Taming/Damping Electrostat Backwave


In my understanding of the physics of the situation, the signal coming off the back of an electrostat panel is the same signal that comes off the front though in opposite phase. If there are reflections off a back wall, they cannot be a better signal than the one off the front of the panel. It strikes me that in a strict sense, if one could COMPLETELY eliminate the backwave on electrostatic speakers (a giant silent sound vacuum, sucking in the sound off the back of the dipole), this would be, in the words of the once famous and now infamous [:)] Martha Stewart, 'a good thing'. Am I missing something? Is there any argument to support not trying to eliminate the backwave through all means possible?

My Martin Logan SL3s sound reasonably intolerable when too close to the back wall, great when a certain distance away, and in my limited, ad hoc, distinctly non-scientific (not to mention bad WAF) experiments, even better when I put a variety of dampening material between the panel and the back wall (even when the wall is 6ft back).

Does anyone have a view or experience on the "complete backwave elimination" strategy? Do you try to eliminate it entirely? Do you leave some backwave in for 'flavor'? How do you deal with it? Put shag carpeting on the wall? Hire tall sheepdogs to sit on stools calmly for hours on end a la Fay Ray? I would love to know how other people deal with the backwave issue...
t_bone

Showing 1 response by rives

The back wall reflection of E-stats is very important. For one it comprises probably 35% of the total sound that you hear (in the panel frequency range). Secondly, the back wall reflection is short in terms of timing. You can calculate this by taking the distance (ft) from the panel to the back wall and dividing it by 1130 ft/sec. So let's say you have 6 feet from the back wall. The reflection will arrive at the listener approximately 20 milliseconds after the first wave. Since this is less than 35 milliseconds your ear/brain integrates this up as part of the original signal and it's probably 50% or so lower in db from the original signal. This ads to the ambiance and spaciousness. If you eliminate this reflection you will probably not have enough high and mid frequency energy and the speakers will likely sound rather dull and the sound stage will collapse, but if you have 2 extra mattresses in the house and some down and wool blankets--there's a pretty easy experiment you could try.
So you probably wonder why did the speakers sound bad when they were close to the back wall. The panels are curved and when they are two close they may have focused at near a point on the back wall, creating a very strong point of reflection that might have had near the energy of the first wave that reached you. This would do two things: throw the speakers out of balance with too much high frequency energy, and 2 while the reverberation time is okay, the second point source with nearly as much energy would not be okay. It would mean rather than your ear perceiving ambiance, it would hear notes that were not clearly defined because of two signals near the same strength.