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Altaic 10th October 2002 12:02 PM

Sealed ESL Design for DIYers
 
So, I'm making ESLs (yay) for my room. The acoustics in my room suck, but a discarded roll of wall-to-wall carpet will fix that =)

The main things I worry about these days when designing an ESL panel are bass and arching. So I modified my design (note this is my plan of action; it's not tried or tested):

A = 1" acoustic insulating foam with 1" clearance from frame
D = diaphram
F = frame
G = 3/16" rubber insulator gasket
G2 = 3/16" rubber insulator gasket with two 1/2 inch sections removed opposite eachother
S = stator
C = cloth cover
R = conductor ring with wire attached to the outer edge

-= original =-
|A| |C| |F| |S| |G| |D| |R| |G| |S| |F| |C|

-= new and improved =-
|A| |C| |F| |G| |D| |G2| |S| |G| |D| |R| |G| |S| |G2| |D| |G| |F| |C|

Assembly:
1) Stretch the three diaphrams on three separate bike-tube jigs
2) Coat one of the diaphrams on both sides with Elvamide (http://www.quadesl.org/Area_51/area_51.html or http://www.polymart.com)
2) Spray adhesive on one side of the Gs, G2s, and the conductive ring
3) Attach each of two Gs to a frame, each of the remaining two Gs to a stator (smoother side), and each G2 to a stator (the bumpier side), making sure they all are aligned
4) Attach the conductive to one of the Gs attached to a stator
5) Place the pieces/layers as shown above (without the A and C)
6) Make sure everything is aligned and clamp the frames on each corner (or more; I used 1/8" thick aluminum as my frame, so I'm not worried about the pressure being uneven)
7) Drill a hole in the center of each corner of the assembly, taking care to not to dislodge the clamps
8) Bolt each corner, tightening until the inner side of the stators are 7mm apart
9) Remove the clamps
10) Glue/seal a small hose to each of the openings the gaps in G2 created
12) Fill a balloon full of N2 or your favorite innert gas
11) Kink one of the hoses and slowly pump out the air for one side of the assembly until the diaphrams almost touch the stator (try to get it as close as possible)
12) Attach the balloon full of N2 to the hose you used to pump the air out of the chamber, and slowly/carefully refill the chamber with N2 (you don't want the diaphrams bulging outwards)
13) Unkink the other hose, flush the chamber with N2 and kink both hoses
14) Repeat steps 11 - 13 on the other chamber
15) Remove one hose at a time, filling the hole in the gasket with silicone sealant
16) Let sealant set/dry
17) Cut diaphrams from jigs at the edge of the gasket and blow-dry so the jaggedness goes away
18) Finishing touches go here. For instance the cover cloth and the acoustic foam with 1" spacers, along with a baffle if you so wish, a box of some sort, or a stand/mount

Notes:
- One could attach two valves to each chamber instead of two hoses, and leave them in.
- Flushing the chambers is not necessary, but it will increase the concentration of N2, which will reduce the chance of arching.
- Since the stators are so far apart (most bass ESLs are a max of 6mm) and there are three membranes (3 times the weight), the bias will have to be greater. Hopefully the resultant sound have richer bass while keeping the highs crisp.

Edit: I have a full parts list with online distributors, part numbers, prices, and so forth I can post if anyone wants it. It's costing me around $100 per 18"x36" ESL. It is the original list, but plenty of diaphram and gasket material is already included, so it should work fine.

Enjoy!

MRehorst 10th October 2002 02:50 PM

Balancing the gas pressure on the two sides of the diaphragm might be a little tricky. A pin hole in the diaphragm would allow the gas pressure to equalize itself. What will happen on days when the barometric pressure increases or decreases?

I suggest drilling before you attach any diaphragms. The drill is going to drop sharp edged crumbs into the speaker, which can't be good, and operating a relatively large, heavy, sharp, motorized tool within a couple inches of a tightly stretched, 6 um film is a recipe for disaster.

What is the purpose of the acoustic insulating foam on the back of the speaker? I suggest you try the speaker without it first. You'll probably like the sound better...

Is all this really necessary? What problem are you trying to solve with all this extra effort/material?

MR

Altaic 10th October 2002 03:20 PM

Barometric pressure should not make any noticable difference, because the two chambers will be the same pressure. The equal pressure is achieved just before the silicone sealant is applied, when the chambers are open, at equal pressure with the outside world, so matching them should be easy.

I thought the accoustic foam would absorb the inverted wave created by dipole ESLs. It would probably eat up my bass, though, sort of opposing what a baffle accomplishes.

The point of this is to get better sound out of a panel. For not too much more effort and hardly any more cost (maybe $20) arching would be prevented, and max voltage and travel increased. If the volume is increased, then I could put a quazi-enclosure on it, and save some of my bass (maybe... I'm not too sure).

MRehorst 11th October 2002 02:47 PM

On a day when the barometric pressure is higher than the day when you sealed the speaker the outer diaphragms will be pushed toward the stators. When playing music, the outer diaphragm will move in step with the driven diaphragm. If the outer diaphragms touch the stators they will buzz. Make sure you have large gaps between the outer diaphragms and the stators. Baromateric pressure changes can create quite a large change in the surface of a diaphragm.

I once considered making a "mirror" out of metalized film by stretching it over a large circular frame, then sealing the chamber behind the film. The result would be a mirror that on some days is convex and other days concave, due to variations is barometric pressure. I even considered making a crude, but very large telescope mirror that way, where the focal length could be changed by changing the air pressure in the chamber behind the film. The mirror so constructed would tend to vibrate with air currents and maybe even sound, but it might provide huge light gathering power (how many amateurs have scopes with 1m mirrors available?) for observing the milky way or looking for comets where you don't need a lot of magnification. The light weight would make the thing easy to handle. I don't know if the shape would be right for a telescope mirror. Maybe I'll try a small one some day. One could most certainly make a dandy solar cooker or light concentrator that way...

The outer diaphragms are very close to the driving diaphragm. That means there is a relatively small air space between them. That means that the driven diaphragm will be moving itself and the air on either side of it plus the two outer diaphragms, which due to the tension on them, will resist such movement. I think the effect will be equivalent to an increase in the moving mass, which should both lower resonance of the driver and maybe also reduce high frequency output (not necessarily a bad thing).

When you build these, build one without the external diaphragms so you can compare the sound from the two types. It should be interesting...

MR

Altaic 12th October 2002 07:57 AM

I'll definitely build a normal one and a sealed one. You're totally right about negative chamber pressure. For some reason I was only thinking about positive pressure. One way to get around this is find out the barometric pressure that day, calculate the maximum it could drop and add that much more N2 to each side. of course, that means one would have to balance both sides. A quick and easy to do it would be to loosen the bolts holding the assembly together (the rubber gasket is compressed quite a bit), add the N2 to both sides, and retighten it.

As far as sound-design goes, that would not be good because the sound directionality would change from day to day. You could always have an oiled tube with a snugly fitting ballbearing in it to slowly equalize pressure. As long as the ballbearing responds more slowly than the lowest frequency, bass response wouldn't be affected.

I'll try all these ideas out and report the results. I don't want this to be too complicated, though.

phishead8 13th October 2002 07:51 PM

An experimentalist at heart.


Altaic,

I am glad that you are looking for improvements in ESL design. The idea of a full range ESL that is smaller than 4' x 8' sure is tempting.

I don't want to discourage you from building the contraptions that you are mentioning. But, here are some of my experiences and ideas about ESLs (I am not an expert).

1. The construction of ESLs is always more difficult than I expected. I consider myself a relatively careful person, but sometimes the job requires a lot of patience and it is easy to rush a little faster than I should and end up making a mistake. Usually it was cosmetic, thankfully. Either way, a simpler ESL is necessarily simpler to build. My suggestion is to start with the more standard ESL first. If only for practice. If you don't like the sound, I'm sure that someone will be willing to take it off your hands.

2. The infrequent arc is simply not a problem. Unless you are using a low resistance coating (metalized Mylar for example) you should be okay. I suggest that you spend a little bit of time on making an adjustable high voltage supply. When the weather is humid the membrane may arc more. It is easy to simply lower the bias after the fact. This will also allow you to keep the ESL at the loudest music per Watt from the amp.

3. A flat panel behind the ESL will reduce the sound immensely. I haven't tried an acoustic dampening panel, but those things do not work 100%. I concur with MRehorst, allow yourself the ability to try it both ways. I was simply amazed at the difference when I did it.

I am eager to hear about your results.

I am about to begin another pair of ESLs and wouldn't mind some ideas before I get started. Otherwise I'll just stick with my method from last time.

On another note: where did you get your Mylar? I found some .5 mil Mylar in town, but the minimum order turns out to be $75. On the other hand, I'll probably never need more.

Good luck,
Dan

Altaic 14th October 2002 12:13 PM

"On another note: where did you get your Mylar? I found some .5 mil Mylar in town, but the minimum order turns out to be $75. On the other hand, I'll probably never need more."

I use this place for most of the parts (frame, mylar, gasket, stator, etc.):
www.mcmaster.com

Notes:
-Raw materials are at the bottom right on the main page.
-When browsing the sections, do not attempt to use the back button to get from the top level back to the main page, as it will autoforward you back to the page you were just at. Do this three times and it will think you are doing a DoS attack on it and lock you out of the site for a while. It's a total pain in the ***, and is my only complaint about the site. Otherwise, their prices are good, they deliver fast, etc.

phishead8 14th October 2002 03:54 PM

Rats!

I was hoping that you found a different source. Their .5 mil Mylar only comes 27" wide. My speaker design (and thus materials) is 20" wide. This doesn't leave me much room to wrap around a bicycle tube and piece of wood for the stretching mechanism.

Thanks though, mcmaster-carr is good for everything else.

Get them to send you a hard copy of the catalog. It's a great thing to have around as a DIYer. Sometimes I just flip through it and find all of the stuff that I didn't know existed. Sometimes problems that I was going to try and tackle myself have a ready made solution for extremely cheap. It sure can alleviate headaches.

Good luck on the project.

-Dan

MRehorst 14th October 2002 08:39 PM

Is the dielectric strength of nitrogen much higher than that of air (which is mostly nitrogen, anyway)? One of the old commercial ESLs (was it Beveridge?) used a sealed enclosure flooded with a gas with a very long name that DID provide an increased dielectric strength over air. Check the patents- that's where I saw the gas named. I looked up a source for the stuff once- welding supply houses carried it.

I am inclined to agree with phishhead- arcing really isn't problem, but if you're going to chase it, chase it well...

I think that if you make a few small drivers you'll get a good feel for the sorts of mechanical difficulties you may run into assembling, handling, mounting, and wiring ESLs. The mechanical issues are the real challenge . Finding a place to hide the step-up transformers and bias supply(ies) is some of the trickiest stuff you will face.

I wasn't aware that McMaster-Carr stocked so much useful stuff- thanks for the pointer!

MR

Altaic 15th October 2002 02:54 AM

I'd agree that arching isn't so much a problem. The thing is, though, that give it 2x or 3x the voltage, and it will do nothing but arch. Put an innert gas in it and it won't, which means you've made the output of a small ESL pretty decent.

I think you're thinking of the gas they use in mig-welding or is it tig-welding? I forget the differences between them. But I think it's really easy to get; my school has tons of it. I'll get the actual name and report back later on, maybe this week.

Would a torroidal mains transformer be better than a normal one for use as a step-up? I'm not going to but a few hundreds of dollars into an audio transformer just yet.


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