there will be no difference between aluminium and steel worth mentioning. BTW: make sure that the sheet´s holes are rounded on both sides of the sheet. Sanding and/or etching is strongly optional.
I wouldn´t recommend the usage of aluminium sheets for any other than very small tweeter-panels. Besides beeing too soft for larger panels a serious problem will be to create a long lasting stable electrical contact point. As you know aluminium oxidizes fast. A simple single bolt forming a contact by pressure on a pair of contact clips won´t last long and would ´compress´ the complete panel. A contact failure is preprogrammed.
Rather a 2-bolts solution, one bolt for each stator sheet could be feasable, since You could tighten each bolt very hard without exerting pressure on the panel itself. Additionally the 2-bolt solution must be manufactured before the spacers and membrane are glued to the stator while the single bolt solution will be manufatured as one of the last building steps. This creates the problem of how to test within the building process. This means: You´d like to test e.g. the membrane coating on function. Therefore You take just the one stator to which the membrane is glued and contact the membrane and stator to the HV-supply. When energized the membrane should bow towards the stator sheet. If not, of if its not doing so completely and evenly, you can correct for by doing a recoat.
So, I recommend using always 2 independent contact points, one for each stator.
With aluminium it could be a solution to use press-in bolts, since those deform the material and cog with the sheet material. Nylon is out of question here since the forces needed for a good contact are way too low.
What´s Your intention to anodize the aluminium? You have to insulate the sheets properly. Just anodizing -though it forms a isolation barrier- is way too less. You need to coat the sheets with a thicker and proper insulating material.
With steel sheets as stators on the other hand you can use either the bolt solution very well or You can solder a wire or contact point directly on to the sheet (ML and I do it this way). Another solution could be to weld a solderless express crimp terminal (3/16" or 1/4") to the sheet.
Copper tape is fine as diaphragm contact. If its pure copper than oxidizing can create problems over time. When putting the copper onto the diaphragm You should wet the tape with the coating fluid so that the fluid ´glues´ the tape to the membrane thereby creating a good solid large area contact on one hand and forms a anti-oxidizing barrier (top coat, like a laquer) on the other hand.
Why do You have just the 2 material-choices? Both don´t look optimal to me, because of their hole sizes. 0.1875 (+5mm) is slightly larger than You´d like to have for a mid-high-panel with a d/s around 1-1.5mm and 0.078" (~2mm) is very small. While smaller holes are basically a plus, they can create problems with the insulating coating. Besides the possiblity of completely sealed holes the reduction in open-area-percentage is higher than with larger holes. It seems that 1/8" (~3mm) holes on a 1/6" grid is the best compromise with regard to strength, openness, efficiency, coatabilzy and even optics. If You want to build something serious, I recommend to ´rethink´ certain points of Your design. If You just want to do a ´test´, than go ahead. ;)
Re: Stator material selection, preparation and contacts
Yes electrical forces are much stronger than gravitational forces, my frame to support the panels is made of oak that will be treated to avoid warping they measure 1.9cmX3.8cm *length (6feet).
I was looking at steel perforated panels with specs
hole diameter 1/8"
% of open area 40%
Center to center spacing 3/26"
From McMaster-Carr Product number 9255T661. Your absolutely right one bolt would compress and compromise my D/S spacing. I will be using the 0.08" acrylic, would this be ok to use with the above metal?
IF I follow Sanders guidelines he says to satisfy:
2*stator thickness<hole diameter<D/S spacing
The steel I currently have (0.078" hole diameter 0.03" thick with a D/S spacing of 90 mil (acrylic plus glue layer) satisfies the inequality nicely, I understand that the lower bound is required so that the perforations look like holes not tunnels (Helmholtz’s resonators) the upper bound I thick is a result of not having the film be damped by reflections off the non-perforated part.
You are more experienced in building ESL's So I ask you, should I make one more trip to McMaster Carr, get the above metal (product number 9255T661) to be used with the 0.08" acrylic or stick with the steel I have that satisfies the Roger's inequality?
I will probably just solder a good contact directly to the steel.
You can have a look at the metal at:
and place product number,9255T661 in search box
Thank You Very Much,
You still haven´t specified any design criteria about what You want to do or I have overseen it somehow. Anyway with such little input please don´t expect answers from me about the usability of materials or their physical dimensions.
The system is a hybrid, with transmission line woofer crossed over (passive) around 400Hz. The panels will not be full range just operating mids and highs. Phase cancellation will be reduced with a baffle design. The panel will measure 12"X40". It will be played in a small room (10'*12'), I usually don't listen loud, max SPL around 85dB a meter away. I will probably measure the frequency response and build a simple equalizer if needed, the baffle should be enough. I’m sure I left something out, just ask can provide.
ok, thats what we needed. ;-)
Well, playing from 400Hz on up, the spacers should not exceed 1mm (1/25") thickness. 2mm (0.078") is too much already.
Your panel size and width is large enough to omit a baffle completely, which is always preferable in sonic terms. Use a thin but strong frame instead (besides: such a thin frame assists the acoustics witth optics, as it adds to the ´light and airy´ impression)
Keep in mind that even in a small room the panels need space around them. The min distance to the back wall should not be less than 40".
If You can manufacture, try to curve the panel. This will increase the stability of the panel very much --> less rattle, less resonance. This could be of elevated relevance if You use those thin MasterCarr sheets.
Since the d/s will be small and the panel area large, the capacitance will be high (between 1nF and 2nF, but so will be the efficiency. You will need a transformer or a pair of with a rather low transformer ratio of 1:50 to 1:75. The polarizing voltage can be quite low too (1-1.5kV). Lower voltages are very desirable because it means less problems with insulating and less losses, longer lasting materials and improved dynamics. You won´t need a very powerful amp either, if the bass doesn´t ask for.
Have You spent some thoughts on how to insulte the stator sheets? If not, then start doing and keep us informed! ;)
I agree with Calvin, about 1mm diaphragm-stator distance should be good and give a nice sensitivity (aprox. 95db/2.83V/1m with a stepup of 1:75, 1500V, depends on equalisation). This is good. But with 1mm, construction will be more difficult as you will need more diaphragm supports to get a stable diaphragm. The advantage of more supports is increased structural integrity, which you will need to get good enough tolerance (diaphragm - stator distance). A disadvantage is increased dead capacitance.
If you want to go ahead with the 2mm spacers then you can get aproximately the same sensitivity with a stepup of 1:100 and 5000V polarisation. As Calvin pointed out, this might give you electrical problems, but it can be done. And the higher the step-up, the more difficult it is to make a good transformer.
But I see two other problems with this design.
First, without segmentation a panel this size will beam like a laser! Unless you want to listen with your head in a vice, you need some form of segmentation. A popular way to achieve this (with perforated metal stators) is to split the stator into two pieces. One normal piece that covers most of the panel which handles the mid range, and a second piece in the form of a 1" wide vertical strip, running down the length of the panel on one side to handle the high frequencies.
Second, crossing over as high as 400Hz will make integration between the woofer and the panel much more difficult. At least this is my experience. I prefer to cross over below 100Hz.
Thanks for getting back. I like the idea of using closer spacing so that I don’t need as much potential and less possible for electrical insulation problems to spoil the show. I already have this acrylic cut, they are double the size you suggest 80mil. I know I can still use it, just need greater voltages. SO to avoid the electrical problems my stator prep is as follow:
Using a dremil (low speed) with cone shaped sander attached that just fits outside the hole diameter. This will round off the holes. This is tedious and time consuming.
Step 2. Use dremil to remove any sharp edges around corners from shearing.
Step 3. Sand both sides of the steel
Step 4. I have experimented with pc-board etching solution. This stuff is used to make circuit boards out of sheet copper. Agitate for an hour. This really does a nice job of cleaning the steel, dulls it out.
Step 5. Some kind of coating, maybe epoxy, went to home-depot they had some type of spray called plastic dip, works on steel, and gives insulation coating. Need to research its dielectric breakdown voltage per thickness of coating. Will get back.
Step 6. Testing, hook up stators separated by 2*D/S to high voltage supply, repeat steps if problems occur.
Or do you thick it would be wiser to just use 3M foam tape at 1mm thickness? It seems less work is involved. But I have Sanders book too much on my mind. I know it is not a good thing to stick hard to instructions, where’s the fun.
I don't know if you guys have RadioShacks near you, but I went to several near me and RadioShack Just isn't the same. I couldn't find any pots, or hook-up wire. Just batteries and cell-phones. Its a sad thing.
Some people, I thick older a-philes, like laser imaging. You have a tiny sweet spot but the imaging is fantastic, give and take. Curving the cell tries to widen that sweet spot, but too much dispersion you will lose imaging, because now the room is more involved. I opted for a slight curved cell. I have access to metal shop, so curving the steel is no problem. The toughest part will be tensioning the diaphragm just right. Maybe mech. jig.
So keep the acrylic or just use 3M?
Forgot to ask you, the sensitivity you mentioned 95db/2.83V at a meter away is this something you measured? That sounds really nice.
About, Segmenting… The geometry of the spacer pattern (length and width of each cell) along with the films tension should define the films normal modes of vibrations, i.e. what frequencies they like to play. The way I understand it is that you design your spacer pattern, you suggested 1" wide tweeters down the whole length of the cell. The film is glued down along the spacer thus defining the segmented cell, is this what you are referring? If you look at ML's Spire or vantage you may notice that the width of each cell is something like, from top to bottom, wide, thinner, wide, thinner, etc... If you wanted to vary the tension of each cell, then were going to have to use several separate films, which means more contacts. Or do you just set the tension say a 100N below the tensile limit?
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