Panel horizontal tension

MartinLogan Audio Owners Forum

Help Support MartinLogan Audio Owners Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.
Status
Not open for further replies.

beanbag

Well-known member
Joined
Dec 29, 2013
Messages
150
Reaction score
0
Location
sf bay area, CA
This thread is for discussion regarding my "controversial" statement that the ML curved panels don't have much horizontal tension in the diaphragm. Maybe after the moderators move some posts over, I will add more explanation and reply to certain points.
 
Just one solid, accurate, realistic, correct and terse explanation will do beanbag. No need to bother the bosses, surely.
 
Agreed Justin.

Please proceed beanbag.

As they say in tennis, your serve.

The mods have better things to do.

PS: May I respectfully request that the basis for discussion begin on this thread with your following statement made on the "Neolith" thread which, I believe, is the initial source of the controversy.

Horizontal tension goes away quickly after the panel is assembled.
 
Last edited:
BTW, the panels only have tension in the vertical direction. They can't have tension in the horizontal direction because then you couldn't maintain a curved shape.
So I don't see how it is particularly harder to set the tension in this panel vs any other.

Sure they apply initial horizontal tension to take out the wrinkles and get the film to lay evenly over the clear spars. But that horizontal tension goes away quickly after the panel is assembled.

This thread is for discussion regarding my "controversial" statement that the ML curved panels don't have much horizontal tension in the diaphragm. Maybe after the moderators move some posts over, I will add more explanation and reply to certain points.

Instead of progressively running away from your bold (and incorrect) initial proclamation that "the panels only have tension in the vertical direction", wouldn't it have been easier to just admit it was wrong and move on? Surely that would have been less embarrassing than this. Starting a whole 'nother thread to discuss an irrelevant topic? Just so you can backtrack on your initial statement in order to defend it and proclaim you were never wrong? Sad.

The funny thing is, your salient point of the original post (that it shouldn't be any harder to set the tension on this panel vs any other) was correct, but not for the (incorrect) reason you stated. It isn't any harder because ML no longer applies the Mylar solely by hand as they used to. They now use a mechanical contraption to set the tension (in both directions) and press it onto the frame assembly (as seen in the second video RUR posted).
 
Here goes:

This is a free body force diagram on any particular chunk of mylar:
flat-element.PNG

Thin films can only exert forces in the plane and not perpendicular to it. The two horizontal tension forces sum into a net force that pulls the chunk down into the rear stator.
Nothing opposes this force, which is why I said that "it is theoretically impossible for a curved...."
The way to maintain this shape is to have zero horizontal tension.

But if you DO have some horizontal tension, the only way to oppose this force is to have a vertical tension. This pulls the mylar chunk into a saddle shape, with two edges pulled downwards, and two pulled upwards:
saddle element.PNG

The "angle" that the force pulls at is approximately equal to the inverse radius of curvature in the horizontal or vertical direction. So the final force balance equation is Tv/Rv=Th/Rh. Usually the radius of curvature in the vertical direction is very very large since the panel is flat in that direction. To get the equation to balance that means you either need a very very large vertical tension, or a very very small horizontal tension.

For now, suppose the vertical and horizontal tensions are equal. (I'm going to show why this is wrong.) The radius of curvature in the horizontal direction is... umm, I dunno, but for a 9" wide panel, the edges are recessed by about 1" or so. In the vertical direction, take a spar spacing of 3". For the same radius of curvature (required if the tensions are the same), the droop is approx the square of the length of the segment. So (3/9)^2=1/9". Nope that would jam the film into the into the rear stator. The horizontal tension has got to be wayyyy less.

The ML has a stator spacing of... I dunno, it was either .04" or .06" but in any case, a reasonable upper bound for the sag might be around .01" or so between the stators. That works out to about 12x less horizontal tension than vertical in this case (using very approximate math). So check your own speakers to see how much sag you have between the spars and that will tell you how much horizontal film tension you have. Whatever it is, it is still a very small fraction (or none if the film is totally flat) of the vertical tension.

You can check the total tension by lightly poking at the film with an insulating stick and see how much it deflects. On mine, it deflects fairly easily, so I know the total tension is low. That means the horiz tension is very very low.

As to the videos showing them laying out the film. Yup, you can see the sag between the spars as they lay it out. But then they take the panel out of the jig. The horizontal tension is going to cause the perforated sheet metal to unbend slightly, while the vertical tension mostly remains because the sheet metal is really hard to bend or compress in that direction. Or maybe the tension settles out very quickly because the glue they use has flow properties. (why do some speakers develop wrinkles, and why do panels slowly slide down?) Whatever is the reason, I only look at the final assembled product to gauge the tension and not the assembly procedure.

To answer some questions:
The panel doesn't need any horizontal tension to operate - vertical tension alone can provide a restoring force to drag the film back to the center.

Rich: I don't distinguish between "none" and "very very small".
 
I am afraid the response from you Beanbag is of no interest whatever, given that, as pointed above you have recanted on your original statement that people found ridiculous, (and that ML video evidence contradicts). As you have already been reminded, you stated

"What is the difference in sound between a panel with high tension and low tension anyway?

BTW, the panels only have tension in the vertical direction.They can't have tension in the horizontal direction because then you couldn't maintain a curved shape."

Hence any response to your thread other than to repeat the above would be a collusion with your shifting your initial premise.
As such this Dragon declares "For that reason, I am out". I won't waste my time on this thread and genuinely hope you won't either, Beanbag.
As we say in Scotland "Your bum's oot the windae, pal"
 
Nice long post combining mathematical formulas with a heap of speculation, unsupported assumptions, and outright guesswork in order to come to the conclusion that you didn't really mean exactly what you said in the first post that started all this discussion. That is the longest, most tortured, and weakest admission of "my statement was inaccurate" I have ever seen!

You don't distinguish between "none" and "very, very small"? What? Mr. engineer? Mr. mathematical precision? I find that statement laughably sad coming from you. What's more, I find the "very, very small" comment completely unsupported by the evidence. If you look at that video, it is clear that the person is putting more than just a "very, very small" amount of horizontal tension on the Mylar. She is stretching it pretty hard.

Is there less horizontal tension than vertical tension? Could be. Wouldn't surprise me. Although nothing you have said is sufficient to convince anyone of that from a scientific perspective. And your unsupported explanations, like that the horizontal tension goes away quickly while the vertical tension remains, are enough to give one pause in believing anything you say. The panel un-bends when they take it out of the jig? Well, it gets re-bent and re-tensioned when they put it in the speaker frame. The glue flows? If that were the case, then it would lose much more tension in the vertical dimension since you contend that tension is much greater to begin with.

Ultimately, it is clear in the video that the worker is applying a lot of force to create horizontal tension as well as vertical tension, and that fact alone means your original comment that led to the entire discussion was in fact wrong. It would be harder for a worker hand-pulling Mylar over a panel the size of the Neolith because they do have to apply tension over the horizontal, as well as vertical, planes. Had you just admitted your mistake when you were called on it, you would have saved yourself a lot of back pedaling and jumping through hoops to try to prove the validity of an incorrect statement.
 
Last edited:
The horizontal tension is going to cause the perforated sheet metal to unbend slightly.....
I understand the physical principle, but this will depend upon the amount of tension and the stiffness of the stator, no? Tension may or may not be zero @ equilibrium.
 
How about you Gordon? Comments?

The serve appears to be a good one - now's the time to return it.

Adam,

No. It still doesn't provide a reason, that I can understand, for a quick loss of horizontal tension after the panel is assembled.

Seems to be pretty good serve but the line person called a "foot fault". ;)

Maybe the second serve will be better.

Gordon

PS: Could be my inability to understand is because I'm not a physicist or a mechanical engineer.
 
Last edited:
Ah, I realized something this morning. The reason there is very little horiz tension in the film is because mylar is relatively inelastic (more like PAPER than Saran wrap), so what happens when you put it on the tension rig, is that all the horiz tension / force goes into squeezing the spars down into the back stator. The unsupported bits of film are able to relax and thus have low tension. In the vertical direction, the tension does not squeeze down on the spars and thus can go towards film tension.

Rich: I already said very early on that "if there are remnant bits of horiz tension, then that will cause bowing...". And then later I went on to explain why horiz tension is very low anyway, both from observation and also why it is still low even if you really lean on the tension rig. So quit complaining about the difference between none vs very little, and not caring about the difference between very little and a lot.

More later...
 
Beanbag, you seem to be spending a lot of time chasing straw-men and explaining away your initial comment, so lets just bring this full circle. Your initial comment was:

BTW, the panels only have tension in the vertical direction. They can't have tension in the horizontal direction because then you couldn't maintain a curved shape.
So I don't see how it is particularly harder to set the tension in this panel vs any other.

Based on what you know now, was that statement correct or incorrect?
 
I understand the physical principle, but this will depend upon the amount of tension and the stiffness of the stator, no? Tension may or may not be zero @ equilibrium.

yes
If I assume the film has no shear-type deflections (not true), then there still would be a little tension at equilibrium.


In practice, it would depend on the elastic properties of the mylar. For hypothetical example, you could lean really hard on the tension rig, and tug on the mylar in just the right way, such that the film will pull so hard on the spars to unbend the stator, that the segments between the stators might actually lose all or significant horizontal tension. The way to tell is the sag profile in between the spars. Maybe it initially sags down quickly from the spars, indicating high-ish horizontal tension, and then be flat in the middle section, indicating no tension.
 
Last edited:
I'm not really sure. In principle, depending on the elastic properties of the mylar, you could lean really hard on the tension rig, and tug on the mylar in just the right way, such that the film will pull so hard on the spars to unbend the stator, that the segments between the stators might actually lose all horizontal tension. The way to tell is the sag profile in between the spars. Maybe it initially sags down quickly from the spars, indicating high-ish horizontal tension, and then be flat in the middle section, indicating no tension.

If I assume the film has no shear-type deflections (not true), then there still would be a little tension at equilibrium.

But are you considering the tendency of the curved stators to want to maintain their profile, and the force (pulling) they apply to the film? In fact, the horizontal tension is only limited by three things:

1) Ability of the film to resist being pulled apart
2) Ability of the stator(s) to resist being deformed
3) Tendency of the film to dip in between the spars, and ultimately contacting the inner stator (which is insulated (electrically))

The third factor listed above can be mitigated by increasing tension in the vertical direction, limited only by factor 1. In fact, you may be able to have significant tension in both directions, but I would assume and would argue that the vertical tension exceeds the horizontal tension by a factor of two or more (a guess)

Good. Lets talk about Subwoofer enclosure materials now :D
 
but I would assume and would argue that the vertical tension exceeds the horizontal tension by a factor of two or more (a guess)

Oh good, another reasonable reply.

How did you come up with the factor of two? I came up with around 12 or so.*


*Note that I asked a question regarding somebody's supposition without questioning their motives or making commentary on their personality. A shining example of civil discourse, LOL.
 
*Note that I asked a question regarding somebody's supposition without questioning their motives or making commentary on their personality. A shining example of civil discourse, LOL.

I won't question your motives or comment on your personality, except to question whether you have any ability to admit you made a mistake. Absent that ability, it's difficult to give any credibility to anything you say.
 
A shining example of civil discourse, LOL.

Civil discourse was well displayed by comments you made in the "Shatki stone" thread, which resulted in the thread being closed. LOL indeed.

PS: Lest anyone forget, here's beanbag's last post on that thread prior to its closure.

I have a response to this, but it would take a long time and feelings would get hurt along the way. So it will have to come out in little bits at a time, at opportune moments. LOL
 
Last edited:
Oh good, another reasonable reply.

How did you come up with the factor of two? I came up with around 12 or so.*


*Note that I asked a question regarding somebody's supposition without questioning their motives or making commentary on their personality. A shining example of civil discourse, LOL.

Actually, I believe the ratio of tension would be proportional to the ratio between panel width versus height. The reasoning behind this theory is as follows:

When applying tension to a film, or anything else, such as applying rotation torque to a fastener, what you are trying to achieve is a uniform amount of stretch to satisfy a number of requirements:

1) Useful tension to generate crisp sound waves (for the panel), Clamping force (for the fastener, an example),
2) Giving the ability to maintain that tension over time by bringing the film or fastener near to its' yield point, or tensile strength

The ratio I mentioned is simply that when applying tension to any object in a measureable or precise way, what you are really doing is stretching the object X amount per unit of distance (EX: 0.050" per inch). Therefore, to achieve the same surface tension on the vertical and horizontal plane of an electrostatic panel, you would be required to apply proportionally more force in the vertical stretch because there are more "inches" to stretch to achieve to desired surface tension, and that requires more force (directly proportional).

Sorry. All that is wrong.

The force needed to achieve uniform surface tension over the horizontal and vertical plane is the same, because the same number of square inches are being stretched. I imagine the actual amount of tension was determined largely through trail and error, gaining insight through the results thereof, and then making informed modifications using the information gathered in the initial phase of trial and error. It would be a very delicate balance between sound quality and efficiency, I think. I suspect the strength of the film is not the limiting factor. If efficiency were not a factor in the equation, then it is logical to me that with more surface tension in the panel film, sound quality would be increased because the film would react more quickly (push and return to rest), but efficiency would be reduced because more energy is required to move/excite the panel.

I don't want to carry on too far because these paragraphs that I've written are ALL theoretical, and written during my first cup of coffee. Also, I'm getting carried away thinking of ways to re-invent the wheel, so to speak. Now I want a ton of surface tension and a 10000 watt amplifier.
 
Status
Not open for further replies.

Latest posts

Back
Top