A few questions...

[jamieblue]

Hi all,
I am a student in my second year of sixth form, doing Physics. For my coursework I decided to make an ESL from scratch. At school we have a Power Amplifier (a Peavey CS800X), some very basic transformers (coils of wire, I use two 600 turns transformers, Philip Harris make), two perforated galvanised steel metal sheets (originally 500 x 1000mm, but someone cut them down in the Design department to around 350 x 1000mm), and EHT supply capable of delivering DC voltage up to 5kV (PHillip HArris make, can be found on website), two spacers (about 3-4mm in thickness, about the same as the stators in width), clingfilm used for the diaphragm and diluted washing soap used to make the clingfilm have a resistivity etc. The same person who cut the metal designed a unit made from wood so I could slide everything everything in. I used cables that you find in school labs, e.g. the black and red cables. For the contact I used household alluminium foil.

I tried the experiment out on Friday, but with no success (what a surprise!!!). I have searched on Google looking for these answers for some time, and I'm sure they are there somewhere, but I really have spent a long time searching so I thought I'd post. Here are my queries... :

- I don't understand how the diaphragm is attached, or not, to the spacers. I always thought that the diaphragm needed to be able to vibrate, so the way I figure glueing it to anything will cause it not to vibrate. Unless the perimeter of the diaphragm is glued to the spacers, and if so, how much of the diaphragm should be glued?
- Is it possible to not actually have spacers, just have the two stators? I have read somewhere that the spacers just dampen out the fundamental frequency and stop resonance or something, but is this their only purpose? I know that they are there to stop the diaphragm from being pulled in to the stators, but if the stators are placed far away enough, is there any need for them (with a high enough voltage)?
- If spacers are needed, how thick should they be? Under 1mm, over? I know the dimensions of the stators for definite, but with the stators I am not so sure... :S
- Does the contact to the diaphragm need to touch the whole perimeter or just a part of the diaphragm? I am fairly certain of this one, but just checking (only a part I think will be ok... )
- Is it best if the alluminium foil contact is flat or rolled up like a sausage? Or does it even matter that much?
- I am not able to get access to Mylar film in time. Has anyone had any experience with the film you would use to give car windows a tint, it can be found at Halfords ? Otherwise, any other suggestions for a membrane?
- How far does the diaphragm need to be from the stators? I have read around 2mm, but am really not sure :S
- Would just standard washing up liquid do the job for giving the membrane a high resistivity? Powdered graphite I know also works, but I doubt I'll be able to get any of that

I would be extreeeeeeemely grateful for any comments or replies at all to this post. I hate asking for help, and asking for this much help! But I feel it has to be done Thanks again in advance,
Jamie

[Bazukaz]

Hi, you need to read a bit and understand the basics of ESL construction.
There is sufficient information on the web , google is your frend

Instead of mylar you can use baking film(or bags). It is transparrent and strong - has similar properties to mylar , but usually comes in >=12 micron thickness. The film must be streched quite hard , otherwise it will tend to stick to metal sheets.

Graphite is not good for coating - difficult to apply , contaminates spacers , etc. You can experiment with mix of PVA gue / Water / Black ink. Some types of ink work , some won't.
For experimenting , you can use hand soap as well.

Regards,
Lukas.

[Few]

- I don't understand how the diaphragm is attached, or not, to the spacers. I always thought that the diaphragm needed to be able to vibrate, so the way I figure glueing it to anything will cause it not to vibrate. Unless the perimeter of the diaphragm is glued to the spacers, and if so, how much of the diaphragm should be glued?

Glue the diaphragm to the spacers on all the perimeter edges. If the stator is 350 mm wide, you'll probably need to rigidly space the diaphragm from the stator at some other points as well. Dots of silicone have been used to good effect for this purpose. You might space the silicone dots about 10-20 mm from each other and from the nearest stator edge as a way to get started.

- Is it possible to not actually have spacers, just have the two stators? I have read somewhere that the spacers just dampen out the fundamental frequency and stop resonance or something, but is this their only purpose? I know that they are there to stop the diaphragm from being pulled in to the stators, but if the stators are placed far away enough, is there any need for them (with a high enough voltage)?

You need spacers so that the diaphragm has room to move before it hits the stators. I'd start with around 2 mm of space between each stator and the diaphragm. If things go well you can reduce that gap and improve the sensitivity, but from the sound of things (or lack of sound) you might be better off giving yourself a little safety margin and using a larger gap. I'm not sure I understand the second part of the question: generally the spacers are used to set the diaphragm/stator distance. Without spacers, there would be no gap and the diaphragm would just be sandwiched between the stators with no room to move.

The spacers do little to dampen resonances. The distance between spacers will influence the frequency of the fundamental resonance, but they don't really damp it.

- If spacers are needed, how thick should they be? Under 1mm, over? I know the dimensions of the stators for definite, but with the stators I am not so sure... :S

See previous answer, but in short: start with 2 mm spacers and drop to 1 mm if all goes amazingly well.

- Does the contact to the diaphragm need to touch the whole perimeter or just a part of the diaphragm? I am fairly certain of this one, but just checking (only a part I think will be ok... )

You can probably make everything work with electrical contact to only part of the conductive coating on the diaphragm. For long term performance and to reduce charge leakage problems you can try an electrical contact around all the edges, but it isn't necessary just to get the thing to make sound.

- Is it best if the alluminium foil contact is flat or rolled up like a sausage? Or does it even matter that much?

I wouldn't thicken the aluminum foil too much by folding or rolling it. That will affect the stator/diaphragm spacing. Use the thinnest contact you can make work reliably.

- I am not able to get access to Mylar film in time. Has anyone had any experience with the film you would use to give car windows a tint, it can be found at Halfords ? Otherwise, any other suggestions for a membrane?

You can try baking bags, or heat shrinkable plastic used to cover windows in the winter. You'll likely cut off some high frequency response if you use heavy plastic, and something that gradually stretches will be hard to keep tensioned, so you might keep those constraints in mind. Again, do something simple just to make some sound and then refine it later if you time to work on it.

- How far does the diaphragm need to be from the stators? I have read around 2mm, but am really not sure :S

This distance should be set by the spacer thickness. See above.

- Would just standard washing up liquid do the job for giving the membrane a high resistivity? Powdered graphite I know also works, but I doubt I'll be able to get any of that

You can experiment with a variety of coatings. I think graphite isn't such a bad way to start. I've been listening to graphite coated diaphragms for years now. If you can't find a hardware store with graphite lock lubricant, you can use a pencil with large diameter lead and some coarse sandpaper to generate some graphite powder. It's not very high tech, and you might have to be careful to wipe off graphite particles that are too big, but it sounds like you're in a rush and just want to make something work. If so, the pencil trick ought to enable you to make some sound.

I hope these suggestions help. The real key to success with these things is to get some cheap and readily available materials that have some hope of working. Put together a couple of really simple and ugly prototypes to figure out what works, and then go for it with the "real" materials. Experience is the best teacher...as long as you don't electrocute yourself in the process.

Good luck!
Few

[a.wayne]

Originally posted by Bazukaz
Hi, you need to read a bit and understand the basics of ESL construction.
There is sufficient information on the web , google is your frend

Instead of mylar you can use baking film(or bags). It is transparrent and strong - has similar properties to mylar , but usually comes in >=12 micron thickness. The film must be streched quite hard , otherwise it will tend to stick to metal sheets.

Graphite is not good for coating - difficult to apply , contaminates spacers , etc. You can experiment with mix of PVA gue / Water / Black ink. Some types of ink work , some won't.
For experimenting , you can use hand soap as well.

Regards,
Lukas.

Why not graphite , isn't this what has always been used to coat diaphragm's when doing electrostatics ....

[frank ziel]

Yes, but it is a mess, it will go everywhere and you have to
rub hard. Why not go the easy way: take Tesa glue, thin it down,
and apply somehow (brush, spray...) and have the right coating. Lukas is right here.

Regards
Frank

[a.wayne]

OK,

Will any glue work ?

[bear]

PVA glue is "white glue" the most famous brand name in the USA is "Elmers Glue-All". But the dollar stores have chinese copies.

Not any glue. PVA glue.

_-_-bear

[frank ziel]

For Europe Tesa is good. It is allready very liquid and christal-
clear, very flexibel when dry and sticks good to most materials
(not to PVC, I think)
Regards
Frank