Basic geometry conundrum with scope screen - AVS Forum
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post #1 of 9 Old 02-23-2011, 09:53 PM - Thread Starter
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Been doing a lot of reading the past few weeks, tossing a lot of ideas around in my head. A few basic ones clash. Here's one in particular...

It seems that "general consensus", whatever the heck that is, believes the optimum viewing angle for 2.40:1 scope films is somewhere around 53 degrees based off of THX etc. viewing height and working back to the width. Lots of threads, lots of discussion.

I'm a fan of AT screens with matching center, horizontal stretch anamorphic, and usually curved screen, so let's go from there. Pretty standard L/R speaker placements are at +/- 30 degrees. Guess where I'm going?

My problem comes from trying to reconcile keeping the optimum screen size and geometry for video (or, perhaps we can just say "as large as possible within reason") and trying to avoid things that will cause audio quality issues. Like, say, having a hard screen edge with masking motor and mechanism right up against a speaker causing acoustic diffraction and reflection issues.

Possibilities as I see them...
- Mount all speakers in a baffle flush with screen so no diffraction issues. Puts the center right up against the material. Or offset the center rearwards with delay on the others.
- Mount all speakers substantially behind the screen. Still have a hard screen edge/mechanism, now just a foot or a few in front of the speaker... now even worse for people sitting on the ends of seating rows.
- Pull all speakers substantially in front of the screen and not use a matching center. Bleh. And also narrows the potential seating area width.
- Put L/R substantially in front of the screen and center offset behind with lots of delay. Bleh for delay and spl adjustments much less good tonality matching as power response, direct/indirect ratio, driver/driver integration, lobing etc will be vastly different if there is significant path differences.
- Not use motorized masking at all so that the profile of screen frame is as unobtrusive as possible. ?? Or just invent some acoustically transparent frame and masking system?!?

Now consider this issue if one was also investigating dipole mains, where you really don't want something right beside the speaker. Very few "good" options come to mind. So few that I can't think of any. The best I can do is having screen edge aligned with baffle edge and having as small a frame profile as is possible. Masking would have to be ingenious. I think I can tackle the ingenious part. Now if the speaker baffle is curved on a radius equal to listening distance, you can only match screen and baffle edge for some of the speaker, not all. Presumable HF as that's where diffraction/reflection artifacting will be most notable.

Or, one could simple narrow the "optimum" speaker separation angle, though to have any improvement geometry-wise you'd need to further inside the frame than you would otherwise be outside the frame. So narrower than 53-7 deg, or <= 45 degrees. Probably <40 deg to be any significant improvement. Getting pretty narrow. Not an option I'd likely consider.

Or, you could widen the separation angle to say 65-70 degrees. To be honest I've never experimented with such a wide separation, though I occasionally see pics with at least this much, sometimes what looks like as much as 90 deg separation. Any thoughts on how detrimental this is to imaging, stability, etc.?

Anyone else given much thought to this issue? Obviously a bit more critical when dipoles are in the mix, but still a source of audio artifact with monopoles.

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post #2 of 9 Old 02-24-2011, 03:06 AM
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I'll solve one of your problems - don't use dipole or bipole main speakers.

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post #3 of 9 Old 02-24-2011, 10:19 AM - Thread Starter
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Lol, yeah, dipoles adds a bit of complication. Not a fan of bipole as it makes one problem much worse while making another only marginally better. Going that route I'd rather just step all the way to omnipolar but that's more appropriate IMO for two channel or possibly surrounds.

There are very few negatives of dipoles for theaters once you get past the displacement requirement for ht level spl. One negative is a narrower on axis angle that could potentially narrow seating area, but with attention and measurement aggressive toe-in can actually provide a more consistent l/r spl balance over the width of the seating area than a very wide dispersion speaker.

Oh, and there's this issue I've asked about here.

Dennis, even with monopoles, do you tend to mount at the screen edges, or keep all behind? Or do you generally not use scope screens as often?

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post #4 of 9 Old 02-24-2011, 10:30 AM
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That's why I am staying away from AT screen. I don't want to comprimise my sound in any way. The only advantage an AT screen has over non-AT screen for me would be that I can mount the center speaker at ear height but that's it.

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post #5 of 9 Old 02-24-2011, 02:26 PM - Thread Starter
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I'm confused. I've always considered the inability to use identical speakers for the front three that are oriented properly for desired dispersion and minimal lobing and at matched heights to be a serious compromise in the audio quality. The issue I'm tackling is fairly minor in comparison.

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post #6 of 9 Old 02-24-2011, 04:19 PM
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Quote:
Originally Posted by Bigus View Post

I'm confused. I've always considered the inability to use identical speakers for the front three that are oriented properly for desired dispersion and minimal lobing and at matched heights to be a serious compromise in the audio quality. The issue I'm tackling is fairly minor in comparison.

I don't think that having the center speaker lower than ear height is that big of a crime. You can always tilt it up or down if needed. Heck, when your at a movie theater the center speaker can't be ear height for everyone.

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post #7 of 9 Old 02-24-2011, 08:45 PM - Thread Starter
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Quote:
Originally Posted by stepyourgameup View Post

I don't think that having the center speaker lower than ear height is that big of a crime. You can always tilt it up or down if needed. Heck, when your at a movie theater the center speaker can't be ear height for everyone.

Well, that's why we all get to do things the way we like them!

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post #8 of 9 Old 02-24-2011, 08:52 PM - Thread Starter
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Original problem solved. Problem was caused by having a short throw ratio resulting in large pincushion needing a curvature radius close enough to the listening position distance such that the arc of the front three speakers (if trying to keep distance to LP equal) was followed the arc of the screen close enough to cause the L/R to be very near the screen frame.

Solution, use a long throw distance, like 2x distance to LP. Pincushion shrinks dramatically, and either no curve is required or the radius becomes >4x distance to LP. Thus, if the center is nearish the screen the L/R are pulled far enough in front of the screen edges to avoid terrible diffraction effects. If using dipole, the frame can be placed in the rear wave, which will shift the diffraction artifacts into the power response/room contribution which will be a drop in a large ocean.

Having a thin screen frame with minimal masking mechanism is still important. Creating a motorized masking mechanism with zero additional frame width at the edges and/or mechanism beyond the screen edges should be easy enough to tackle.

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post #9 of 9 Old 02-24-2011, 11:07 PM - Thread Starter
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Quote:
Originally Posted by stepyourgameup View Post

That's why I am staying away from AT screen. I don't want to comprimise my sound in any way. The only advantage an AT screen has over non-AT screen for me would be that I can mount the center speaker at ear height but that's it.

I thought this was worth another response. As many many people consider an AT screen to be a compromise in video quality but still use it BECAUSE of the numerous acoustic benefits, perhaps it may be worthwile to reread some of the discussions over the years and give a second look at whether you think your current setup is maximizing audio as much as possible. Never hurts to step back and take another look at things.

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