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Rear Wall Diffusion in RFZ Control Room

Discussion in 'Room Acoustics / Isolation / Treatment' started by ChrisH, Apr 14, 2015.

  1. ChrisH

    ChrisH Active Member

    Hello fellow Acousticians,
    After finding an extra room that will for mixing/mastering, fallowed by then picking the right monitors for your room and ears, finding the best monitor placement and listening position within the room, next step is creating an RFZ, right?

    My question is whether or not to treat first reflections on the rear wall with QRD Diffusor's or with broadband absorption ..
    I know probably to an extent it is a room by room dependent type situation.

    My new control room will be 19ft 6in long X 9ft 10in wide X 8ft 11in tall.

    The reason I'm not just automatically opting for diffusion on the rear wall is due to the fact I've heard conflicting opinions with some saying that "treating the rear wall with diffusion will make the mixing environment too lively and that you're better off just treating those rear-wall-first-reflection-points with absorption so that you're brain and ears will have an easier time deciphering the sound from the monitors and the sound of the room".
  2. kmetal

    kmetal Kyle P. Gushue Well-Known Member

    Well I'm not an acoustician, but I know some basics. i wouldn't plan on the diffusion, for two basic reasons. 1 a room that size isn't large enough for a truly diffuse enviornment. 2. I really woulndt want to be scattering sound around like the behind me an a narrow (relative to length) room like that, without a purpose. Iike in a lede design. Add to the difficulty in 'predicting' the results of this sort of thing and it just always seems to me, more trouble than it's worth.

    Instead of a diffusor, or absorbsion, why not use a bit of both, and make the rear wall a bass trap with some serious depth. Add some nice finish grade plywood, and decrotive, purposeful holes, or beveled slats, or what ever you like, and you've got a tuned brodband trap, that looks good, and doesn't kill all the acoustic energy in the room.

    I know room ratios are a basic guideline. But the dimensions of this room aren't exactly fantastic, it's almost as tall as it is wide, and almost twice as long as it is wide. Have you done any calculations to see where your problems line up? Is this an absolute improvement over your current area? How flat are your monitors?
  3. DonnyThompson

    DonnyThompson Distinguished Member

    I'm not an acoustics expert either, but have to ask, have you analyzed the room?

    Room dimensions do matter, there's no doubt about that, and there are some very good books out there that can help you to figure out what those dimensions will translate to acoustically -



    ... but having an actual sonic measurement of the room (RT60, etc) would also be quite helpful as well.

    It's difficult to treat your room for particular issues - until you know what issues you are treating for.

    I guess what I am suggesting - or asking - is.... how do you know you need a diffuser in your room?

    I'm not saying that you don't need one - I'm asking you how you know that you do?

    IMHO of course.
  4. JayTerrance

    JayTerrance Active Member

    I'm interested in this same situation. My control room is a bit larger, but I have been looking at various room layouts from documents produced by various "acousticians" and many show a diffusor centered on the back wall. So I logically assume I need a diffusor at some point. Donny raises a great question for me to think about in "how do I know I need a diffusor?". The problem is I dont know how to begin answering that question because I don't even know how I "determine" that I should install a diffusor. Is there any kind of testing process for diffusors that exists that I'm not aware of?
  5. kmetal

    kmetal Kyle P. Gushue Well-Known Member

    Your basic RFZ is gonna be what it is. You cover your 'mirror points ' on the wall. Your buddy moves a mirror along the wall and ceiling, you sit in listening position Where're ver you see the speakers in the mirror, you place absorbsion.

    An average RFZ design is roughly %40 absorbsion. The idea is that you don't make it dead, you make it relatively neutral. The idea is to hear only what comes out of the speakers.

    I'm not being a jerk intentionally, but the drawings you've seen from acousticians most likely weren't for a room of this size and nature. In other words magazine acoutsic based adds aren't showing my rooms from mom and dads basement. There is no reason to believe that because you diffusion in some pictures, that it's relevant.

    The bottom line from everyone I've ever talked to, is that in small rooms, diffusion isn't effective, or predictable, as other things. Trying to figure out dispersion patterns, on a frequency dependent basis, is above my ability right now. There's software out there that can setup 'models' of the rooms acoutsic energy and predicted response. This not your typical 'free' Aurelex thing where they tell you what foam to slap in the reflection points.

    It's not that it can't be done, or would look bad, it's that it's unpredictable, and unpredictable isn't good science, it's a good experimentation. Personally if it were me I'd go with something more predictable.

    There's no saying you can't add diffusive typ things. But my line of thinking, is tame the reflections first, see what ya got, then work on what's left.

    When you weigh up the cost benefit of a couple basic panel absorbers/traps vs the RPG pre fab stuff, it's not something I have the money to gamble with. I've never seen any diffusion sections on the room/material calculators I've used. I believe the reason is, it's not that simple, and in an RFZ design, I don't belive diffusion plays a large role, if any at all. If anything in a small room some sort of resonator like I said before, is where the rear wall can, be both semi reflective, and usefully 'tune able' if you need to.
  6. ChrisH

    ChrisH Active Member

    Thank you everyone for your input.


    Great questions!
    First off, here's the actual room with accurate size measurements..
    http://i295.photobucket.com/albums/mm150/ch-holt/control room.png

    As you can see, it's not symmetrical (so no mode calculations), the measurements I provide were if I were to square off the room by adding a wall going N & S along that left side divvy.

    Due to the fact the room is not perfectly rectangular like the measurements I provided, calculations are unpredictable.
    However, if I did add that wall, calculations predict the main problem areas would be 28.97 hz axial isolated and 58hz axial.
    I've went back and forth with throwing up a wall in the room the make the room symmetrical within the entire length of the room but I've been told keeping all the cubic footage would be superior to making the room smaller and symmetrical, help!!
    I could also square the room off by throwing a wall running E to W at the top divvy, however there were more problem areas with that route, whatya think?

    From what i've been told from an acoustician, the 19ft 6 in length of the room is long enough for effective diffusion, but you have a great point and concern.
    He also suggested a mix of diffusion and thick bass trapping for the rear wall.
    My monitors are Event Opals, I have the studio eq tuning kit that I will use to help execute the best placement for the monitors and listening position, then treat from there, fallowed by tuning the monitors as final tweak/last resort.


    Great points.
    I read and own both of those books, I've read Rods from front to back multiple times, and I did my best to comprehend the Master Handbook of Acoustics.
    I really don't know if I need or could effectively use diffusors in this room, that's why I'm coming to the people of recording.org :)

    I'm thankful for the Event Studio EQ tuning kit, that way I can see where the problems are and instead of tweaking the eq on the monitors I will try to treat the problem areas the best I can with absorption first, thought I am worried that by not making the room symmetrical one way or another, that my frequency response will differ from my left to right monitor :/
    At the same time I've heard conflicting op

    Keep it comin guys! I appreciate all the help
  7. kmetal

    kmetal Kyle P. Gushue Well-Known Member

    The 19' dimension isn't the one I question diffusion wise, it's the 10'. If the diffusor was centered on the rear wallthere would only be 5 feet or so to the left or right before hits a hard boundary. If that is covered with absorption then you're altering the dispersion pattern of the diffuser. That's my main concern.

    Any basic mode calculations will get you in the ball park. The asymmetry is more of a concern with phase relationships and stereo field imaging.

    Have you tried breaking some rules and firing the speakers the short way? I'm guessing this is what you meant by finding the listening position.? Also are the boundry walls in the picture outside walls? Are they concrete foundation.? Is is possible to alter the existing room in any way?

    I like the idea of keeping it open for the cubic footage.

    Between deep bass trap on the rear wall, and maybe a rack or couch in the left sidealcove type area might work. It really depends on where the listening position actually lands. Maybe you could make a baffle type wall large enough to keep at symetrcial at least to your ears are in the room, and use the backside of it for cable storage or what have you.

    I want to really love the idea of diffusion, but I have a hard time rationalizing something that I don't know how to predict, In a room that is also not as predictable as it could be. A lot of this is just the basic space restrictions, and my lack of expertise in diffusion.

    The pic I attached is a place I work at that has a layout similar to what yours would be like firing the 'short' way. It's kinda tough to tell. But fwiw.

    Attached Files:

  8. ChrisH

    ChrisH Active Member


    Those are some fantastic ideas.
    Best bet to decide which way the speakers should fire and the remainder of the positioning would be by measuring the frequency response, correct?

    As far as the asymmetry being a concern with phase relationships and stereo field imaging, from what I've read symmetry only matters up to your listening position, givin that you've created a RFZ, what happens behind you with the side walls will only effect low frequency behavior, but maybe I'm wrong.?
  9. kmetal

    kmetal Kyle P. Gushue Well-Known Member

    Yeah man. I'd grab some blankets a cable and my phone and play some tunes. See how the lows move and what feels better. Maybe it's sound nice with the alcove behinds you with a couch in it. A little guerrilla acoustics and an hour will show you what's going on. Flipping my cousins room 180 made a big difference in tightness and overall feel.

    Ideally you want symmetry in the whe room, and yeah I think the idea is its most critical in the front side of the room. Lol I dunno man I just don't want weird $*^t goin on behind me, ya know? if any of the remaining energy is coming back at different times / frequencies there's a potential for cancellations.

    I wish i had a better grasp on everything myself, but that's definatly a concern of mine. The second phase and time align stuff happens, the sound goes 2d. The 3d collapses.
  10. DonnyThompson

    DonnyThompson Distinguished Member

    So are you saying that you ended up getting better response by short-firing your cousin's room, Kyle?

    I'm asking because the wheels in my head are now rapidly turning as I'm looking at my own room... and wondering about the possibilities... LOL. I'm firing length-wise now... but I really don't care for the way the room is laid out this way, I never really have... I'm not saying acoustically - I'm saying logistically.

    It wouldn't break my heart to be able to short-fire my speakers in this room, if I thought I'd get better results - or for that matter, even just as good would be fine ... ;)

  11. kmetal

    kmetal Kyle P. Gushue Well-Known Member

    Hey d sorry for delayed response, I had many many hours this weekend on a home theatere project. In my cousins studios case, it was fairly symetrcial and a a rectangle. It was in a basement, and about half the studio had convete foundation behind the studio walls the other didn't.

    When we set up the speakers firing, both directions, in other words with the speakers firing into, and away from, the foundation side the response was different. Also, since the entry door, would have been behind the speakers, making it a little weird, we went with the better sounding and logistically better position. We also tryed firing them the short way, but it wasn't pleasant in this case.

    If you have a chance to try a different layout, try it, especially if it's logistically uncomfortable. In residential space constrained areas, the acoustics are what they are, but if it's annoying to move around, or it takes too long and you forget an idea, that defeats what home studios are best at. Being there in the moment of inspiration.

    The thing with low end is its lack of directionality. It's going to fill up the room no matter what. the relationship of where the source originates and its boundaries help determine overall response. I say it's worth a shot.

    As I learn this stuff, I've been noticing over the years just how many of the big pro control rooms are seemingly wider than they are long. It's interesting to me. I wonder what they're reasonings were, and how they worked it into the design.

    If I had to give up a couple db of accuracy in a room that's not particularly accurate anyway, but I was more comfortable, I personally would.

    It's very good to know the rules, and adhere to tried and true, in acoustics and construction. It's usually the best place to start, once I've tried it that way, I see what else might work acoustically. Construction wise, there's strict defined ways and methods. I don't do 'bright' ideas in this realm, because it's already defined.
  12. Kurt Foster

    Kurt Foster Distinguished Member

    it's more about (room) volume than which way the room is fired. the idea that you need to fire the length of a room comes from too small rooms and the desire to mitigate the limitations. i also do not know where this idea that diffusion is wasted in a small space comes from. look up acoustic ramp. i have seen lots of good control rooms fired sideways and with short dimensions and diffusers on the walls.
  13. kmetal

    kmetal Kyle P. Gushue Well-Known Member

    My question is how do you predict the effect of diffusion reliably, in general, and in small rooms. Also the OP was talking about a pre made diffusor, not one built custom to suit a known need/problem area in the room. It's the same thing with the classic bookcase diffusor. How do you quantify the effect it's having?

    I can't seem to get a straight answer in diffusion from anyone, and I'm not sure there is one. There's a mountain of material out there to sort thru. I'm eager to learn all I can about diffusion, because I belive that it's the combination of all the principles and elements, not a one stop. How to fairly reliably predict the effects is what I'm after.
  14. DonnyThompson

    DonnyThompson Distinguished Member

    Just guessing here, but maybe it's because diffusion is so dependent on the individual room, and because this varies so dramatically, it's hard to come up with a one-diffuser-fits-all solution, which would also support your approach of questioning how one would know - with any amount of accuracy - if diffusion is needed, and, if it is, what type and how to reliably predict the results?

    I'm not doubting what Kurt is saying here, either - I can't, because I really don't know.

    Although I would agree, based on personal research experience, that info on diffusion isn't as prevalent as info on absorption style treatments is, and, the methods don't seem to be as "cut and dried" as those of trapping and absorbing.

    For example, if you take a room, any room really, and add a certain amount of bass trapping or broadband absorption, you can feel secure that the amount you add will pretty much react the same way... But with diffusion, and because as Kurt has mentioned, is based more on volume than actual dimension, it's much tougher to predict - not only what type you need, and how much, but also the outcome.

    And, would this not also be dependent upon the current absorption and trapping treatments that may already be in place? And I'm asking, not stating it as fact...
  15. kmetal

    kmetal Kyle P. Gushue Well-Known Member

    Well, this is gonna be fun...


    This came up 2nd in a seat for one of the references in the 2nd edition of BTLP. The update of polycyndrical stuff is cool, need more time and brain power. This is gonna be a wild ride!!!!

    I'm not doubting what Kurt is saying. I'm tying trying to reinforce how necessary predictability is, where we can't actually hear the room, and small rooms are both the most common type of room these days, and the most difficult acoustically. In my dream world, the whole wall itself is one big conglomerate. Nice stone work with wood. Maybe no grout, and I'll tune it like that?

    Phil (Greene, sound engineer) always says his old echo chamber didn't get live until the grout was in. Before that he said it was quite dead for a comletely tiled room. He attributed the grout as the significant factor in the decay time.

    Reflection is such an intriguing part of sound.

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