# Somewhat unique acoustics questions



## crackyflipside (Jul 8, 2006)

For reference, the room is 15 by 22 and has a soffit (4' x 1.5' high)

1.Ok, if I have an AT screen and the LCR behind the screen, do I put absorbtion just in between the LCR or do I treat the whole wall from top to bottom with absorbtion?

2.On the side walls, when treating reflection points, does the absorbtion go all the way from floor to ceiling or is it only treated from ear-level to the ceiling with reflective surface underneath?

3.I was planning on a star ceiling, with the star ceiling I could also use rigid fiberglass board and use it as absorbtion on the ceiling, would this risk making the room too dead sounding or would it create a sound as if the ceiling was higher? (I heard this trick of using absorbtion on the roof when recording because it makes the room sound larger I'm not sure if it was all ** or what)

4.On the rear wall, I have diffusion planned by making the back of the HT DVD shelves and some absorbtion. Should the rear wall be treated like the side walls or should it be as dead as the front wall?




5.So guys, am I generally OK to think like this or am I setting myself up for an acoustical nightmare? :dunno:


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## bpape (Sep 14, 2006)

Front wall should be dead completely for multichannel.

Diffusion on the rear, that's a whole nother topic. Don't think a DVD rack is going to provide that - it won't.

The star ceiling can be made from a mix of materials so only selected panels are 703 and the rest are hard - that's the way I'd go - just do the ceiling reflection panels in 703 - maybe [email protected] Have done this many times in my designs and it works well. IMO, a dead floor and a dead ceiling is too dead.

Side walls - hit the reflection points and then scatter additional absorbtion around the room evenly. Don't have to cover a lot of it but you want it equally distributed around the space for even control - though I prefer to leave the rear a bit more lively.

Bryan


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## crackyflipside (Jul 8, 2006)

bpape said:


> Front wall should be dead completely for multichannel.
> 
> Diffusion on the rear, that's a whole nother topic. Don't think a DVD rack is going to provide that - it won't.
> 
> ...


Well the dvd rack covers about 85% of the back wall 










I was thinking of making the back part of the shelves large panels of rigid fiberglass absorbtion. and the rear corners to hold the bass traps from floor to ceiling. Not to mention the AV closet in the back of the room is a giant, ugly diffusor. Should I treat that as well or leave the surface hard?



I was also thinking about the ceiling, and I thought since it was soffited, leaving the bottom and sides of the soffit with hard surfaces and treating the higher part of the ceiling completely with absorbtion would be ok. I neglected to mention there would be two ceiling fans on the higher part of the ceiling, maybe that would provide enough diffusion to completely treat the higher part of the ceiling.

For panels, would the same thickness, let's say 2" of rigid fiberglass would be good for all the absorbtion?

I'm not sure what 703 is but I also forgot to mention that I am huge on DIY and spending the least amount of money for the best value.


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## yphs_mst (Nov 11, 2006)

> For reference, the room is 15 by 22 and has a soffit (4' x 1.5' high)
> 
> 1.Ok, if I have an AT screen and the LCR behind the screen, do I put absorbtion just in between the LCR or do I treat the whole wall from top to bottom with absorbtion?
> 
> ...


Hello, please allow me to give my opinions and thoughts ...

1. Treating the whole front wall with absorbtion will help if the rear speakers is facing to the front wall (eliminating echo), but if you use a dipole speaker for the rear speaker and one of the side poointing to the front wall, absorption on the front wall will degrade the speaker's response. There're other thoughts, but this is the one that comes to my mind right away.

2. Treating the side walls for eliminating strong reflection can be done just at the ear's height only, but please notice that the greater absorption surface (and aslo thicker), the lower frequency it can handle. I guess ... it's just a matter of interior design too ... diffusion can replace the absorption if the room is dead already.

3. An absorptive surface will create a sense that the sound goes away (like if the ceiling is an open area) ... it doesn't make the room small/bigger. Well,, it might make the room smaller on the other thought, but the more absorption, the better imaging you'll have. Some people want the imaging to be really clear from the screen, and some people want the imaging to be wider/bigger. It's a compromi with the acoustician i guess ^__^ 

4. Rear wall treatment, again .. do you want it dead or live ? so either diffusiion or absorption ... 

Hadi


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## bpape (Sep 14, 2006)

Just for clarity...

1. The front wall being dead in a multi-channel environment has nothing to do with the style of the surround speakers. The point is to stop any reflections from either the side or rear surrounds from coming off the front wall and mixing with the front soundstage. Having the front wall dead with dipole/bipole side surrounds will do nothing to degrade their performance. 

The whole point of that type of speaker is to sit in the null of the direct sound. That will be accomplished whether or not the front wall is dead. Realistically, most people have 2 rows but only 1 set of side surrounds and don't sit in that null anyway.

2. Diffusion can help provided the space is large enough and the distance to you is far enough. However, as far as absorbtion goes, there is more to it than just reflections. It's more critically a matter of getting the overall decay time across the spectrum into the target range based on the size of the room and it's intended usage. 

3. Agreed on it making the surface appear to go away. Disagree on more absorbtion always equalling better imaging. One can get excellent imaging with a small amount of absorbtion and then not gain anything in terms of imaging while increasing absorbtion to bring the decay times into line. 

On the other hand, I can put up TONS of absorbtion and not have it necessarily in the right place - or not obey the rules when it comes to speaker/sub/seating placement and have awful imaging. Absorbtion/diffusion are tools to be used along with proper setup - not an end-all by themselves.

Bryan


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## yphs_mst (Nov 11, 2006)

> I can put up TONS of absorbtion and not have it necessarily in the right place - or not obey the rules when it comes to speaker/sub/seating placement and have awful imaging


Ic where you're going, the "placement" of everything. I don't doubt it, but still, listening a something in a room with a high absorption (approaches an anechoic chamber) WILL have better imaging than a room with more reflections, especially early reflections, but yea the placement ... that's why I said it's a compromise with the acoustician if there is one.

Hadi


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## bpape (Sep 14, 2006)

I guess it's a matter of preference.

To me, when the room get's too dead (as you said - approaching an anechoic chamber) it starts to LOSE the imaging and sound more like headphones - but body sized! That's more of a binaural presentation where one is 'inside' the performance rather than a stereo image where one has the presented soundstage in front of you.

Bryan


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