Mixing theater design

Hi, I'm moving into a new space soon - Toronto, Canada. This room is mainly used for mixing for 5.1 television, small films . . . very occasionally music. The room is setup according to Dolby specifications for mixing film/tv and everything is mixed at 79dbSPL on near field monitors (BlueSky). It is in a self contained isolated space, so there aren't any sound isolation issues to deal with, just room treatment. I've planned on building the front wall inside the shell that consists of 2x4 framing filled with OC704 covered with transparent cloth. The ceiling is a drop ceiling, acoustic tiles hung from an existing wood plank ceiling. The floor is industrial carpet. The side and rear wall are drywall and I will treat them as necessary after the rest is done, I'm afraid of making it too dead. I'd appreciate any opinions on the placement of the front walls of the room, and anything to look out for or be concerned about with this design. TIA, jim

Hi, Any reason for no symmetry on the rear wall? Steve

That's the existing shape, I haven't formulated a plan for it yet. I can't build out at all, could only build up the inside . . . didn't really want to lose the area, since that area is the entrance to a producers table on a riser.

We needed to build up the left wall - it was against a hallway and another office room. We added to the inside wall with a layer of greenboard, green glue and a layer of drywall. This worked really well in killing any leakage. Moving along . . . finished the front wall, 4" 704 covered in cloth, with airspace behind it. Sounds very nice - tight bottom end. May want to add some insulation to the top of the drop ceiling - especially at the front of the room (there is a lot of airspace above the ceiling. I'm working on the back wall now. There is a raised platform with a producers desk at the back. The rear left corner is a bass trap and the rest of the walls in the rear and rear sides is covered in 1" OFI72.

I'm still wondering why the room is not symmetrical at the back. Why not put another bass trap in the other rear corner, to even things out? You'll probably need more bass trapping that what appears in your sketch anyway, so why not just keep the room symmetrical, and put it in that other rear corner?

I don't want to loose that much physical space. The symmetry would only be in the drawing anyway wouldn't it - since one corner is bass trap and the other is fibreglass right on drywall.

The symmetry would only be in the drawing anyway wouldn't it

No, it would be acoustic as well, I think. Apart form the lack of bass trapping in the room, there is also the issue of acoustical symmetry at the rear of the room. You say that there is going to be a director's desk back there? So he will have a huge amount of absorption on one side of his head, and very little on the other side. So how is he going to get a good stereo image and judge EQ like that?

Anyone sitting back there is more interested in their laptop than critical listening :mrgreen: After all they're behind the rear speakers.

That doesn't affect the lack of bass trapping.... But if you don't want to, then you do want to. so be it.

But there doesn't seem to be a problem with lack of bass trapping. The bass is very even in response all around the room, and sounds very tight. It doesn't sound as if there needs to be more bass traps. What do you see in the dimensions that would indicate a lack of bass trapping? I'm curious.

Well, the room is not small, but it isn't huge either, so it will need bass trapping. It also doesn't seem to be based on any known ratio: The inner leaf hard surface dimensions on your diagram give a ratio of 1 : 1.08 : 1.9, which comes up as being close to "A worst case scenario calculated by RPG", according to Bob Gold's calculator, and it fails 2 of the 3 tests proposed by Walker, Hoeg and Christensen (EBU paper). Between 24 Hz. and 94 Hz. the modes of the room itself are the dominant acoustic characteristics, and there are a total of 21 modes below 100 Hz, (and about one hundred modes below 150 Hz.), so yes, I'd say that bass trapping is important in such a room. The Bonello diagram for those dimensions is none too encouraging either. Or did you use another ratio as the basis for the room? I just calculated the above based on what I see in your diagram. You say that the room "seems" to have even bass response, but how did you determine that? I mean, is that just from subjective listening with ears, or did you actually measure it objectively with something like ETF, and generate waterfall plots and accurate response curves, plotted at 1 Hz intervals in the low end? That will tell you if you need more bass trapping or not. If you already have those plots, could you post them here? I'd be interested in seeing them. I'm also confused about that large black triangle thing in the rear corner: I'm assuming that the black triangle IS a bass trap, right? But when I look at the dimensions of the room, it seems like that actually ISN'T a bass trap, and is just an angled wall, since you mark the black surface as the limit of the room (inner leaf hard finish surface). But that makes no sense, since if that is an angled wall, then it means that your room has practically no bass trapping at all, so I doubt that the room response would be very even. Could you clarify what that black triangle shape is? Angled wall, or large superchunk bass trap? - Stuart -

Thanks for your response, The rear right is a hard surface, the bass trapping is on the rear left and the front. When I plug those numbers into Bob Gold's calculator I get a ratio of 1 : 1.44 : 2.53 ?? 22'10" x 13' x 9' I've only done subjective testing so far, I hope to get more results as things progress. jim

The rear right is a hard surface,

That's what I feared. So you don't actually have any real bass trapping in there at all, just a couple of inches of absorption on the walls, and that small thicker section on the left rear.

When I plug those numbers into Bob Gold's calculator I get a ratio of 1 : 1.44 : 2.53 ?? 22'10" x 13' x 9'

Well, according to the numbers on your diagram, the height of the room is 12 feet, not 9 feet (taken to the inner leaf finish surface). Even if you take into account that "angled blue shape" on the ceiling (what is that, anyway? Is that the "acoustic tiles" you mentioned?) the height of the room only gets down to 9 feet just a few feet from the front, so the average height is well over 9 feet, probably more like 10. I also noticed you said:

The floor is industrial carpet.

Why would you want to carpet the floor, if the ceiling is already absorbent? That will make the room too dead, most likely. The general rule is that opposing surfaces should have opposite treatment. So if you have an absorbent ceiling, you should have a reflective floor. Also, industrial carpet is probably doing unpredictable things to the acoustics. Do you have the acoustic specs for that specific carpet, to tell you what frequencies it absorbs best, and which ones it absorbs least, to take that into account for the rest of the treatment? In any event, it will only be affecting the highs, not the lows, so it is contributing to making the room "duller", leaving the bass boom in.

I've only done subjective testing so far, I hope to get more results as things progress.

That sounds like you are doing things backwards! You need to test the room BEFORE you can decide how to treat it! If you just put up treatment and then test the room, you won't know if you made things better or worse with the treatment! In fact, you should really have test it when it was just an empty shell, before putting any treatment in at all. As it is now, you really don't know what you ahve treated and what you have not treated, or if you made things better or worse!

Thanks, I'll give some thought to your comments and advice. jim

jimlongo wrote:

The rear right is a hard surface, the bass

Why? What's behind it. Secondly and more importantly- What are you asking for? If there's no bass problems and the room sound good, then I don't get it. you've got the dolby 5.1 spec's- build as per that. Lose the carpet and find out what behind that wall.