Hi again, I've been trying to respond to your queries for a couple of days but up until now have not been able to get anything through. Maybe I'm doing something wrong or perhaps because of my long absence from this forum, my posts need to be monitored and accepted by the administrator.
We have not calculated the nominal Tesla field in our electrostatic speakers, as it is not a parameter we are really interested in. ESL's are different to ribbons in that the operating field strength varies greatly with signal levels due to the very high signal voltages used.
The air gap that is chosen depends upon the frequency range you wish to cover and the max SPL you wish to obtain. Low frequency response requires larger diaphragm excursion so requires a smaller gap. Larger gaps affect sensitivity, which is partially improved (but not completely) by increasing the polarising voltage applied to the diaphragm.
Wide panel ESL's do not normally have HF roll-off (in fact quite the reverse) as long as they use good transformers etc. However, HF does become very directional with wide panels, hence the common use of multi panels in commercial designs.
Our products have the air gaps optimised for the individual panels and application. As an example, the ESL III has an air gap of approximately 2.3mm on the bass panels and 2.0mm on the treble panels. While the difference seems small, the field strength is subject to an inverse square law so small changes in gaps create quite large changes in sensitivity (output for a given input).
Martin - Jan's comment about keeping the air gap small for your application is quite correct. If you don't require the speaker to develop deep bass then you don't need large excursion so a smaller gap can be used. Needless to say there is always a trade-off. Smaller air gaps for a given panel size increases the capacitance of the panel. This is reflected back to the driving amplifier by the square of the turns ratio of the audio transformer. So, an increase in say 500pf panel capacitance will mean that there is an additional 5uf capacitance to drive with a 1:100 turns ratio transformer. This is a significant increase in load for the amplifier.
However, as the sensitivity is higher, you may be able to reduce the panel size, reducing capacitance.
Treble dispersion is affected by the width of the surface radiating the high frequencies. Narrow transducers have better horizontal dispersion but have reduce area to create the sound. A reduction in air gap helps to improve output for a given input.
We have selected an active width of 40 mm forr our treble panels as this gives a combination of good dispersion and output.
Hope this helps
