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Which early reflection worse - ceiling or side wall?

post #1 of 27
3/22/12 at 8:26am
Thread Starter 
I ask because my far side wall reflection is dangerously close to my flat chair rail. Discussing this with Dennis E, he said that there's no issue so long as the chair rail is below the ear, but my chair rail is 2" above the first row's ear. That said, I plan to have my speakers centered with the screen (pretty high), so the actual reflection point is just above the chair rail, into the Linacoustic. Others I've discussed this with say it's no big deal, but I don't know...

My question about which is worse stems from the fact that most HTs do not seem to treat the ceiling and so an early reflection from all three LCRs is guaranteed. Also, the energy from the ceiling bounce will be higher, as there is less distance traveled compared to the distance for the far speaker. And, for the same reason, the delay off the ceiling will be less than the delay from the side wall.

That said, I'm wondering if the reflection's direction makes a big difference. Could it be that our ears can discard ceiling bounces, whereas a side bounce is more readily accepted as an echo?

Please see the very end of my build thread for pictures describing this issue - post 119. It's not too late for me to move the chair rail down an inch or two, but I'd rather not if there's no need...
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post #2 of 27
3/22/12 at 9:15am
JMHO, but I would classify early arriving reflections as destructive, independent of the source. If you have been following discussion on that topic, you track them down and either absorb, diffuse or whatever depending on your goals.

In my room, I have measured early ceiling reflections that approach the intensity of the direct sound , while side wall reflections are much less.

I'm guessing that you can't make a overarching statement that one is worse than the other without actually measuring.
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post #3 of 27
3/23/12 at 5:51am
^^^

Great stuff there....
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post #4 of 27
3/23/12 at 11:47am
Thread Starter 
Great responses indeed - thanks!

But I guess I'm confused then why so many HTs not treat their ceilings, and yet the owners claim they sound great.

I'm sure cost/esthetics have much to do with it, but even the Bacon Race theater, with a treatment plan from Bpape didn't call for ceiling treatments...

I hear people claiming that all these reflections are destructive, and I'm sure they are, but how noticeable are they to average ears?
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post #5 of 27
3/23/12 at 12:03pm
Yes it is puzzling. Two projects I'm working on with DE acoustical plans leave the ceiling bare as well.
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post #6 of 27
3/23/12 at 1:53pm
Quote:
Originally Posted by Morph1c View Post

Great responses indeed - thanks!

But I guess I'm confused then why so many HTs not treat their ceilings, and yet the owners claim they sound great.

I hear people claiming that all these reflections are destructive, and I'm sure they are, but how noticeable are they to average ears?

Perhaps it's because we (I) don't know any better! I'm fairly fat, dumb, and happy right now with my audio with an untreated ceiling, yet I have some very high gain reflections (measured) coming from it. After I treat it, I'll either come back here and say wow what a difference, or hmmm, didn't really notice much of a difference. Hard to say until you actually do it.

Quote:
Originally Posted by BIGmouthinDC View Post

Yes it is puzzling. Two projects I'm working on with DE acoustical plans leave the ceiling bare as well.

+me as well for puzzlement. Although not a "traditional" approach to treatment, Dennis definitely does it in this build http://www.avsforum.com/avs-vb/showthread.php?t=1373052 and some others he's documented here.
Why not for the other plans?? Maybe tied to unknown speakers/polar response and such?
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post #7 of 27
3/23/12 at 1:56pm
Possibly WAF as well
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post #8 of 27
3/23/12 at 2:19pm
Thread Starter 
It might also have to do with speakers mounted center of screen needing to be tilted down (away from ceiling) to face the listeners

Perhaps the off-axis energy is reduced to the point that the destructive waves are sufficiently diminished.

If this is true, then maybe the same logic applies to lateral/side reflections...?
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post #9 of 27
3/23/12 at 2:31pm
Deal with all of them. I have diffusion at ear level on the side walls and above and absorptive panels on the ceiling at the first reflection points. Too much treatment, however, can make the room sound dead.
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post #10 of 27
3/23/12 at 3:37pm
My last theatre (acoustics by Rives Audio) had a low ceiling (7 ft.) with a difuser built in. It consisted of 2x6 blocking mounted between the ceiling joists at 45 deg. angle (alternating slant for each joist bay). It had 1" OC703 behind the blocks, and was covered with GOM fabric. It worked really well.

That theatre also had BAD diffusers on the walls.
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post #11 of 27
3/23/12 at 3:41pm
Quote:
Originally Posted by Brad Horstkotte View Post

Possibly WAF as well

I think that's a big one. My wife said the theater room was mine to do with as I please. That lasted about half way through the planning stages.... I should have kept my plans to myself and kept the theater room door locked until the grand opening.
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post #12 of 27
3/23/12 at 5:59pm
Quote:
Originally Posted by dragonfyr View Post

Actually you had absorbers on the ceiling.

Placing a porous fabric membrane in or over a 'QRD style' diffusor renders it into an absorber.

Don't want to quibble over definitions here -- I know that Richard Rives Bird thought it was a diffusor. Also, the RPG BAD panel diffusors I had were covered with GOM fabric (as they came from the factory).

So I question whether a layer of AT fabric turns a diffusor into an absorber. But I'm sure there is some absorption there, so it's a matter of degree, I guess.
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post #13 of 27
3/23/12 at 8:20pm
I nearly always treat the ceiling unless WAF gets in the way....

My rationale goes as follows...
- speaker designers should and most often do pay good attention to ensuring the lateral off axis response is smooth and matches the on axis response
- very few designers even consider the vertical dimension
- nearly all speakers are made up of vertically stacked drive units which have very poor vertical off axis frequency response due to lobing and constructive / destructive interference as the distance between the drivers changes in the vertical dimension
- psychoacoustically a vertical reflection with the same spectral content as the direct on axis sound has been found to distort timbre. The same is not true of lateral reflections, which if they match the direct sound spectrally add spaciousness, which some like, and others do not

So, nice thick absorbers go on the ceiling! If the speakers are behaved vertically off axis due to, for example, good MTM design or a tall 'line source' type speaker, then diffusers are possible too.
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post #14 of 27
3/24/12 at 5:24am
Could an MTM speaker ever be designed to perform well off axis along the axis of the drivers? I thought this was an inherent limitation of the design?

Aesthetically I think people could at least consider strategically placed reflectors on the ceiling if they don't like the look of thick absorption. Or custom wood coffered ceiling if you have Dennis Erkine designing it
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post #15 of 27
3/24/12 at 5:36am
Quote:
Originally Posted by kromkamp View Post

Could an MTM speaker ever be designed to perform well off axis along the axis of the drivers? I thought this was an inherent limitation of the design?

Aesthetically I think people could at least consider strategically placed reflectors on the ceiling if they don't like the look of thick absorption. Or custom wood coffered ceiling if you have Dennis Erkine designing it

Yes. Very difficult though.
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post #16 of 27
3/24/12 at 6:48am
Here is a fully treated ceiling which has high WAF. Just one approach.
LL
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post #17 of 27
3/24/12 at 9:13am
Quote:
Originally Posted by Dennis Erskine View Post

Here is a fully treated ceiling which has high WAF. Just one approach.

Wow. That is one high ceiling.
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post #18 of 27
3/24/12 at 10:28am
Thread Starter 
The heavies are weighing in on treating the ceiling, but is this difference only measurable using a tool like REW or can average (less than perfect) ears hear it? Of course, even my ears (subjected to some loud music, engines, and explosions) can hear the difference between a live and dead room, and I'm sold on needing to treat the sidewalls, but I don't know to what extent one should worry about a single sidewall reflection in my space, especially given the number of DE/Bpape plans without ceiling treatments...

Expressed another way, assuming there are no ceiling treatments, how critical is treating the "far" speaker's sidewall reflection? All my other LCR sidewall reflections land fully on GOM-covered 1" Linacoustic, it's just this one reflection that comes so close to my chair rail that I'm worried about. If the consensus is that this reflection is significant and worth addressing, then I'll need to decide between lowering the chair rail (a step backwards, but doable as only ~6' of wall is currently done), raising the speakers to be a few inches above center of screen (not sure if this ok), and raising the floor for both tiers a inch (likely more expensive than lowering the chair rail)

Again, please see the very end of my build thread for pics of the reflection points
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post #19 of 27
3/24/12 at 10:43am
Quote:
Originally Posted by Morph1c View Post

The heavies are weighing in on treating the ceiling, but is this difference only measurable using a tool like REW or can average (less than perfect) ears hear it? Of course, even my ears (subjected to some loud music, engines, and explosions) can hear the difference between a live and dead room, and I'm sold on needing to treat the sidewalls, but I don't know to what extent one should worry about a single sidewall reflection in my space, especially given the number of DE/Bpape plans without ceiling treatments...

the ceiling high-gain indirect reflection will still superpose with the direct signal at the listening position to produce a comb-filter interference pattern as measured via the frequency response.

and you will not "hear" the early arriving signal reflection - see post#3 of this thread for further explanation.
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post #20 of 27
3/24/12 at 12:31pm
Quote:


number of DE/Bpape plans without ceiling treatments..

I cannot speaker for Bpape; however, there are multiple ways to skin that animal. You can look to procenium and soffit designs which block and/or absorb and/or redirect the energy before reaching the ceiling which would otherwise be redirected by the ceiling into seating positions most affected by those reflections. In the pictured example, the ceilings/proscenium was too high and therefore ceiling treatments were dictated.
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post #21 of 27
3/24/12 at 12:36pm
Quote:
Originally Posted by Dennis Erskine View Post

I cannot speaker for Bpape; however, there are multiple ways to skin that animal. You can look to procenium and soffit designs which block and/or absorb and/or redirect the energy before reaching the ceiling which would otherwise be redirected by the ceiling into seating positions most affected by those reflections. In the pictured example, the ceilings/proscenium was too high and therefore ceiling treatments were dictated.

I can't count the number of times Dennis has posted something and I've thought to myself, "yeah, that makes sense. Why didn't I think of that."

And that was another one
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post #22 of 27
3/24/12 at 12:36pm
Quote:
Originally Posted by Dennis Erskine View Post

I cannot speaker for Bpape; however, there are multiple ways to skin that animal. You can look to procenium and soffit designs which block and/or absorb and/or redirect the energy before reaching the ceiling which would otherwise be redirected by the ceiling into seating positions most affected by those reflections. In the pictured example, the ceilings/proscenium was too high and therefore ceiling treatments were dictated.


one data point: bpape was a big proponent of ceiling panels for my room -- not a sales pitch mind you, this was advice about repositioning existing panels for maximum benefit.
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post #23 of 27
3/24/12 at 12:49pm
why not just measure the acoustical behavior of the space before blindly applying treatments or assuming the ceiling is a boundary incident of high-gain, early arriving indirect specular energy?
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post #24 of 27
3/24/12 at 2:48pm
Quote:
Originally Posted by localhost127 View Post

why not just measure the acoustical behavior of the space before blindly applying treatments or assuming the ceiling is a boundary incident of high-gain, early arriving indirect specular energy?

Assuming other people are like me, doing the measurements is tough to get right -- and interpreting them is even harder

Case in point, I'm still getting this wrong, and even for those aspects that my bad technique did not screw up, it's not clear what to do next, based on what I can see in these REW results...

http://www.sendspace.com/file/uh8mx8

---

So I think the honest answer is, for those of us struggling with measurement and analysis, we're trying to improve things, anyway, where it's possible. Maybe what you are saying is it's impossible without good measurements and good analysis. In which case, I gotta pony up for an expert.
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post #25 of 27
3/24/12 at 7:19pm
Quote:
Originally Posted by dragonfyr View Post

Nathan, how about defining exactly what you want on terms of the application/use, and desired response.

Do you want a wide slightly fuzzy but 'big' image, or do you want a tightly defined image?

Do you want pleasingly vague, or a more analytical accuracy?
Or somewhere in between?

Do you have a dimensioned diagram of your room and preferably corresponding pictures of each wall?

And what limiting factors are you imposing on the solution?

Your freq response looks a bit 'interesting in its rather substantial tilt, but assuming all is OK, the ETC indicates quite a bit to work with.

And pardon me for not remembering all of the details of each space of the myriad I peruse each week, but is this of a previously treated space or a 'virgin' space?

Its a 13x17 foot space about 7.5' tall,

When listening to two channel orchestral music I like a deep, realistically wide presentation that lets me hear into the space and have a visceral sense of the place the recording was made.

But more than fifty percent of he time I listen to surround film soundtracks.

There's a wall to wall bass trap on the front screen wall two feet tall and six inches thick, Owens Corning sheets. Two two by four by four inches thick panels on the ceiling. Two such panels on each side wall at the first reflection points and two more on the rear wall. A final two are near the front corners near the screen.
LL
LL
LL
LL
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post #26 of 27
3/24/12 at 10:36pm
Quote:
Originally Posted by dragonfyr View Post


You definitely do not have a diffuse return enabling any widening of an image.

And yet with all of the absorptive trapping you still have the sparse reflections.

Personally I would remove much of he side and rear wall panels and measure
in order to determine the ACTUAL high gain sparse reflections and surgically ONLY treat them.

What energy remains from the rear and sides would be diffused.

As it stands, if you like how dead it is, the only real opportunity you have is to further damp the isolated sparse reflections, as the later arriving sparse reflections can contribute to localization errors.

The fundamental problem is that your goals and treatment methodology are at cross purposes. You are essentially making a non-environment (dead) space while wanting a behavior dependent upon densely diffuse low level early reflections as well as a later laterally arriving densely diffuse decaying soundfield...

This is why my thought has been to replace the side wall absorption with qrd diffusers.

What is a "sparse reflection"?

I'm not attached to the room being as dry as it is.
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post #27 of 27
3/25/12 at 9:36am
Quote:
Originally Posted by dragonfyr View Post

QRD diffusors, in order to be effective over the broadband will need to be about 11-12" deep. And one will need to be located about 9 feet from them in order to being in the far field and avoid nearfield polar lobing. And this assumes a lowest octave that scatters rather than diffuses.

Thus, one should have ~ 9 feet on either side of the listening position, plus the ~2foot total depth of the diffusors.

Well then, that's a non-starter since my room is only 13 feet wide and we're talking side wall first reflection point placement which is less than 9' from the MLP.

I don't mind the depth/size of the device but it sounds like that doesn't matter given my distance from it.

Quote:


But again, I would suggest against simply thinking in terms of what treatment you 'want', rather than first examining the actual behavior of the room and THEN determining what treatments would best compliment the actual behavior in order to achieve the desired response.

Agreed. I'd like an easier time hearing into the space of a recording, and to hear a proper sense of width and depth with natural acoustic recordings (think orchesteral works).

I'm willing and able to consider any room treatments that might achieve that. This is a dedicated room, no WAF to contend with, though budget is not unlimited
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