Taming/Damping Electrostat Backwave

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.

Duke

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?).