Ah, didn't realize you guys continued this party without me
Originally Posted by localhost127
so you are concerned about "understanding the engineering and science" yet you place a porous fabric directly in front of your reflection phase grating diffusers...
Your graph is right but your complaint, not
. Let's cover each. The graph comes from Cox and D'Antonio's book on diffusers and acoustics. What it states is absolutely true. Due to high air velocity near the wells, if you stick a resistive material in front of it, you will create an absorber. Hence the reason some people sell combo diffusers/absorbers by following this research.
You did not provide attribution/context for the graph. Had you done that, folks could read this bit after it by the author: "Any cloth covering should be placed at least a well width away from the front face."
While not that easy to see, our side walls are quite deep. The acoustic material is shallower than them. Further, there fabric is on tracks that stand out past the maple wood framing you see on the face of the wall. All in all, there are good number of inches of space between it and the diffusers. In addition, Keith spec'ed and we used material which has less resistance than the typical GOM fabric people use (sadly the graph above does not stipulate what material was used). All of this helps to reduce the effect. How much? Well, there is some data we can draw up in the form of measurements by RPG for their diffusers:
We see that as the material moves away from the very front of the diffuser, the absoption coefficient reduces. The top purple is what Cox and crew showed in their graph. The red is what happens when you put good bit of distance between the fabric and diffuser. Sadly RPG doesn't spec either what cloth they used.
Bottom line, don't go wrapping your diffuser as I see many times with fabric hugging the material. Also note that the diffuser even without the fabric has a non-unified absorption.
So yes, we have an undesirable effect here. And it is not the only one. That fabric so far away from the side wall acts as an absorber even when it is not in front of the diffusers. Heck, why stay with audio? Our front wall uses an acoustically transparent screen to hide the speakers and acoustic treatments. The woven pattern certainly is not the ideal material for video. Why did we do all of this then? Because good engineering is a matter of good compromises!
The goal for a reference class home theater is to have you be lost in it. To make you forget you are in a theater but in the movie scene itself. There is no way that illusion works if when the screen brightens, you see all that structure and acoustic material on the wall. That may warm the heart of whoever spent money on them, but it completely deters from that one important goal: "suspension of disbelief" as Keith calls it.
To be sure, I had to be taken there kicking and screaming
. Two years ago you wouldn't catch me dead with an AT screen. Solid, 1.0 gain matt had to be it. But just spend one or two minutes in our theater and you forget where you are. You get drawn into the picture in a way that no front wall with speakers showing would ever do. When I am showing it to my friends and such, as soon as I stop talking, I see them mesmerized by the *total* experience* and I have to force them to look at me and listen. You think right in the middle of that they are hearing that increased absorption that you so worry about? I think not. You have to optimize for the entire experience and that might require compromise in a few details.
This brings us to another important point I have been making which is how easy it is to screw up these acoustic material. Which consumer who buys one of these diffusers would think twice about covering them? Heck, I have seen people do that and even write up their build threads that way.
So no, our theater design didn't come from taking a two day class or reading a book. The audio was designed by Keith Yates who is a master at the perceptual effects I am talking about there. I did the video but happy to continue to explain our design choices.
Can we see your listening room by the way?