New way for absorbing back wave?

Through this forum, I have become a believer in absorbing the back wave of the electrostatic panels of my M_L speakers. It became particularly evident as I repeatedly ran the YPAO compensator from my Yamaha RX-Z11 and had to move the fronts way out into the room to avoid getting the "phase error" message. As others have observed in this forum, I must be getting all kinds of comb filtering if the setup compensator is so bothered by the "phase".

The best solution I could find would have been to use a RealTraps Mini HF trap behind each M-L Ascent i, mounted out from the wall 4 in. But try as I might, the asthetics police (wife) would not accept ANY absorber panel on "her" walls. Then a brainstorm. I purchased the 2' x 4' RPG foam absorber sheets and cut them down the middle, lengthwise. A stack of three of these half-panels friction fits exactly behind the electrostatic panels of the Ascents. As best as I could tell from the data sheets, a stack of three of the RPG diffusors has the same attenuation as the minitrap HF in the frequency range of the electrostatic panels.

The Mk I ear test had the back wave inaudible in music play. I ran YPAO with the Ascents near the front wall, toed in to the sweet spot. The level compensator showed +6db, or so on each side. I put a triple layer behind my M-L Theater center speaker (which is behind the screen) and it showed the same approx +6db boost. The "polarity" check was correct, according to YPAO, where in the past placement this close had a "polarity error". Best of all, all of this was invisible to the wife.

I am offering the opinion that stuffing the back panel of the Ascents has more or less eliminated the back wave. Imaging is terrific. Anybody see any downside to this placement?

How about a picture?

I'm curious to what others might think.... If this speaker sounds better by stuffing absorption material directly behind the panel as noted above... Why didn't Martin Logan just enclose the stat - and stuff the cabinet full of foam? I don't think any electrostat is built like that - I may be mistaken - but, I thought part of the magic from this speaker was from its dipole design.

timm said:

I'm curious to what others might think.... If this speaker sounds better by stuffing absorption material directly behind the panel as noted above... Why didn't Martin Logan just enclose the stat - and stuff the cabinet full of foam? I don't think any electrostat is built like that - I may be mistaken - but, I thought part of the magic from this speaker was from its dipole design.

Click to expand...

My thoughs as well at first reading ted99's post. What ran through my mind was "so essentially make it a box speaker?"

But maybe I'm not getting his drift - I agree with amey01, how about a picture (it's worth a thousand words you know).

What's old is new again! See...
http://www.martinloganowners.com/~tdacquis/forum/showthread.php?t=4815&highlight=aerius+foam

ted99 said:

Through this forum, I have become a believer in absorbing the back wave of the electrostatic panels of my M_L speakers.

Click to expand...

Really?!

It became particularly evident as I repeatedly ran the YPAO compensator from my Yamaha RX-Z11 and had to move the fronts way out into the room to avoid getting the "phase error" message.

Click to expand...

I seriously doubt the Yamaha room analyser software can handle a dipole speaker's radiation pattern. So why turn your stat into a box speaker just to satisfy the needs of your (rather limited) analyser software?

If you can't get the stats out at least 5 feet from the wall behind them, then you should think about another kind of speaker. Also, you will need a wall mounted flatscreen to avoid degrading the image with a large inert surface/object placed between the panels. Dipoles need breathing room because the backwave contributes to their overall SPL, but they also need good, bust-it-all-up diffusion (NOT absorption) of that backwave. This is necessary to minimize the diffraction effects that inevitably occur when the backwave (by now in a multitude of busted-up phase reflections we hope) finally meets the still-coherent frontwave.

I totally disagree with Neil on this one. I have tested good diffusers behind the MLs vs. good absorbers and there is no question to me that absorption is the way to go. For most listening rooms, there is simply no way to diffuse the back wave of the speaker sufficiently to avoid reflections and comb filtering, which will diminish your clarity, imaging and soundstaging.

It is a myth to say that absorbing the back wave turns MLs into a box speaker. My speakers before MLs were Def. Techs. They were dipole box speakers. But they were still box speakers. My Summits with back wave absorption are still electrostats, believe me. There is a huge difference between the two. The back wave isn't what gives electrostats their great sound. It is the size and light weight of the mylar diaphragm and minimal crossover points that gives them their speed and incredible sound.

The method you have chosen does have potential problems, though. If you stuff absorptive material too close in behind the diaphragm, it can't breathe. It needs to be able to move air quickly in both directions in order to respond properly to the signal. I would be concerned that you are smothering your panel a bit by stuffing the absorption so close to the mylar diaphragm. I would recommend some space, at least a foot or so, between the speaker and the absorber so that the panel can breathe.

It seems that I recall that absorbing the back wave should only lower your speakers' output by about 3 db, so the 6db boost sounds high. Maybe JonFo can give us some input on this.

nsgarch said:

If you can't get the stats out at least 5 feet from the wall behind them, then you should think about another kind of speaker.

Click to expand...

Five feet?! I disagree. Circle gets the square.

Rich said:

I totally disagree with Neil on this one. I have tested good diffusers behind the MLs vs. good absorbers and there is no question to me that absorption is the way to go. For most listening rooms, there is simply no way to diffuse the back wave of the speaker sufficiently to avoid reflections and comb filtering, which will diminish your clarity, imaging and soundstaging.

Click to expand...

I suspect your results are what they are due to room constraints rather than the relative merits of absorption vs. diffusion. In a 24' long room with speakers 8' from a wall behind which has only moderate diffusion (e.g. a half-full bookshelf) there is simply no way for the rearwave to meet up with the frontwave in time to cause comb filtering -- the path length is just too long.

Now, it may be that rooms are too small, and/or people can't bring speakers into the room 8 feet or more. I've been in that situation myself and yes, I covered the entire wall behind the speakers with quilted furniture blankets, but I considered it a compromise, not a solution. It certainly doesn't change the science in terms of what the optimum configuration would be.

My current setup is almost optimum If you look at the picture, the speakers are about 4 feet from (the back of) a freestanding bookshelf/partition (which is a pretty good 'abfussor') but the bookshelf is itself 8 feet into the room.

When you are thinking about speaker placement, it's good to visualize things from a bird's eye view. With dipoles, there is a 20 degree, V-shaped 'dead zone' extending left, or right, off each edge of each panel. As long as the path length of any reflected wave is at least 8 feet longer than the direct sound reaching your listening position (i.e, 7 microseconds later than the direct sound) it will not affect imaging. Comb filtering will occur if the front and rear wave can combine at launch -- and the dead zone prevents this from happening, but not if the speaker is too close to the wall behind. If on the other hand, you take steps to eliminate the rear wave, you eliminate the dead zone too, and then the front wave can behave much like with any standard speaker, wrapping around the enclosure and interacting with all four walls. It is the opposite of letting a dipole 'breathe'.

First, I’ll address the OP’s setup:

Putting the foam that close is not advisable. For one, it is reflective to some degree, so that foam is indeed causing some comb filtering in the mids and highs as they are reflected back through the panel at 1ms or so.
The foam is blocking most of the mid-bass and other lower frequency rear wave, so those frequencies should appear much cleaner.

However, as Rich points out, that’s still not ideal. One should have the absorbers on the wall behind, with enough offset to trap the lower frequencies well, and of a type that also absorbs enough higher frequencies to reduce their level by 3db or more relative to the front wave.

Secondly, the diffusion vs. absorption thing is both confusing and controversial, I understand.

I won’t argue with those that prefer a diffuse soundfield from their dipole speakers. For 2ch systems, it has some relative merit if positioned just right. If that’s what floats your boat, then fine.

It just is not the most accurate reproduction of the recorded music. It can’t be, as we are taking a single phase signal and reproducing it with a device that spits it out in-phase and out of phase (the rear wave) into the room at the same time.
The differentials in arrival times of the two signals will cause destructive interference of the original wave forms. Please play with the tank simulators referenced here. Make sure you pick a dipole radiator.

Now, I do espouse the use of Diffusion on my rear channels, as I feel the added ‘ambience’ is helpful for those speakers and the fact that they are located closer to the listener, and the added reflections from the diffusion are helpful.

So for front speakers, Absorption will absolutely deliver a cleaner, more accurate signal to the listener.

I seem remember an article in Stereophile about 10 years ago concerning suppression of the backwave. In the article one of the reviewers had a a pair of MLs for review (not sure which model) and Gayle Sanders came over to help with setup.

I think the reviewer put some kind of damping material behind the panel and mounted on top of the woofer cabinet. Apparantly Gayle Sanders was surprised at how good the sound was without the backwave and was going to look into this idea further.

JonFo said:

.................we are taking a single phase signal and reproducing it with a device that spits it out in-phase and out of phase (the rear wave) into the room at the same time.......

Click to expand...

Are you saying, then, that there are similarities between Martin-Logan and Bose ? :devil: I have my flak jacket on in anticipation of the responses

BTW I agree with you that absorption is the way to go.

In a large hall or auditorium, the sound of any one instrument or voice radiates in all directions; and its radiating wave IS phase coherent in all directions, Most likely though, it will be out of phase with all the other sound waves of the other instruments/voices. We don't worry about comb-filtering in an orchestra! Soundwaves from a hundred and fifty instruments are bouncing all over the place. In phase, out of phase, and yet we still know where to look when the brass or strings or woodwinds make their entrance. Why is that? Because it's a LARGE room, and there's just no way any secondary reflection will arrive anywhere close (in time) to the direct sound.

So put away your fiberglas, stop whining, and buy a bigger house :bowdown:

Image!

Well my time to chime in. My system in this current house is bordering the edges of a very large picture window. Not my choice of course, the last house had a dedicated audio room in a finished basement. We are currently working (meaning the early planning stages) on putting a room above the garage for such a space.
ANYWAY. After listening to all this, I have had trouble with a nice image with my system in its current location. The image is not as tight and focused as I like. I have of course narrowed this issue to a problem with the window. (Curtains are out guys. The window overlooks a 20,000 gallon koi pond/waterfalls so its a great visual feature.)
I just tried this AM hanging a thin folded blanket on the back of my Ascents. There is what appears to be a handle jutting out at the top of the Ascents. I used that in a sense as a towel rack. The effect was not subtle, I'm still not sure I liked the sound
I have been listening to some old favs that I remember having a nice image presentation at my last dedicated room. It did seem to "muffle" (just a tinge), the upper partials I am still in the evaluating mode. A friend of mine is on his way over to help with the e-val.

As a side note his system consists of the Gallos Ref #3. A unique speaker in its own right. For its size it throws a huge sound-stage with very adequate bass. I have been trying to convince him to bring-em over to do a A-B comparison, but he hasn't bit on that one yet. I would take my Ascents over to his place, but as you know they are quite large to move. Besides his listening ares is more conducive to the smaller Gallos..........Off topic! sorry guys. Has anyone with Ascents tried this blanket idea out there? Would someone? Just to see what you think. It only takes a 2 min. and is undone in about 10 sec. I only draped the blanket to the top of the woofer cab. And remember its a thin blanket, not a comforter.

Doug - out

nsgarch said:

We don't worry about comb-filtering in an orchestra! Soundwaves from a hundred and fifty instruments are bouncing all over the place. In phase, out of phase, and yet we still know where to look when the brass or strings or woodwinds make their entrance. Why is that? Because it's a LARGE room, and there's just no way any secondary reflection will arrive anywhere close (in time) to the direct sound.

Click to expand...

This is a red herring argument. Listening to a recording reproduced on a speaker is a completely different beast than listening to a live performance. First of all, the sound in an orchestra comes from lots of omnidirectional point sources, not from a true dipole stereo setup. Second, the hall in an orchestra is designed for perfect acoustics. The room is not just large; it is huge. And it is designed for perfect acoustic reflection & diffusion. I would also imagine that the ratios of distance from sound source to rear wall to listeners are way out of proportion for most orchestra listeners as compared to the standard listening room.

Although orchestras sound great, I have to admit I have never experienced the kind of pinpoint imaging and soundstaging with an orchestra that I get with my Summits. It is just an entirely different experience.

I will agree with you that with a much larger room, you can use diffusion and minimize the comb filtering effects of the rear wave. But to say that you shouldn't own ML's if you don't have a huge room is just ridiculous. If your theory held true, the majority of people on this forum would not own MLs. Most listening rooms are just not that large. Proper absorption of the rear wave allows Martin Logans to sound their best in any room.

Rich, I'm not sure why the kind of "hyper-imaging" you describe would be desirable -- if indeed it's even on the recording (so much depends on the mic-ing.)

But I didn't mean to imply one needs an enormous room to let their dipole 'run free' so to speak. A listening room need only be big enough to keep direct and secondary reflections far enough apart in time so the brain will recognize the reflection as a reflection. If the brain combines the two, that's when imaging goes to hell. The standard that has been established is that an echo, to be heard as an echo, must arrive at least 7 msec after the direct sound. If you divide 1180 ft/sec by .007 sec, you get 8.25 feet (difference in path length.) Not a hard thing to achieve in a room of roughly 17 x 27 feet or more (is that ridiculously large?). Perhaps your last sentance should say, "Proper absorption of the rear wave can allow MartinLogans to perform acceptably in smaller rooms."

Photo is attached of the top of the Ascent, with three thicknesses of RPG foam behind panel. So, as I thought, it is not a new idea, but it is a more recent way to use a currently available generic material. I will not comment on the diffusion/absorption/bi-pole arguments. Others have covered these arguments is some detail and I suspect that this is a subject much like "high-end" cables and power conditioners: there are as many opinions as there are users.

I'll comment on the idea that there may be a form of loading of the backwave because of the proximity of the foam. I offer the opinion that the manufacturer of the foam says that the foam is porous and therefore allows the backwave into the material, where it is attenuated, and that additional thickness allows for greater atttenuation. The manufacturer further states that the foam is shaped in the way it is to allow some backspacing above the retaining wall so that there can be a "flow" of the propagated soundwave. My application is completely clear at the back, so the effect could be at it's maximum. I'm open to informed opinion on this.

As I said, if one accepts the notion that back wave absorption is desirable (and some disagree with this), the ideal would have been a mini-trap HF spaced out from the wall behind the speaker. Since the room "owner" vetoed that, I looked for an invisible methodology.

This seems to be an old idea using a new material. As it happened, the minimum order quantity of the 2' x 4' foam is 6 sheets: exactly enough to put 1 1/2 sheets (cut to 12 in width) behind each Ascent, and three full size behind the Theater center speaker.

nsgarch said:

Rich, I'm not sure why the kind of "hyper-imaging" you describe would be desirable -- if indeed it's even on the recording (so much depends on the mic-ing.)

Click to expand...

That's funny. I would refer to it as proper imaging, not hyper-imaging. And it absolutely depends on the recording. If it wasn't mixed right, the imaging won't be precise. I just know that my speakers in my listening room image much better than any concert hall orchestra I have ever listened to. Ultimately, I think that is one of the differences between stereo reproduction in an acoustically-treated listening room and live reproduction in a large concert hall.

nsgarch said:

Not a hard thing to achieve in a room of roughly 17 x 27 feet or more (is that ridiculously large?).

Click to expand...

It's not that I think that is ridiculously large. I just think probably 90% of the listening rooms of ML owners are much smaller than that. My room right now is 14' by 19'. My room in my last house was 20' x 22'. Maybe we should do a poll and get a feel for what size the average listening room of the ML enthusiast is. I thought JonFo's room was pretty big, and even he saw a great benefit from absorbing the back wave.

nsgarch said:

Perhaps your last sentance should say, "Proper absorption of the rear wave can allow MartinLogans to perform acceptably in smaller rooms."

Click to expand...

No. I stand by my earlier statement. I have heard Summits in several rooms and systems. I have heard Prodigies, Ascents and Monoliths in multiple rooms and systems. And I have heard what absorbing the rear wave can do for the Summits and Ascents. Proper absorption of the rear wave can allow Martin Logans to perform their BEST in ANY room, in my humble opinion.

Ted,

Looking at your photo, I have to say that I think that method is detrimental to your sound. First of all, you are not letting your speaker breath. The mylar is very light and has to be able to move freely back and forth between that stators in order to reproduce all frequencies evenly. By putting that much foam right behind the stator, you are creating a pressure barrier that is not allowing your mylar to respond correctly to the signal. Also, with that foam you are probably not absorbing lower frequencies very effectively, so you are still getting some reflection and comb filtering at the lower midrange/upper bass frequencies. I can't imagine that you are going to get your best sound from your speakers using that method.