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Old 5th January 2012, 05:51 PM   #1
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Default A Back Wave Baffle for Martin Logans

I (like many planar speaker owners, I suspect) are forced to have my Martin Logan Sequel IIs too close to the front wall. After some experimentation, I began to understand that the upper midrange glare and diffuse imaging I was hearing was due to the back wave from the dipoles. And, after some other mods documented on this site, this back wave seemed to become more energetic to the point of making the speakers tiring. I was able to prove the need for damping the back wave by clamping a couple of sheepskin rugs to the back of the speakers. I needed a more permanent solution to damping the back wave that would also address the WAF. While in my local Lowe's I spotted a product called ProVent by ADO Products. It is thin plastic, 14" by 48", costs $1.88 each and looks like this:

[IMG]Click the image to open in full size.[/IMG]

The ProVent is a perfect width for affixing to the back of the Sequel II. I suspect that a few other models of Martin Logan have the same 14" width so ProVent will fit also fit those models without modification. Also, the ProVent has wide flanges where it would attach and those flanges could be cut down to accommodate speakers down to about 11" wide.

For the Sequel II the ProVent must be cut down to 42" length. Scissors are adequate to cut the vent. You can try using the ProVent as a rear baffle just as it is. For temporary attachment I used sticky putty. In my situation, placing the ProVent behind the Sequels helped, but they still sounded bright overall. I was also concerned about cancellation effects from sound reflecting back from the ProVent.

I considered various materials for absorbing the back wave. I rejected fiberglass for health reasons, rejected rigid board sound insulation because it would be difficult to work with and due to expense; I rejected Sonex-type foam again due to cost and also because it deteriorates over time. At my local craft store, I found 1" loft polyester batting at a cost of $14.99:

[IMG]Click the image to open in full size.[/IMG]

To cut the batting to fit into the cavity of the ProVent, cutting the batting along its 96" length into three strips of 9" wide, 10" wide and 11" wide. I then cut each strip into two 48" lengths. Before cutting, its a good idea to put the batting in the dryer for 10 minutes to increase the loft and take out creases.

[IMG]Click the image to open in full size.[/IMG]

I affixed the three strips to the inside of the Provent using spray glue: first the 9" width then the 10" and finally the 11". I let the extra 6" length of the batting trail out what would be the bottom of the baffle. The completed baffle looks like this:

[IMG]Click the image to open in full size.[/IMG]

I then temporarily mounted the baffles on the back of the Sequels using sticky putty. With three layers of 1" polyester, the speakers sound great. BTW I also tried the baffles with only one layer of 1" batting and they sounded absolutely terrible--worse then without any damping at all! Here you can see the reason for the extra 6" of batting on the bottom of the baffle:

[IMG]Click the image to open in full size.[/IMG]

As you can see, the baffles are quite handsome mounted on the back of the Sequels:

[IMG]Click the image to open in full size.[/IMG]

I just received self adhering velcro tape that I ordered from Ebay and will be using that for the permanent attachment of the baffles.
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Old 5th January 2012, 06:06 PM   #2
tyu is offline tyu  United States
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WOw Rob.....looks like it would work well....like it was made for the ML speakers......but do you need to kill all the back wave..?
Hay it your setup an your ears an if this works for you .....vary cool....
If You Dont Go You Dont NO...Thanks for the time an info an pic.
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Old 5th January 2012, 11:17 PM   #3
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I have done a little experimenting with dampening the back wave before and this is what I have found,

Beveridge vs traditional ESL?

There maybe another thread where I describe my setup in more detail and if I find it I will post the link for you.

The change in the sound was subtle but noticeable.
I didn't much care for it as it takes away from the sound of them being free and open and effortless, persay.

But if this doesn't bother you then I would say it is a viable alternative as it is a very subjective topic and I will do some more studies on this sometime again.

I use my little panels on my computer desk and they sit back in the cubby holes and there is quite alot of of reflection and diffractions going on.
I used the corner of the little woofer box as a beam splitter and that worked very well for my nearfield situation.

The idea of reducing the back wave appeals to me very much in some applications as my room and listening space is much too small to properly setup a dipole speaker.
I am not new to this as I also have a set of Apogee Duette's.

But as I mentioned the ability for the free movement of the diagphram and the loading of it effects how it sounds and is what concerns me the most.

I have been investigating the possibilty of close proximity reflecting surfaces behind the diagphram.
It works as well and needs to be investigated further as Charlie's Beam Splitter setup seems to work very well for him.
I have also done a few experiments of using some glass panels directly behind my little panels of 3.25" X 9.75" at different angles.

All of my tests were done in mono to check for such lobing characteristics so listening in stereo will be interesting for me as I am quite behind on my projects.
But the main reason for listening in mono was to not be distracted from having two sources and other reflections caused from multiple sources.

I have been thinking more of a cylindrical type of reflector directly behind the diagphram and I am think about trying this with my next build of some 6" wide by 8' tall panels.
Although the diffractions could cause the lobing effect to worsen.

It really depends on your room as mine is 10.5 wide by 18.5 deep and the side wall reflections reverse my right to left information by the time it reaches my listening area that is 12' to 14' away from the speakers, as there is no room treatment at this time.
So in my case, them being very directional is a good thing.

jer

Last edited by geraldfryjr; 5th January 2012 at 11:19 PM.
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Old 6th January 2012, 08:01 AM   #4
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A cylindrical diffusor will do the job. I've done that in the past.
I got my inspiration from E. Fikier who presented its hybrid design in the magazine Elektuur; it consisted of three pvc tubes, one of them being the backwave diffusor.
Unfortynately such large (pvc) tube doesnt't look very pretty and you have to mount it somewhere below and above the panel. I don't know whether the tube still causes some backwave reflaction as I didn't perform a/b comparison with a totally free standing esl, but the tube can placed against the wall and will sound better than without a tube in that case.
heavy damping with baf seems to worsen sound quality to my ears.
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Old 6th January 2012, 11:21 AM   #5
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Very cool,MJ !!
I would like to see more info on that technique if it is possible.
What is the ratio of the tube diameter compared to the width of the panel.
This is something that I have contemplating for quite some time as I don't need too wide of a dispersion.

I have used flat surfaces at different angles with some success.

I had also tried closing up the back of the panel in very close proximity with some glass and it had an interesting effect as well.
It seemed to raise the efficiency by some factor due to the instant combining of the back wave to the front wave from the reflection off of the plane that was only no more than 3/8" away from the diagphram.

That is about all of the experimentation I did as I was working on transformers and diagphram coatings at the time.

I know I had expressed my findings in more detail on trying to stuff the back of the panel with some fiberglass insulation, But I just spent 3 hours trying to find the post and failed to find it.
I think it was part of a thread where the last half of the thread some how for some reason got deleted.
The "Different idea's for esl's" thread as I had posted alot of info there and now it is gone and I remember when it happened as I was not to happy.

jer
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Old 6th January 2012, 11:47 AM   #6
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What I remember is that the diameter of the pvc pipe was around the same as the width of the mylar film. The distance between the pipe (outer edge) and film is approx. 20 cm. See Do It Yourself - Electrostatic Speakers - Project: ESL 175 by Martin-Jan Dijkstra for a picture.
Please note the construction techniques described are outdated; these were some of my first projects.

According to E. Fikier the tubes can filled with sand (he put the dynamic woofer on top of the tubes) , but using them only as diffractor this is not needed.
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Old 6th January 2012, 02:41 PM   #7
tyu is offline tyu  United States
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Rob
One thing about geting more or what should be output of the Any panel, an the top being tipup......all one has to do is lean the speakers back....kant...i think the seq dont lean back much.. an at a 90dg you do get more out put...But if you look at the SL3 on up you well see thay can lay back....the new ones like summit i had here for a time are almost at a 45dg....sounded way rolled of..thay were dark...the owner was freaktout when i pulled them up...thay open up....looket weard the base was facing more to the floor....anywho...there are pasives way to deal with the top being to hot...keep on diying...thanks
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Old 6th January 2012, 03:18 PM   #8
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Thanks,MJ, I remember now.
Your project was one of the very ones that got me inspired to finally start building ESL's since Roger Sanders original articles.

jer
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Old 6th January 2012, 07:39 PM   #9
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Quote:
Originally Posted by geraldfryjr View Post
I use my little panels on my computer desk and they sit back in the cubby holes and there is quite alot of of reflection and diffractions going on.
I used the corner of the little woofer box as a beam splitter and that worked very well for my nearfield situation.
Jer: Can we see a pic of these little ESL's of yours?
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