Baffle wall with constant directivity waveguide speakers, and toe in - AVS Forum
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Old 12-08-2013, 06:44 AM - Thread Starter
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I've already posted this over on the HT design forum, but have only had a little feedback, so I'm posting here since it's speaker related, and probably more speaker than build:


I've noticed that not many people have a curved baffle wall or have toed in the waveguide speakers as per some recommendations - Bill Waslo has an interesting paper on the subject:

http://libinst.com/PublicArticles/Setup%20of%20WG%20Speakers.pdf

This paper also mentions toe in on a baffle wall:

http://www.triadspeakers.com/cinemaplus/partners/pdfs/pmi/040601_baffled_again.pdf

Dolby (Atmos) suggests that we can toe in the waveguide but leave the woofer flat on the baffle - part 2.6 Aiming:

http://www.dolby.com/uploadedFiles/Assets/US/Doc/Professional/Dolby_Atmos_Specifications.pdf

As I understand it (what little understanding I have), the 2Pi data of a speaker tells us how it performs in a baffle. Procella speakers work well in a baffle wall and are one of Dennis's recommendations, but he doesn't toe in the speakers as far as I can tell. Is it because the kind of spaces we use at home aren't big enough for it to be that big an issue or something else?

I'm going to attempt a baffle wall at some stage, so would like to get it right if possible - as Dennis has said, once those speakers are in, they're not going anywhere smile.gif

Gary

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Old 12-08-2013, 07:23 AM
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once you understand how/why controlled directivity speakers work, angled in baffles make sense. they just aren't practical for most folks.

klipsch discussed the concepts in the paper you liked up 60 years ago.

here is a nice implementation in a home environment:

http://www.cowanaudio.com/finale.html

pnw's octagon theater is quite nicely done as well. not sure if his site is up or down at the moment though.

and another article on the topic:

http://audioroundtable.com/forum/index.php?t=msg&th=10619
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Old 12-08-2013, 09:28 AM - Thread Starter
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Thanks for the reply and links.

My set up will be for a home theatre with an AT screen, all speakers behind the screen. Everything seems to point towards toe-in, but most baffle wall set ups seem to be flat with no toe in, even those with waveguides, which is why I'm wondering why more people don't do it - maybe you hit the nail on the head with the practicalities of making a baffle wall, let alone one with toed in speakers and how you get the screen on the wall (a curved screen seems another option as I'm using an anamorphic lens).

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Old 12-08-2013, 09:34 AM
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That's what has me interested in Parham's cornerhorns. Just makes sense to my untrained mind.

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Old 12-08-2013, 10:36 AM
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I asked Wayne Parham about putting my 4pi speakers in a baffle wall and here is what he said on his forum

"All of my speakers respond well to this kind of placement. They are all designed to be used in quarter-space or eighth-space, and I don't use baffle step compensation filters. My thinking has always been that the baffles are rather large, so the transition happens in the lower midrange, close to where room modes take over. That's not where you really want to bump up the power, but then again, you don't want it to be lean down low either. So instead of including baffle step filters, I've suggested placements that reduce the problem acoustically. And as for room modes, a different but related matter, they can be effectively mitigated using a multisub approach.

Placing the speaker close to the boundary reduces baffle step, among other things. The closer the speaker baffle is to the back wall, the less the baffle step. If you can recess the speaker in the wall flush, the baffle step goes to zero. It also makes the reflection off the back wall go to zero too. Those are both important goals.

Some people try to reduce the negative influence of the back wall reflection by moving their speakers out away from the back wall. That's reasonable, a good idea, I suppose. It reduces the amplitude of the rear reflection because of distance. It increases the delay of the reflection too. There's a window of time where reflections are most troublesome, and by moving the speakers away from the back wall, they hope to decrease the early reflections, and to delay them out of that window.

But I go the other way, for several reasons. It is hard to space the speakers far enough away from the back wall to do much good in most living spaces. Most rooms in peoples' homes aren't big enough. So I use another approach. I prefer to use directional speakers that limit the rear wave in the first place, which decreases the amplitude of the rear wave by virtue of directivity. Instead of trying to make the rear reflection late, I try to make it so early it is indistinguishable from the direct wave. As you decrease the distance from baffle to boundary, when the distance becomes zero, so does the reflection delay. So that's the direction I go.

That's why I like using directional speakers designed to mount right up against the rear wall or in corners. The rear wave reflection becomes less of a problem and baffle step is nearly eliminated, because there is so little transition region. If the speakers are mounted with their baffles completely flush with the walls, or their drivers mounted at the apex of the corners, baffle step and rear reflection are completely eliminated."





On another question I shared with him some test results that Ruben posted on his SMX screens and the measured frequency response and he said:

"In truth, there are a lot of things in the environment that cause 10dB swings in what would be an otherwise perfectly flat response curve in an anechoic environment. Heck, just the reflections off the walls do that in the region below 200Hz or so.

Then again, I'm not sure I would be willing to accept response swings so large in that frequency range, right where our hearing is most sensitive. Here are a couple things I'd try:

First, angling the speakers might help because the much of the reflected energy would be directed away from the speaker instead of back towards the source. The self-interference ripple is caused by a reflection that combines with the source, and is therefore strongest when it is straight on.

When a boundary is a quarter-wave away from the source, the reflection is a half-wavelength late, which forms destructive interference. When the boundary is a half-wavelength away, the reflection is a full-wavelength late, which forms constructive interference. The relationship between source and reflection changes with wavelength i.e. frequency, so the resulting response curve develops ripples, as some frequencies combine constructively and others combine destructively.

If the reflection isn't directed back towards the source, the ripple would be reduced. So angling them inward should help. On-axis sound is at an incident angle that reflects away from the source. Far side off-axis is even further angled away from the source. Only near side off-axis sound is reflected back towards the source. Of course, that's the problem with this approach. These speakers generate sufficient output off-axis that there will be a fairly significant reflection back to the source even with a lot of toe-in. But it's worth trying. Try the recommended 45° toe-in to see if it helps reduce screen ripple.

Second, putting some open cell foam or other semi-absorbant material between the speaker and the screen may smooth the ripple. It would probably only be required in front of the tweeter if the screen is placed close enough. Increase tweeter output slightly to compensate. It would make the reflective "boundary" formed by the screen be somewhat "fuzzy". Instead of having a partially reflective boundary that is at a single distance from the source, you'll have a region of semi-transparent, semi-reflective material for the sound to pass through. The self-interference reflection could not line up with well-defined quarter-wave nodes."



I asked about the open cell foam from Tropicals

"I think this is a good application for this open cell foam. I'd place it between the horn mouth and the screen, perhaps attaching it to the screen in front of the horn so sound has to travel through the span, not just the screen. Don't leave a gap because we're trying to eliminate transitions, having only one, where the sound leaves the screen towards the listeners. You can fill the horn with it as Geddes prescribes too, which will cause the sound to travel through the same sort of space all the way from the phase plug to the screen."



One last thought, with acoustically transparent woven screens you need to be aware that the level of transparency decreases with an increase in the angle.of incidence.
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Old 12-08-2013, 11:12 AM - Thread Starter
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That's a blumming good read Jeff, thanks for taking the time to post it.

So, the only fly in the ointment may be that too much of an angle with a woven screen (will be Seymore XD in my case) would mean the screen becomes less transparent because the angle effectively loses sight of the holes in the weave. I'd read that but forgotten it, so thanks for the reminder because if I'd experimented without the screen in place and found an optimum set up at say 45 degrees toe in, the screen may have a considerable effect on what sound actually comes through it - unless the screen is also curved. Could have been a time consuming error.

The foam sounds interesting - have you tried it and heard a difference?

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Old 12-08-2013, 01:33 PM
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Foam is on my list of I'll get around to it one day. The source is swisstropicals.com I think. It is used for aquarium filters. There are some threads on other forums about filling the wave guides with open cell foam as Geddes does in his designs.
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Old 12-09-2013, 08:59 AM
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Quote:
Originally Posted by Gary Lightfoot View Post

That's a blumming good read Jeff, thanks for taking the time to post it.

So, the only fly in the ointment may be that too much of an angle with a woven screen (will be Seymore XD in my case) would mean the screen becomes less transparent because the angle effectively loses sight of the holes in the weave. I'd read that but forgotten it, so thanks for the reminder because if I'd experimented without the screen in place and found an optimum set up at say 45 degrees toe in, the screen may have a considerable effect on what sound actually comes through it - unless the screen is also curved. Could have been a time consuming error.

The foam sounds interesting - have you tried it and heard a difference?

Gary

I have my SEOSs angled with intersection just in front of the MLP as outlined in your above links.

I have not measured any significant issues with my angled incidence on the XD material. From this I can only assume the difference to be of the magnitude of a couple dbs or so and correctable with EQ in my case since its not obvious in my measurements.

I did not angle the baffle wall although its probably the best practice. I angled the speaker in the baffle and kept the baffle wall through hole tolerances as close as possible and filled the remaining gaps with Roxul.

The baffle wall attempts to keep the speaker radiating half space down to the schroeder of the room. This is mostly beneficial in mitigating the Allison effect as Parham eluded to in the quote above.

As long as the monopole speaker doesn't have extensive( more than a couple dbs) BSC in the crossover it will benefit from the baffle wall installation below baffle step frequency of the speaker IMO.
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Old 12-09-2013, 10:30 AM
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I did some testing of the XD material including distance from material here: http://www.avsforum.com/t/1454505/acoustic-transparency-of-spandex-the-truth
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Old 12-10-2013, 04:58 AM
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Can you still get the benefits of having no baffle step due to having the LCR's mounted in a baffle wall if they are toed in? I would have assumed that the benefits of a baffle wall with regards to no baffle step, are only going to be had when the LCR's are flush within the wall, right?

I would love to have a curved baffle wall with a curved screen, as that seems to be the most simple way of achieving this standard of having zero baffle step. I realize that having a curved screen may not seen necessary at first glance, but, if your L & R mains are towed in behind an AT screen, doesn't that pose a problem with regards to the woven screen fibers blocking some of the waves from the speakers due to them not firing straight into the room?
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Old 12-10-2013, 08:15 AM
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Quote:
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Can you still get the benefits of having no baffle step due to having the LCR's mounted in a baffle wall if they are toed in? I would have assumed that the benefits of a baffle wall with regards to no baffle step, are only going to be had when the LCR's are flush within the wall, right?

I would love to have a curved baffle wall with a curved screen, as that seems to be the most simple way of achieving this standard of having zero baffle step. I realize that having a curved screen may not seen necessary at first glance, but, if your L & R mains are towed in behind an AT screen, doesn't that pose a problem with regards to the woven screen fibers blocking some of the waves from the speakers due to them not firing straight into the room?

You get the maximum baffle step losses when the speaker is in the middle of a large field. The minimum (zero) is with a speaker flush mounted in the wall. A speaker mostly flush mounted but toed in some would have little spreading loss but you would still have all the normal issues with cabinet edge diffraction.
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Old 12-10-2013, 07:33 PM - Thread Starter
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Some interesting reading there.

So far, I think I'm leaning towards a either a curved wall, or a wall with the centre part flat, and the L & R sides toed in, and probably a curved screen to try and keep the material as close to the speakers as possible.

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Old 12-11-2013, 06:15 AM
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Why are you putting your LR speakers behind the screen anyway? Is there really no space for a "frame" on the sides? I find that the wider front sound stage really adds to the theater experience....

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Old 12-11-2013, 06:23 AM
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Quote:
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So far, I think I'm leaning towards a either a curved wall, or a wall with the centre part flat, and the L & R sides toed in, and probably a curved screen to try and keep the material as close to the speakers as possible.
The curved wall is how it should be done but it seldom is, because it's not as easy as a flat wall. A curved screen can result in a distorted picture, so that's probably not a good idea.

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Old 12-11-2013, 07:32 AM
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Why are you putting your LR speakers behind the screen anyway? Is there really no space for a "frame" on the sides? I find that the wider front sound stage really adds to the theater experience....

+1

I like the L/R speakers just to the sides of the screen, but I don't see that done often. I think most people put all speakers behind the screen because the screen takes up most of the width of the room.
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Old 12-11-2013, 07:36 AM
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If I had to do it all over again I would have bought a bigger screen to cover the whole front wall and had the speakers in the corners toed in a bit or whatever gave me the best soundstage. JBL pro speakers have an angle bracket built in to toe in the waveguide. I can angle my speakers a bit(maybe 5 degrees) in my flat baffle wall.

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Old 12-11-2013, 10:23 AM
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I personally wouldn't toe-in behind an AT screen....those horn mouths need to be pressed right up against the front of the screen (but not touching).

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Old 12-11-2013, 10:35 AM
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I should move my screen closer to the baffle, I wonder how the response would look after that. It is probably why the DR's behind the screen rolloff more than the SEOS and JTRS(not behind the screen) after 10khz. I can't measure until I get the EMC 8000 back in the chain.

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Old 12-11-2013, 11:05 AM - Thread Starter
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Quote:
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Why are you putting your LR speakers behind the screen anyway? Is there really no space for a "frame" on the sides? I find that the wider front sound stage really adds to the theater experience....

All documents I see show that for accurate imaging, the LR speakers should go inside of the 2.35 image, but outside of the 16:9, For example:

http://www.thx.com/professional/cinema-certification/speaker-layout-and-baffle-wall/

And the image from the Dolby Atmos paper I linked to above:



Speaker height will vary depending on the seating arrangments.

If the audio is mastered assuming the above to be the way it will be reproduced, then we should get better results than if the speakers are placed elswhere. Having said that, my previosu CIH set up had the speakers outside of the screen and toed in, but seemed fine, even with the center directly under the screen. I want a tidier looking set up this time, with nothing but the screen on show, but also want to try and get it right (ish).

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Old 12-11-2013, 11:09 AM - Thread Starter
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Quote:
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Quote:
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So far, I think I'm leaning towards a either a curved wall, or a wall with the centre part flat, and the L & R sides toed in, and probably a curved screen to try and keep the material as close to the speakers as possible.
The curved wall is how it should be done but it seldom is, because it's not as easy as a flat wall. A curved screen can result in a distorted picture, so that's probably not a good idea.

Hi Bill,

Thanks for the reply.

I will be using an anamorphic lens so a curved screen will help remove the pincushion, as well as follow the curve of the wall. I agree that using a curved screen without an A lens would result in barrel distortion that would have be dealt with, probably by overscanning of the image onto the black borders. There may be focus issues as well. Overscanning is often how pincushion is hidden when using flat screens with an A lens.

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Old 12-11-2013, 11:12 AM - Thread Starter
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I personally wouldn't toe-in behind an AT screen....those horn mouths need to be pressed right up against the front of the screen (but not touching).

The Dolby Atmos paper I linked to above suggests that toe in can be done in much the same way that MKTheatre says. I agree that the horns should be as close to the screen as possible though. I'd like to see the results of things like baffle step within a baffle wall when speakers are toed in on a flat wall. I wonder how much difference it would make, especially if there is no BSC in the crossover. There won't be in mine.

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Old 12-11-2013, 11:16 AM - Thread Starter
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I'm still curious as to why Dennis Erskine never seems to curve his baffle wall projects. He knows a lot about this subject and there is a lot of science involved in his designs, so there must be a good reason for it. He's just not telling. smile.gif

If I could go flat wall and flat screen that would be by far the easiest way to build it, so I'd really like to know why many aren't (other than what Bill says which is probably closer to the truth in many cases).

Gary

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Old 12-11-2013, 11:24 AM
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I personally wouldn't toe-in behind an AT screen....those horn mouths need to be pressed right up against the front of the screen (but not touching).

Good point, however with controlled directivity waveguides such as the SEOS, one can utilize time/intensity trading and employ a large toe in and significantly widen the sweet spot for adequate spatial imaging. Although the CC energy remains well represented, for anything other than dead center listening, the spatial collapse of the L-R image can be a big problem with off axis listening of typical set-ups.

The perceived subjective depth of the recorded event, and the immersiveness that should be commensurate with the wide visual image, likely isn't as important as other elements of the experience. I'm guessing it's more akin to the icing on the cake, but possesses value none the less.

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Old 12-11-2013, 11:29 AM
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I have my L and R outside the 16:9 and inside the 2.40 and the sound stage is plenty big for me.

Also FWIW angled speakers behind a flat screen are not really a problem as long as the AT properties of the screen are decent at the incidence angle. It actually should help the reflected energy off the screen material fall outside the baffle onto the treated baffle wall and produce less comb filtering.

This is why many recommend facing the baffles of the speakers with absorption and certainly treat the baffle wall with absorption.

Here is an image of what I mean:



As far as the center I'm not sure if it should be pushed up as close as possible to the screen or as far away as possible. I would assume it depends on the fabric. Greater distance reduces the intensity of the reflection contamination and closer proximity pushes the affected frequencies higher up in the passband. Measurement would likely be necessary in individual cases to determine best practice.
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Old 12-11-2013, 11:40 AM
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I'm not sure how much of that Atmos paper applies to the HT. Commercial theaters don't typically have space on the sides of the screen. If you go a few pages past that you can see that it is assuming a screenwall that covers the entire front wall. Of course, some homes do this as well and then it is the only choice.

I generally don't hold THX, Dolby or DTS recommendations as absolutes. There are so many variables to take into account that I think experience and experimentation should trump their guidelines. The Atmos stuff has to account for so many things that don't apply to HTs. Most Dolby, THX and DTS stuff I see has to account for speakers ranging from Bose cubes to multi-way horns. Then you have the fact that mastering studios all seem to do things a little differently.
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Old 12-11-2013, 12:43 PM
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Originally Posted by Gary Lightfoot View Post

I'm still curious as to why Dennis Erskine never seems to curve his baffle wall projects. He knows more about this subject than anyone on the forum (or perhaps anywhere for that matter)
Speak for yourself. rolleyes.gif
Where you usually see baffle walls are full sized theaters, and they're flat because at the listening distances involved toe in would be detrimental. If I was to do a baffle wall for a typical HT it would be curved, not only for the reason of toe in, but because a curved wall is more resistant to flexing, would result less difference in distance between the various speakers and the LP, and would smooth room response by eliminating a flat boundary.
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Bill Fitzmaurice Loudspeaker Design

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Old 12-11-2013, 01:14 PM - Thread Starter
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Originally Posted by Bill Fitzmaurice View Post

Quote:
Originally Posted by Gary Lightfoot View Post

I'm still curious as to why Dennis Erskine never seems to curve his baffle wall projects. He knows more about this subject than anyone on the forum (or perhaps anywhere for that matter)
Speak for yourself. rolleyes.gif

Apologies Bill, and thanks for your comments.

I'll edit my post accordingly smile.gif

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Originally Posted by elmalloc
Who says Cameron is "right" and why do we care about him so much - lol!

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Old 12-11-2013, 01:43 PM - Thread Starter
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Originally Posted by coctostan View Post

I'm not sure how much of that Atmos paper applies to the HT. Commercial theaters don't typically have space on the sides of the screen. If you go a few pages past that you can see that it is assuming a screenwall that covers the entire front wall. Of course, some homes do this as well and then it is the only choice.

I generally don't hold THX, Dolby or DTS recommendations as absolutes. There are so many variables to take into account that I think experience and experimentation should trump their guidelines. The Atmos stuff has to account for so many things that don't apply to HTs. Most Dolby, THX and DTS stuff I see has to account for speakers ranging from Bose cubes to multi-way horns. Then you have the fact that mastering studios all seem to do things a little differently.

My screen will be pretty much wall to wall as well, as unfortunately here in the UK our room sizes aren't on the same scale as those you lucky guys seem to have. The THX image does have space to the sides, but the speakers are still within the screen.

I agree that sometimes experimentation and experience can help determine what works best at home for individual circumstances (many of us on these boards spend a lot of time with meters etc measuring stuff and doing calibrations), but Dolby etc have been doing this a long time, and along with people like SMPTE, certain methods and standards are determined which guide the way things should be done for best results. They've certainly invested more time and money into doing the research than any of us here have, so I can't just dismiss the white papers etc that are out there unless there is good reason or proof to the contrary. A domestic set up may have good reasons to move away from certain commercial recommendations due to the size differences involved, and I'd like to see that information if it's the case.

Bill has said what he would do and why. If I'm going to move away from what seems to be best practice, I would really like to know why.

Gary

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Old 12-11-2013, 02:08 PM
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I'm not saying those whitepapers are bogus. I'm just saying that they are always a significant compromise unless, like in the case of the Atmos paper you are building a commercial theater from the ground up. The commercial theater world is rather homogenous relative to the HT world. You always have 2,3 or 4 way horns and multiple rows in commercial. Coverage patterns are vital so no seat is missed.

HT applications have such a broad range of possibilities, the suggestions simply don't hold much weight for me. THX and Dolby's home recommendations aren't even considering CD horns. They aren't considering baffle walls and AT screens.

I'm also not suggesting that having your L/R speakers behind the screen is bad. It can work quite well that way. I've just experimented with horns in corners and it is the best approach I've experienced. Of course if it is not feasible then that settles that.

Here is a link that talks about corner placement theory: http://audioroundtable.com/forum/index.php?t=msg&th=16684
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Old 12-11-2013, 02:10 PM
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