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Yep, That's classic Dennis construction right there!


There are two types of walls: Front walls and all others.


Again, this is for HT rooms, multi-channel playback not dedicated two channel.


Ted
 

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For what it is worth, my opinion is that the rear wall is a bit different than the sides.


In smaller rooms, you can use the rear wall as a point to place diffusion. This helps with standing waves a little, helps RT60 a little, and does not overly damp the room making it sound too dead. You can mix a bit of absorbtion ( I do opposite the screen since you can't treat that on the front wall) but not so much that you kill the surround reflections you want/need.


Overall, I prefer strips of absorbtion, say 9-12" wide by 1- 1 1/2 " thick by 5' tall. Put these at first reflection points for the main fronts, and the rest every 3-5' along the side walls with the center of the panel (vertically) being about your ear level. You end up with about the same sq. ft. coverage as the whole bottom up to ear level but it is distributed throughout the room more effectively IMO. Yes, the front wall should be totally dead.


Keep in mind that mine is for music and for theater. Personally, I think HT can sound very good in a 'properly designed' 2 channel room with just a few mods that don't hurt 2 channel that much. I have not heard the same results from 2 channel in a 'properly designed' multichannel room. Usually sounds very dead and the soundstage is severely closed in. If I am going to err to one side or the other, I will err toward the 2 channel design.


As for having to play all my music through multichannel because I did the room for HT, I'll pass. I spent a ton of cash getting components that recreate music well without a lot of processing. I'm not going to pass them through multiple signal modifications.


Don't misunderstand me. Dennis is describing good practice, theory, and lots of experience with DEDICATED HT environments. If that is your goal exclusive of music listening, go for it. If you are like many of us who also use this space for a high quality music listening space, play with it a little and see what you like. After all, that is what it's all about - what YOU like. It's your room.
 

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A couple of minor points: diffusors do not affect RT60 in the manner described nor will they have any impact on standing waves.


While vertical strips can create a moderate amount of diffusion, you cannot cover all the early reflection points to all seats effectively. In the 'ear level' scenario, you do have 'stuff' bouncing around above your head; but, if you watch your angle of incidence against where your speakers are placed (height), you've covered all the bases within the curve. This is also a much more effective method with rows of seating since you'll have more people close to the side/back walls than in a two channel, single seat of excellence environment.


In multi-channel...no bad seats not one good seat.
 

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Not suggesting that you hit all the first reflection points, just the side wall points from the main L and R.


Diffusion (diffraction) does indeed assist with RT60 and elimination of standing waves. Look at any anechoic chamber. There is NO abosrbtion. ALL diffusion. No standing waves in there. What is the RT60 in an anechoic chamber (understand they are all different, rhetorical question.)


If the the absorbtion is placed appropriately (not directly across from other absorbtion) and diffusion allows the wave to be changed from a first axial to a tangential reflection onto various absorbtive features, you are in fact lenghtening the time till it hits another reflective or absorbtive surface, thereby extending the time and distribuiting the wave across more absorbtive surfaces (assuming good polar plots on the diffusors).


Like I said Dennis, don't get me wrong. For pure HT, your way is probably best. I want it all. I want 1 good seat AND no bad ones. Your 'non-bad' ones may be a tiny bit better but my 1 good one is killer when its just me sitting down listening to music.
 

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An anechoic chamber is all absorption (ever done research in one?). Like this one for example:
http://www.bell-labs.com/org/1133/Re...icChamber.html


Sound decays (STP in air) at the rate of 1/r squared. The rate of decay is not affected by reflection (a form of diffusion). It is affected by distance and absorption.


The purpose of diffusion is to randomize reflections which will result in axial, tangential and oblique reflections.


RT60 is the time it takes for the sum of all reflections to decline 60dB once the sound source ceases. The calculation is RT60=0.049(V/Sa). [where V is volume, S is surface area, and a the absorption coefficient). In an anechoic chamber RT60 is statistically 0 since there is no reflection. Sound reflecting from a wall surface or a diffusor will decay at the same rate.


The location of absorptive materials (except in certain cases in small venues) has no impact on the RT60; however, the placement and type of diffusors will have impact on the proper creation of the reverberent field.
 

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Quote:
Originally posted by Dennis Erskine
An anechoic chamber is all absorption (ever done research in one?). Like this one for example:
http://www.bell-labs.com/org/1133/Re...icChamber.html


Sound decays (STP in air) at the rate of 1/r squared. The rate of decay is not affected by reflection (a form of diffusion). It is affected by distance and absorption.


The purpose of diffusion is to randomize reflections which will result in axial, tangential and oblique reflections.


Learn something every day. I always thought the wedges were all edge hardened in those chambers so they acted more like diffusors. Guess I will have to stand corrected on that one.


My comments were only made to present the poster with options, not to rebutt any other previous statements. They were also stated understanding that I was trying to provide a compromise solution not knowing what the poster's use was for the room.


Even though what I was describing may not be significant in pure RT60 terms, I still don't get how forcing a portion of a wave to travel through more air and through more absorbers before bouncing back at the user from the front can't help. Remember, I was only discussing diffusion on the rear wall, not the sides. If it takes the same amount of time to decay but each wave bounces back at my position fewer times and from more different directions within that space of time, how can that not help the situation?


Besides the above stated, it appears to my ears, to provide 2 other benefits. In surround mode, the rear portion of the surround field gets scattered more evenly thereby increasing the sense of envelopment. In 2 channel, forcing the wave to travel a longer distance before coming back off the front wall makes the space appear acoustically larger than it actually is. Not all rooms require this but you don't run the risk of sucking the life out of the sound if you put some up. Like you said, reflection does not effect the rate of absorbtion.


Also, if I the room is a bit live, the only time I would worry about it is when it is just me in there. At that point, I am probably listening to music so it works better that way. When we are watching movies, there are 4-12 wonderful abosobers that get added to the room called people spread throughout the middle of the space.


I have heard rooms done both ways. Both can sound very good when doing multichannel duty. Like I said before, I have just never been able to find that one sweet spot for stereo when you totally optimize for surround, even when sitting in the nearfield of the front speakers. Just my opinion but I'll sacrifice a tad on the ultimate in multichannel quality to get that sweet spot back.
 

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I have a quick question for you guys regarding sound treatment....


I've read and understand that the area below ear level should covered with Insulsheild (or an equivalent) and above should be covered with batting. My question is, for the second and possibly third rows, should you raise the height of the insulsheid to accomodate what would be below ear level for those on risers? I.E. from front wall to first row Insulsheild is installed to the height of 4ft. Once you get to the row with a 12" riser, do you then increase the height of the insulsheild to 5ft or do you go with 5ft from front to back? What is the 'general' rule to follow?


TIA.
 

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Some do, some don't. The idea is to cover a certain percentage of the wall with more absorbtion. If you go higher on shorter walls, you are actually increasing the percentage of the wall that is covered.


I don't know any general rule of thumb about this. Dennis could probably provide a more experienced answer for this type of implementation.
 

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So to follow up on dpape's last question...


What goes above the lower panels? I was thinking of just wraping "board" with polyfill and then covering in GOM. I'm hoping to have all the panels just look the same size and appearace.


My concern is the sound absorption (or lack of) if I dont use the right materials for the foundation of the upper panels.


Anyone have a sugesstion?


Thanks
 

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Polyester quilt batting is the recommended upper covering right on the drywall - the hard part is finding the 1" thick stuff. I have discovered it at Hancock Fabrics. Also check quilt stores.


pam
 

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Thanks Pam,


It seems (from post & pictures) that people are just putting the batting directly on the wall and covering it with fabric? This seems pretty difficult to me?


I was thinking of using 2'x4' sheets of "board" (1/4" plywood?), covering them with the batting and wrapping them in GOM fabric. The trick (and I still don't know how I'll do this) is to then mount these on the wall. My thinking is that this will give me a nice "matched" look of panels (some sized differently for looks), and I should be able to get the upper wall panels and lower wall panels to match - much like the custom theater rooms I've seen.


This method also allow me to build the panels on the floor and glue/staple the fabric perfectly before I hang them on the walls. I think it should give a better result than trying to cover the batting directly on the walls?


Am I missing something here (other than how to attach the panels to the wall), or shouldn't this give a very finished look??


Dave
 

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Quote:
Originally posted by dmcvie
Thanks Pam,


It seems (from post & pictures) that people are just putting the batting directly on the wall and covering it with fabric? This seems pretty difficult to me?


I was thinking of using 2'x4' sheets of "board" (1/4" plywood?), covering them with the batting and wrapping them in GOM fabric. The trick (and I still don't know how I'll do this) is to then mount these on the wall. Dave

Dave,


This is a great idea. I seen a similar project on HGTV the other day. They were using 2X2 panels and alternating colors. They attached them with 2-3 inch wide Velcro. Makes it easy to change patterns?


This was not a theater project but would work just the same.


Just a thought.


Scott
 

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Thanks Scott,


Good idea on the Velcro. I'm wondering if there any any "accoustical" concerns for this type of construction or method of hanging them? (Dennis
)??


Sure it has to sound good, but it has to look good too.


Dave
 

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Dave,


The reason for the polyfill on the upper wall is due to the fact most build out the wall with the firing strips and insulshield on the bottom. Covering the upper, drywalled half with fabric and no polyfill gives a sag in the middle of the stretched fabric.


The polyfill only serves as fill, it theoretically has little acoustical impact (some diffraction, eh?) but still lets the sound waves hit the dry wall and be reflective and alive.


One could due 'hard" reflective panels above and soft, absorbent below ear level (keeps dialog and such clean from first reflections).


There are some heavy duty picture mounting "spikes" which could be used for mounting purposes. We used them to secure our pictures so they wouldn't rattle.


Good Luck,

Adam
 

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Thanks Adam, that helps my understanding. I took a look at your pictures, Great Job! I hope to accomplish a similar look.


Is the entire rear of you screen wall covered in Insulshield? Can you tell me where you got it? I was going to use sound board, but I think Insulshield does a better job at lower freqs.


Thanks again for you input.


Dave
 

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Thanks.


Yes, we covered the entire area behind the screenwall (the screen is actually mounted on a false wall about 18" in front of the real wall. We also covered the side walls next to the speakers, top to bottom (the speakers are another couple inches in from of the screen wall).


Think of a theater stage and my speakers are in the "wings" in front of where the Theater curtains would be (my hope eventually is to mount some drapes).


I actually used the Owens Corning Sound Select due to some issues about getting the Insulshield locally. The specs are quite similar. Let me know if you need it, I have the PDF or web address around somewhere if you can't find it on their website.


Adam
 

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Hello Dennis:


Quote:
Originally posted by Dennis Erskine


The location of absorptive materials (except in certain cases in small venues) has no impact on the RT60



You REALLY shouldn't say that:


Look at picture (copyright) below:

Measured by me in official University Lab.

Method as per ISO 354 - standard reverberation room method (except for position) on the same day with exactly the same rigid rockwool slabs framed with very light metal frame to screen off edges.

You could almost ask: which one do you like best.

So the only varying factor is the position in the room and how it's used.


Also look here:
http://www.ib-neubauer.com/Literatur...001_Rome_1.pdf

Predicting Reverberation Time: Comparison between Analytic Formulae and Computer simulation with CATT room acoustic program.


Sabine and Norris Eyring assume statistical distributed absorption, boundary conditions and reflections.

Millington allows assymetric absorption.


Fitzroy and the improved Higini Aray Puchades approach try to solve the shortcomings of the above approaches in function of assymetric distribution of absorption.


An official Reverberation Room is tuned with added diffusers to the MAIN material sample spot.

There is the edge effect and so much more.


Best regards

Eric
 

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Quote:
Originally posted by Eric Desart
Look at picture (copyright) below:

Sorry I've no idea how to enter the picture.

I resized it it's not too large (nor size nor bandwidth) but it doesn't appear.


If somebody can give a hint (no time to try for hours just for a picture).


Regards

Eric
 

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Well, Eric, you are exactly correct. As it is true that making glittering generalities about any complex process can get one into hot water when shifting through the details later.


However, as I've noted in other postings, as room size decreases, traditional methods of predicting RT60 cannot be considered terribly accurate without use of ray tracing techniques. (In actuality, as architectural and other elements -- people for example -- become larger with respect to total room size. As you are aware, the same would hold true for any attempts to accurately model or predict modal resonances without both ray tracing and finite element analysis. As well, with variances in physical "as built and installed" properties, we find stochasic analysis in monte carlo simulation helpful. These models, however, have the down side of running for multiple hours. In the end, models are useful to prevent expensive errors and the effort is to be statistically accurate such that once we can measure the actual space we're faced with 'tuning' rather than rebuilding.
 
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