Acoustic Transparency of Spandex - The Truth - Page 2

Thank you!

Did you have a chance to check how the comb filtering is affected if the screen material is not set parallel to the speaker baffle but is at , say, 30 to 45 degree angle instead?

I cannot see the weave from my seat, approx 11 ft from the screen.
I am not sure "sharpness" is the right term, but IME with moleskin, it was as if I could not get the picture fully focused. it may be more obvious with text, especially white text over black background. the edges seemed washed out some how.

Yes, I own a set of 4Pi speakers. I did not mean to make you jump through the hoops, sorry. I have the needed equipment and should be able to find out myself. Still it would be great if you find time for this in the next round.

There is another potential approach to fight the reflections, something that Wayne Parham suggested. The idea is to use open cell foam like the one Geddes plugs his wave guides with, but extend it all the way to the screen. It would attenuate the signal somewhat but in more uniform way that can be addressed with EQ or crossover compensation. And in theory this would take care of the comb filtering. I am not sure if the foam could be used with dome tweeters though

I am pretty sure it was never tested. Wayne suggested this in BigmouthinDC HT build thread.
http://audioroundtable.com/forum/index.php?t=msg&th=15168#msg_num_8
It was one of those things that everybody wanted to test yet never got around doing anything to make it happen.

As to the thickness, Geddes wave guides are faily deep, around 10", I think. I do not know how much worse an extra 3" would make it. And I will never know unless it is tried out

Milliskin would give significantly better results, both "AT" and "Image"-wise. It is smoother of weave, and reflects light better when adequately backed with another layer of tightly stretched Miliiskin.

I was hoping there would be enough left from my recent 135" - 2.35.1 White over Silver Milliskin build to provide for a test, but alas, the 135" size gobbled up too much of the White ro match up with the amount of tighter stretched Silver left over.

Looks like the only way this will get done is to order up a full, dedicated yard of each.

Great work, Tux! I'm in the process of planning my next screen (AT) and hadn't even considered the Spandex. Is there a significant cost savings over the XD material?

Thanks again for all the work and information.

Also - Anyone have a link to the white JoAnn spandex? What widths are available? I searched on their site and kept coming up with narrow strips.
Edited by Gorilla83 - 2/27/13 at 8:19am

Great work Tux. Thanks

I am a little confused. What exactly is the "double Moleskin" product in your first graph? And what exactly is "Spandex World doubled" on the second graph?

Just a bit more clarity would be a big help.

I assumed Graph 1 is the Moleskin material that you bought in Canada?

And Graph 2, is the Moleskin from Spandex World?

Thanks,

Hey Tux,

I'm just getting to stretching the material right now. Sorry for the lack of updates but other life projects have gotten in the way. I should be able to send something to you early next week.

-e

I agree that up close, the spandex is definitely smoother than the XD. The XD material is comparatively thicker, but from my viewing distance I can't see the weave at all. I took a pic with stretched moleskin silver, moleskin white and the XD material. If I were doing spandex I would highly recommend going white over grey - the grey moleskin is just too dim. The white stretched over the XD produced a similar brightness which is, to say, nice and bright. As far as picture sharpness, I don't have any hard facts to validate my claim, but the picture is sharper. It's mostly noticeable when there's text - it's almost like a halo around the text that reduces the sharpness not unlike the pj being slightly out of focus. Is the spandex bad? Hardly. In fact, for a DIY it's pretty awesome. Is the XD worth the extra coin? To me it is simply because its a BIT sharper and much easier to install for MY screen. Lots of people have had an easy time installing spandex on a flat screen, so that shouldn't be a deciding factor. If you're on a budget, I would recommend sticking with the spandex. If you can sport the coin, I'd go with the XD. Thanks Tux for the info. Apparently I need to move my speakers closer. I thought they sounded good, but ignorance is bliss!

Done. They are about 5-10 mm away from the screen as opposed to 5". They are front ported, but I haven't noticed any problems with the screen - it's pretty taut.

Tux,

I hope you can get some tests done on the Milliskin. Some Posters are getting both fixated and obnoxious about Spandex's failings AT wise. Even using the heavier Moleskin it doesn't deserve that sort of press.

Well...perhaps so if a dual layer is used. Sheesh...that makes me cringe after having used the Milliskin! So some real test results will serve to prove or discount any AT advantage Milliskin has over Moleskin.

However as far as I'm concerned, visually...there is little question left.

Analogous to how in speaker or subwoofer testing, the levels will be recalibrated in the receiver...

Is it a valid comparison to have three screen samples simultaneously driven by the same projector signal?

Or ideally would the projector need to be adjusted for each sample, so as to not crush blacks or have gray blacks.

 

For instance, on your whiter sample with the blacks not as rich, in calibrated use, one would really lower brightness at least, if not adjust contrast too. 

 

It makes for a more time consuming comparison, because then the images need to be placed next to one another in Photoshop (I could help with this if you wanted). 

 

It is good that you're holding the shutter speed at a fixed 1/3 s. When switching between different samples with different projector settings, I think the aperture should be fixed as well, to make sure that relative brightness/contrast differences are picked up by the camera.

 

Can't wait for the Milliskin test!

If it is at all possible, please put a layer of black cloth or paper behind the CS XD material. That's what Seymour recommends to prevent light that passed through to be reflected back on the screen causing loss of contrast. They sell black AT backer if the wall behind the screen is not painted dark color. This of course is exactly the opposite to what multiple layers of spandex are supposed to be used for.

So if doing in-walls behind an AT screen the wall behind the screen should be painted black?

I think that is what Chris Seymour would recommend for XD cloth. Alternatively a layer of black backer can be used. My understanding the recommendation has to do with what he calls "cross-pixel light contamination".

did you follow the "what is happening" thread? Poster curttard presented a very convincing argument about limited utility of screenshots as a way of comparing picture quality because "any projector and any screen can look as good as any other in a screenshot".

But contrast, according to him, can be objectively represented and compared across different environments if the same procedure is followed:
take two pictures of ANSI checkboard or greyscale ramp test pattern, one over- and one under-exposed, and then "simply compare the lightest and darkest steps that DON'T clip"

Tux,
I finished stretching the screen last night and I've got a bunch of light silver and white milliskin left over. My plan is to send you both unless you think that's a waste. Of course, I can't think of anything I'd use it for.

Quick question (for everyone): I stretched both layers as tight as I thought necessary. I can't see anytHing behind the screen with the light on and I can't see the light silver layer through the white like you can on the one MM just made. I'm wondering if I didn't stretch it tight enough.

Often, much of what you read coming from Mfg Screen advocates is not only biased against the idea that any non-specific Mfg Screen material replacement simply cannot be worthy, often it is simply based upon a total lack of experience. I myself, while limited in experience with Spandex, had no trouble ascertaining that the decidedly smooth weave of Spandex would be more than acceptable "Image-wise" and on par or better than Paint in many instances. Especially when measured against Rolling or poorly done Spray attempts. Once I made actual visual determinations, even the difference between Moleskin and Milliskin weaves showed there was a marked advantage toward using the Milliskin variety.

Throughout DIY Screen making, many have chosen to make issue with the difference between using a White or Gray. Almost always using a comparison between both, with neither really optimized individually for the specific use. White will always show "brighter" of course, but brightness isn't the "end all" many seem to hold it to be...as you seem to have come to realize. The use of a light Gray / Silver has some very definite advantages.

As far as the relationship between Spandex's tighter weave and a Mfg's AT construction (Perf or Weave) there can be no doubting that spandex offers up more surface area for light to reflect off of. But it also is much thinner and absorptive, effectually negating any potential advantage as far as gain. However as far as it having a tighter surface area, that is why it can deliver a sharper, if not indeed a slightly more attenuated image.

Adequate lumen output can absolve one of that loss in brightness. but you cannot effectively overcome a loss of reflective surface area....because the amount of available surface area is what generates the density potential of any given image.

I know you have gotten quite a lot of "feedback" from varied sources. Some perhaps a bit less than congenial. The truth is that those who care enough to really listen and delve into the real, actual needs of any given DIY'ers situation are more apt to be of help in both the long and short run than those who rail about how others don't adhere to what they consider as being the only legitimate way things should be done.

A prime example is the reasoning that a light, or semi reflective backing behind a White Spandex Top covering is detrimental. It is not...and reasoning that goes beyond rhetoric should easily show that anything that can reduce the amount of light lost will offer up a better end result than employing something that effectively absorbs all light that get's passed through the initial reflective surface. White is of course best in this regard...with Silver / Light Gray being good for enhancing Contrast without allowing excessive amounts of light lost.

Personally, I hold the approach you have taken as being quite beneficial and well measured. It is, I'm sure, appreciated by many.

My understanding of the subject is superficial at best. But from what I read, the way you compared black levels may be flawed.
If PJ is calibrated for low reflectivity grey fabric then naturally the blacks on a higher gain white would look washed out. It does not mean that the white fabric is not capable of reaching the same black levels with proper calibration (given there is no ambient light). A good way to compare using the same PJ settings was suggested by poster curttard. The idea is to project a grey scale ladder calibration pattern on both samples and then look at the actual dynamic range each can achieve. To do that just compare the darkest and the lightest steps reproduced on each material without clipping.



Edit:. this is not to doubt your preference, not at all. I wish I was this lucky and found spandex material I liked better than CS XD.
Edited by zheka - 3/11/13 at 12:59pm