When I first heard of elecrostatic speakers, I was astounded at how simple they were, and how I didn't think of them before myself. Basically an electrostatic speaker moves a diaphram using electostatic force, rather than electromagnetic force as in conventional speakers. They are usually a charged sheet of plastic film suspended between two conductive sheets called stators. The stators are idealy acoustically transparent but in reality are perforated metal.
By applying a balanced audio signal to the stators, the diaphram is made to move in time with the audio signal. Because the diaphram is so light (about the weight of a sheet of air a few mm thick!) there is little overshoot and ringing (unlike conventional speakers with heavy cones) so there is verry little distortion.
Of course reality isn't quite that kind, and to make this work, there must be a BIG charge on the diaphram (up to 8000V) and the voltage swing on the stators must also be several thousand volts. Still, the diaphram excursion is minimal so the panels must be quite large to reach respectable SPLs. SPL can be increased by putting the stators closer to the diaphram but then the movement and hence output of the diaphram is reduced. There are a lot of balancing acts like these in ESL construction.
Exploded view of what the panels are like, please note it is not to scale, the holes are actually only 1.5mm in diameter.
I first saw ESL's in a hi-fi shop, they were Martin Logans and they had a price of several thousand dollars on them. This seemed expensive for what they were... Then after I heard a pair of Quad ESL's I decided to have a crack at making a pair myself.
I was quick to realise that building a successful pair of these would mean 90% experience, 10% common sense, So I bought Roger Sander's book 'The Electrostatic Loudspeaker Design Cookbook' which was extremely valuable.
There are three parts you must find that are hard to get: Stators, mylar and transformers. You can use valve amp output transformers in reverse, or buy new ones from the esl exchange for $45. Mylar can also be got there, or you may find a Dupont distributor near you who will sell you a single roll. Mylar is so thin that a roll will have several Km's of it. Crazy. The stators are either tensioned wire or perforated metal. Perf. metal is way easier but hard to find. You must get metal that has small holes (1-2mm) that are REALLY close together (at least 40% open area, staggered pattern) and pretty thin too. It will be quite pricey, but they will probably have a big gilotine there to cut it to your liking. I had a cut plan which left me with enough perf metal left over to make 2 small 2' esls as tests of the construction process. This is a GOOD idea and I recommend you do it if you can cause there is nothing worse than ruining material of which you paid a lot for. You make 90% of your mistakes on the first one, so it is good if you can afford this to be a junker.
The other stuff you need is glue - epoxy and superglue, graphite and spacers. These physically seperate the mylar from the stators, so they have to:
- be a certain uniform thickness as per the design
- be able to stand off thousands of volts
- thats about it...
I used polycarbonate. I got a strip cut off a sheet of 1.5mm polycarb so it was cheap. If you live in any sort of city you will find a plastics supplier that will sell you cut quantities of polycarb or acryllic.
I have only made the small ESL test panels so far, but I have rigged up one and it works! ESL sounds WAY different to conventional speakers, they are crisp, detailed, like a good pair of headphones. They are also VERY directional - it makes for good imaging but more importantly I think the sound goes straight to your ear, rather than with a wide dispersion conventional speaker where it goes straight to your ear and also to the walls and then to you ear, all the different reflections 'smearing' the sound.
Of course they are GUTLESS in the bass, and need to be crossed over at around 500Hz.
Roger Sanders explains this sort of ESL 'theory' a lot better than I do so I suggest you buy his book if you are thinking of building a pair of ESL's. You won't be sorry, the cost of the book is recovered several times because you won't make the usual mistakes!
I was amazed a messy panel of epoxy and plastic could sound to good!
After constructing these ESL's I made an Electrostatic Headphone with quite good results