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...
I have thought of building a large sealed cabinet onto the back of a set of E-stat's or Planars just to see what it would do. While i think that the backwave contributes to the "space" and "air" that you can get out of a set of these speakers, i also think that it creates a whole batch of problems on its' own. Sean
SL3 likes having its back wave diffused rather than absorbed. It also needs a lot of room behind. If you don’t have that much room, SL3 is not the right speaker. You won’t be able to achieve good results by damping the back wave.

The best way to diffuse the back wave is to install some professional diffusers. But they are expensive. If you want something cheaper, you can use plants, either real ones or fake ones will do.
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.
Rives notes an important point about the SL3s -- their curved surface is like a part of a cylinder, the focal area is really a near-vertical line where the axis of the cylinder would be. Consider putting an effective mid-high frequency absorber like ASC Tube Traps at the focal area. Good luck.
Jamesswei's and Sidssp's sugestions are good ones from my experience with the old CLS's and the old Monoliths. I have owned 3 pair of Maggies as well and they benifit from the same thing. They all like space behind them. The fact that they don't have a cabinet is part of why they sound the way they do,no cabinet resonences to deal with. I have a friend who has Sound Labs and the sound is difficult when things get loud. His room is too hard and reflective. When you put on an acoustic jazz trio the potential is revealed. I have used the tube traps with sucess myself and they alow for some tuning where the results with permenent difusers are harder to predict. If you don't like the sound with the trap just rotate it or move it around until you do.

Sean, you might remember Harold Beveridge did exactly what you are proposing, and also went as far as using a bizare JBL style lens in front of the stat panel to control dispersion. I remember that they could sound quite nice but never heard them in a controlled situation. It always seemed to me like it was an approach that added some of the problems associated with box speakers.

I remember the old Dayton Wrights, the Koss electro stats, I had a boss with double KLH 9's, I also sold Quads and Acoustats at different times, they all have an aluring quality, you just need to acomodate their needs with some space around them and some room treatment. I had to go back to dynamic speakers from a stand point of practicality. Good luck and don't give up, you will be rewarded when you get 'em set up well.

Here is a horror story for Quad fans. I ran across some people while working in a HiFi shop who had inherited some speakers the knew nothing about. After talking with them I determined that they were Quad 57's. They told me about how they liked to watch the light show of sparks inside the speakers when the turned them up REAL loud. It about made me cry!
Has anyone ever tried mounting maggies or stats inside a wall that divides two different rooms, so that you get a monopolar wave in each? Just curious if anyone has tried it and what your results were, in particular whether the reflections from one room came back through the panel into the other.
Didn't Genesis or Infinity do something like this? Any one ever try using two pairs back to back to cancel the out of phase rear wave and create an in phase back wave? Just curious.
Room Lenses work wonders behind 'stats. DIY lenses using 5 or 6 tubes are even better or you might want to make noodle lenses. Check out Audio Asylum for info. I recently heard of using bi-fold louvered doors from Home Despot. These are said to be great diffusers and are easy to set up. Just fold them enough to stand up and place them behind the speakers. WAF might be a factor.
T Bone -

I'm pretty much going to say the same things Rives and Sidssp said, just in a different way. So feel free to skip over this post.

Assuming the "holy grail" is to recreate the concert hall experience, let's look at what's going on in the concert hall, and then see what we'd want to do with your SL3's to incorporate some of that into the listening room.

In the concert hall, first thing you hear is the direct sound of the instruments (duh). Then there's a considerable time lag - up to several tens of milliseconds - before the reflected energy starts to arrive. This reverberant energy is very powerful but also very diffuse, and lasts for several hundred milliseconds. This powerful, diffuse, late-arriving reverberant field is largely responsible for the rich timbre and enveloping sense of ambience in a good hall.

The backwave of your SL-3's can be your secret weapon in creating a better approximation of the concert hall than most speakers can.

Ideally, you want to position your SL-3's about 6 feet out into the room. This will allow adequate path length for the backwave energy to arrive late enough (10 milliseconds minimum) that it won't screw up the imaging, but will instead enrich the timbre and the feeling of ambience. Assuming your room's tonal balance is good, you'd want to use diffusion on the first backwave reflection points, to the inside of the speakers along the wall behind them (I use fake ficus trees). You might also want to treat the first sidewall reflection points, though with the fairly directional SL-3's this might not be necessary. Once again diffusion is preferable to absorption, unless your room is too bright. And even then, a little absorption goes a long way. The reason of the diffusive treatment of the first reflection points is that strong, distinct early reflections can skew the imaging, but we'd like to preserve that energy to enrich the timbre and spaciousness.

Okay, now let me come at the same issue again, this time focusing more on voice and instrumental timbre than on concert hall-ish acoustics. The tonal balance of the reverberant field has a significant impact on the perceived timbre. The bass of your hybrid SL-3's is essentially omnidirectional, which means there will be lots of reverberant energy in the bass. The panels are much more directional, and so put less energy into the reverberant field, relatively speaking. If we use absorption to soak up the backwave of the panels, there will be a severe shortage of mids and highs in the reverberant sound field. As a result, the speakers will tend to sound precise, but uninvolving and lifeless, and eventually fatiguing. You see, a significant discrepancy between the direct and reverberant fields contributes to listening fatigue, so we want to keep 'em as much alike as possible.

My advice is always free, and worth every penny.

Although I disagree somewhat with Audiokinesis' theory about ambiance recreation, I do agree with his point concerning power response. (I believe speakers should ideally reproduce the actual ambiance captured on the recording, not introduce spurious additions of their own, something which the listening room is already going to be unavoidably doing anyway - although one can always make the argument that recordings fail to adequately capture all of the natural ambiance, and therefore we are forced to 'add some back' through any means necessary. My problem with that is the fact that the added 'ambiance' will be constant in character [as indeed are all listening room signatures] and unrelated to the soundfield of the actual event recorded, so that while it may sound pleasant in certain instances, it is still a distortion. However, that's a disagreement about ideals, not about panel speakers per se, which can offer fewer side reflections than monopoles, and properly positioned and diffused, I do agree that dipoles can offer their own intrinsic strengths which allow them to more than compete with typical box radiators under the right circumstances.) But to affirm his admonition against over-damping in different words, the speakers you buy are balanced for optimal in-room frequency and power response *without* having to resort to a LEDE (live end, dead end), quasi-anachoic listening room set-up, so if you over-damp by absorbing the backwave, the resulting frequency balance will not be as intended.

From my own (admittedly limited - I've never owned any, yet!) experience with 'stats, I have found that placement is highly critical, especially distance from the front wall, with practically every inch potentially making a difference. I also have found that, even with curved panels, a little judicious toe-in can still have a beneficial effect in redirecting the reflections and altering the perceived direct/reflected sound balance, even when diffusion treatments are being employed.

Unsound, to address your typically creative suggestion about using two pairs of panels back-to-back to achieve bi- instead of di-polar propagation, the phase inversion aspect in a dipole really matters the most not in the reflected waves bouncing around the room - where the phase will quickly become all jumbled up with any radiation pattern or ?-pole design anyway - but at the sides of the panels where the waves wrap around and cancellations occur. If one were to take speakers designed for dipolar operation and alter the figure-8 radiation pattern through the introduction of out-of-phase interference, then presumably the side cancellations accounted for in the original design and voicing of the speaker would be violated, upsetting the correct response. I think.

I've always been interested in the ideas Sean and Karls mention. I believe the Wisdom Audio planar-magnetics incorporate a monopolar panel radiator through the use of a damped rear cabinet (in their current form - the first generation were dipoles with open backs). But considering the inherent problems introduced by all cabinets, the in-wall, open on both faces, infinite side-baffle configuration seems fascinating (and what a cool way to get multi-room sound!).
I experimented this weekend with speaker placement, using my Logan Aerius i. I took the speakers and placed them so that they had about 7 feet from the back wall, and also place some different "objects" (furniture, books, a lamp, etc) behind the speakers to break up the backwave.

I was surprised to find that the sound became thinner. The music didn't have the same rich depth or fullness that I acheived with the panels simply 3 feet from a plaster wall.

This made me think that, as Zaikesman says, Martin Logan designed these speakers with careful consideration to the backwave.

I don't know if all logans have them, but the Aerius have pieces of transparent thick plastic stuck to the backside of the panels. These may act as sufficient breaks in the backwave.
My experience with Quad ESL 63's might be slighly helpful, in that it tends to confirm your main intuition about total absorption of the backwave. At first I built DIY Styrofoam diffusors imitating the RPG Omniffusor, and placed them on stands parallel to the back wall and about 2.5' on average behind the toed-in Quads. That was a big improvement over having nothing behind the Quads. Then I became persuaded that absorption, not diffusion, was the way to go with this backwave, and I filled in the diffusor wells with polyester fiberfil (from a fabric store), and that improved the sound quite a bit. I haven't gone further than that because as far as my ears can tell, I don't have to. I also don't know quite how I'd proceed if I wanted still more absorption, without messing up the looks of the room, making it hard to get at windows, and so on--annoying practical difficulties.

When I believed in diffusion, I once advised a Martin Logan owner to try Argent Room Lenses behind his. I tried them behind my Quads, for which they're a wrong shape, and they didn't work at all well. That figures, given the wrong shape and the fact that absorption, not diffusion, is wanted. Further, the Room Lenses don't absorb at all, while my DIY Styrofoam diffusors--design info supplied on request, and on Room Lens clones too, by t he way--definitely do.
tom, my experience with 63's was some what parallel to your's but with different results. I tried deadening the back wave, partially and totally with poor results. I ended up with the speakers about 4 1/2 feet from the back wall toed in about 15 degrees with the back wave firing into irregular materiels, i.e. books, plants with round plant stands, etc including the corner of the room. the speakers were firing down the length of the room to a listening position 4 1/2 feet from the back wall. Gsreat result w/ flat bass/midrange frequency response.

Dennis, when you pulled your speakers out 7 feet from the wall you may have successfully eliminated any bass reinforcement from the nearby walls - this would emphasize the upper frequencies and make them sound thin. most manufacturers assume that there will be wall reinforcement, especially in smaller systems w/cone speakers(as i recall your speakers have a dynamic woofer?).
I have owned ML's for over 10 years, and currently own ML Ascents. My current room treatment is a combination of four 2'x4'x1" acoustic panels and two ASC Tube Traps. The speakers are 50" from the rear wall, 26" from the side wall and toed-in about 15 degrees. The corner tube traps have the diffuser facing the speakers, with one acoustic panel mounted behind each speaker, but 24" from the side wall so they do not completely absorb the back wave. I then mounted one acoustic panel to absorb the reflection from the back wall. Since the panels are curved and slightly toed-in, side wall reflections from the back wall and the tube trap diffuser will confuse and brighten the image. The results are a very stable image, beautiful detail, but very relaxed with the speakers all but disappearing. Hope this helps.