5.1 mixing room plan (for poking holes in, please)

Hi - I work as an artist-run centre that is moving into a new location later in the year. We are working on putting together a 5.1 mixing room, and I was hoping to get some feedback about this plan: The room is small (328 square feet), as we won't have so much space overall. This is about as large as we can manage, with minimal disruption to the adjoining spaces. We are on the bottom floor. There will be foot traffic above, and an adjoining stairway. The surrounding spaces are used for video editing, production and electronic (soldering, circuitmaking, etc.). We are thinking of putting concrete blocks in the new walls and cutting the floor concrete around the inner edge of the walls in order to separate the floor of the mixing room from the floor of the rest of the building. I know that this means the walls of the mixing room will still be connected to the rest of the floor structure and could therefore be resonant, but cutting on the outside edge is not possible on all the walls, given the existing building structure (indicated by the crosshatched walls). We encountered a significant problem on a recent site visit - there was concrete support post jutting out 46" that hadn't been included on our floor plan. We decided to try reorienting the room so that the post sits between the back 2 surround speakers. I couldn't see any other way to have the room be symmetrical left to right, and there was nowhere else in the building that the room could be relocated. The speakers are 10" deep and 7" wide, so the size in the drawing is to scale. The distance from the mixing position in the drawing is set at 7 feet, which seems to give us 5 inches to move them closer (given 6.5-7.5 feet distance from the mixing position as ideal), as it still looks a little tight in terms of how far the speakers are from the walls (especially the centre one). At 7 feet away, we've got 1 foot between the back of the speaker and the wall, plus the 10 inches from the front of the speaker, so almost 2 feet. It'd be 3 feet 1 inch between the mixing position and the post. We would use a quadratic diffusion along the front surface of the post, so in reality it'll be a bit less than that. As I mentioned, we have a few inches of room to move there, but it'd be tight between moving the centre speaker further from the wall and the mixing position further from the post. The coloured drawing shows what we're thinking about the ceiling. The ceilings on this floor are 10 feet, but the support post also has a beam that comes down 1 foot from the ceiling. As this would mess up the left-to-right symmetry really badly, we're thinking of fanning it out (the pink area in the middle) to make it even on both sides. That whole area will have a 9 ft. ceiling (more on this shortly). We're thinking of sloping the ceiling gradually, so that it reaches 10 feet along the 2 green lines, then sloping it back to 9 feet in the back corners (indicated in pink). This way the ceiling isn't flat and parallel with the floor, and it also helps mitigate the almost-90 degree angles in those back corners by changing the angle of the ceiling to the walls. We'll have two outside-type doors at the entrance, but because of where the entrance sits left-to-right along the front wall, we are thinking of covering the back surface with diffusion and mirroring that shape with diffusion on the other side of the front wall (to the left of the mixing position, whereas the door would be to the right). One thing I've heard recently is that the workers are finding that the building is much more skewed than it seems in the plan. This will probably work in our favour, as those 91-degree angles on the back walls will almost certainly be further from 90 than they appear on the plan. Whether that will be closer to 95 or 85, I can't say. We do plan to add bass-traps in the corners. The ventilation isn't worked out yet, but we would like to have the fan decoupled from the duct-work and as far from the mixing room as we can have it. Here's where I can see some problems, and I'd very much appreciate any feedback anyone can provide: 1) I'm not sure what exactly to do with the very middle of the ceiling (the pink fanned-out area in the centre) - if we keep it flat across, that's not ideal, as it'd make the floor and ceiling parallel in the spot where you'd be mixing, but if the low point extends along the line of symmetry like a fold, that might also be weird if you happened to be sitting slightly on one side or the other, or even just turned your head, etc. We're not doing too badly with ceiling height (it'd be slightly under 9 feet in that area) - would it make sense to use some kind of cloud? 2) What about the back 2 speakers and the post? We're trying to do the best we can about the angles, and it helps that the speakers are small, but is there anything else we might consider? We might get more room if we soffit-mount the front ones, but that'd create problems with the wall thickness and structure and might be more trouble than it's worth. 3) If we got to choose about the position of the ventilation duct (it'd likely have to be the front wall again), how might we mitigate that it'll probably be asymmetrical? That part is a bit confusing to me, as it seems like adding a little reverb tank to a room. More importantly, I'd really like feedback about problems I CAN'T see. This is quite a new endeavour. Please let me know if I've neglected any important information. I know we haven't worked out exactly where all the diffusion should go yet, but if we've got enough to feel that a basic floor plan can be confirmed, then I feel there's room for us to move on and figure that out. Thanks so much for reading, and best of luck to all those in the middle of their own projects.

Hi "T Elliott", and Welcome to the forum! :) There's several issues that I see at first glance with that plan. First (obviously!) is that huge concrete support pillar. Ouch! That really messes things up, big time. The way you have things right now, the listening position is way too close to that pillar, and also too close to the rear wall in general. In fact, it looks like your head is very close to the geometric center of the room, which is the worst possible place for it, acoustically. It places you right in the middle of all the axial nulls. Plus, I honestly don't think that shape of room is workable: there's just to much strange stuff going on a couple of feet behind your head. I think what I would do is rotate the layout about 90° clockwise (so if faces to the left of your diagram), and build a splayed wall on the right side that takes that entire pillar out of the room, then maybe use the space next to it for storage or something. Yes, that does mean that the room would be smaller, but given the current shape, I don't think that a smaller room would be a bad idea at all! If you do that, you would then be able to have the main speakers firing the "correct" way, down the long axis of the room (instead of across the short axis, as you have them now), and you'd also be able to get the L, C and R speakers right up against the front wall, where they belong in a small room, or even better still, soffit mounted. There would be room to do that (it would be tight, but still do-able). That plan would also give you much better symmetry, and solve some of your construction issues that you mention later. (You'd need to move the door a bit further along that side wall, to allow space fo the soffits and/or bass traps, but that's not a big deal.) Overall, I think that would be a much better proposition than what you have right now.

There will be foot traffic above, and an adjoining stairway. The surrounding spaces are used for video editing, production and electronic (soldering, circuitmaking, etc.).

OK, so isolation is going to be an issue, by the sounds of it. What is your goal there? What level of transmission loss are you planning for, with the current design?

and cutting the floor concrete around the inner edge of the walls in order to separate the floor of the mixing room from the floor of the rest of the building.

:shock: Careful with that! You can't just go cutting structural members of a building just like that! The floor is very likely part of the structural integrity of the building, and probably part of the foundations too. While you MIGHT be able to do that, you'd need to hire a structural engineer to tell you for sure, and guide you as to where you can cut, how to dig out the old floor, how to add the new footings, etc. This is not something you can do just like that, without professional advice. And of course, the underlying question is: why do you think you need to do that? What is the noise level in the building, how much structure-borne noise are you dealing with, and how much isolation do you need, in decibels? That's the key question. You might not need to even think about cutting that floor: You are on the lowest level of the building, so presumably the floor is laid directly on the ground, and therefore damped by the entire planet! Unless you have extreme isolation needs, there's probably no need to do that.

I know that this means the walls of the mixing room will still be connected to the rest of the floor structure and could therefore be resonant, but cutting on the outside edge is not possible on all the walls, given the existing building structure (indicated by the crosshatched walls).

If you cannot cut the entire floor, then there is no reason to cut any of it. Resonance is not the problem: flanking is the problem. If there is structure-borne noise in the floor, then just cutting part of it will have no effect at all. You'd have to cut the entire floor, and isolate it correctly, in order deal with that. Even a thin sliver of concrete or rebar that you didn't cut would negate the effects of everything that you did cut. Isolating a studio is an "all or nothing" proposition. If you cannot isolate the entire floor then isolating part of it is pointless.

The speakers are 10" deep and 7" wide, so the size in the drawing is to scale

What speakers are they? (Make, model). That's important.

(given 6.5-7.5 feet distance from the mixing position as ideal),

Not sure who gave you that figure, but the EBU recommendation for 5.1 rooms is 2 to 4 meters, meaning 6'-6" to 13'-2", roughly. (EBU tech supplement 3276). So ideal distance is about 9' to 10'. (But of course, that depends on the room and the speakers, too.)

as it still looks a little tight in terms of how far the speakers are from the walls (especially the centre one).

Very true. With a room that size, the center speakers should be right up against the wall. A one foot gap is already producing SBIR artifacts in low frequencies. Yet another reason to rotate the room.

It'd be 3 feet 1 inch between the mixing position and the post.

Waaaaaay too close! That's just 3 ms! You want your head to be at least 10ms from the rear wall.

It'd be 3 feet 1 inch between the mixing position and the post. We would use a quadratic diffusion along the front surface of the post, so in reality it'll be a bit less than that.

The entire room is too small for most types of diffusion: you can't get it far enough away from your head to allow for the lobing artifacts to merge and smooth. The general recommendation is at least 3 full wavelengths o the lowest frequency being diffused, and best at 7 full wavelengths or more. At 3 feet, you cannot tune your diffusers any lower than 1.1 kHz, and ideally no lower than about 3 kHz, which isn't worth doing at all. Your only options for treatment with that layout are broadband absorption, very thick, and spaced away from the walls. You don't have much space to do that either, with only 3 feet behind your head! The fact remains that the room is too small to benefit from most common types of diffusion, especially QRDs. You might be able to get away with polys, and certainly could use slot walls, but not QRD based diffusers. The door is in the wrong place too: Corners should always be reserved for bass traps in small rooms, since you get the free 12 dB or 18 dB boost in effectiveness in corners, so that door hast to be moved, regardless of which way you orient the room.

As I mentioned, we have a few inches of room to move there, but it'd be tight between moving the centre speaker further from the wall and the mixing position further from the post.

Why are you trying to keep the speaker AWAY from the wall? It's already too close, and firmly in the region where SBIR is going to be a big issue, so the ideal is to put it right against the wall (assuming that it has bass roll-off controls, or that you have another method of rolling off the bass, to get rid of the baffle step compensation). But even if you did that, you'd still only have 4' from head to post. Still much too close.

That whole area will have a 9 ft. ceiling (more on this shortly). We're thinking of sloping the ceiling gradually, so that it reaches 10 feet along the 2 green lines, then sloping it back to 9 feet in the back corners (indicated in pink).

You mean like a reverse gable? That would probably act something like a compression ceiling, only sideways. No idea how that would work, acoustically, but Tom Hidley abandoned his attempts at compression ceilings several decades ago, and that was before 5.1 was even invented.

This way the ceiling isn't flat and parallel with the floor,

What's wrong with that? Provided that you treat it correctly (you'll need a cloud anyway), there's nothing wrong with a flat ceiling.

and it also helps mitigate the almost-90 degree angles in those back corners by changing the angle of the ceiling to the walls.

Once again, what is wrong with 90 degree angles? This is a control room, not a live room, and you are not trying to do an RFZ or CID design, so there's no reason to have a non-rectangular room. It's a myth that controls rooms must be non-rectangular. Treated right, they are great. And it's much easier to calculate the modal response, too.

We'll have two outside-type doors at the entrance,

With a single-leaf wall such as you propose (single layer of concrete block), there's no need for two back-to-back doors. a single Superdoor would do the job very well.

but because of where the entrance sits left-to-right along the front wall,

I already mentioned that the entrance door is in totally the wrong place. It needs to be moved. The best position would probably be the middle of that wall, roughly, regardless of which way you orient the room in the end.

The ventilation isn't worked out yet, but we would like to have the fan decoupled from the duct-work and as far from the mixing room as we can have it.

Fans (plural) not fan (singular). You need at least two ducts: one to bring fresh, cool air into the room, and the other to take stale warm air out. If the building already has an HVAC system then you could tie into that (using silencer boxes, of course), but you'd probably still need booster fans. What flow rate and velocity are you figuring for that room, and how much heat needs to be removed?

if we keep it flat across, that's not ideal, as it'd make the floor and ceiling parallel in the spot where you'd be mixing,

Once again, there's nothing wrong with that. You are going to need am angled cloud there anyway, and worst case you'd just make it hard backed to break up flutter echo and modal issues, if needed.

but if the low point extends along the line of symmetry like a fold, that might also be weird if you happened to be sitting slightly on one side or the other, or even just turned your head, etc.

Exactly. Especially with 5.1, where you have sound coming from the rear/sides of the room as well. I would not do that. Just make it flat, and treat accordingly, in the normal way.

2) What about the back 2 speakers and the post? We're trying to do the best we can about the angles,

Right now your angles are too tight. EBU recommends 110° to 120°, but yours are less than that. ITU goes even further, and suggests up to 135° is OK, but I'm ont aware of anyone who recommends less than 110°.

We might get more room if we soffit-mount the front ones, but that'd create problems with the wall thickness and structure and might be more trouble than it's worth.

I beg to differ! :) Soffit mounting is never more trouble than it is worth! The benefits are enormous. No SBIR issues at all, tight and clean bass, best possible imaging, accurate sound stage, perfect power balance, no need for baffle step compensation, etc. In fact, I'd even mount your surrounds in some kind of soffit, since you say they are very small. Soffit mount is your best option, without any doubt. I'm not sure why you say it would create problems with wall thickness: Maybe you could explain that in more detail? The wall doesn't have to be any different tan a normal wall.

3) If we got to choose about the position of the ventilation duct (it'd likely have to be the front wall again),

The ducts (plural) not duct (singular) should be on the ceiling, if at all possible, not on the walls. Normally you bring in fresh cool air at the rear of the room, and exhaust the stale warm air out the front, or vice versa. so you could a pair of inlet ducts somewhere in rear ceiling, and another pair somewhere hear the front, perhaps over the speakers, roughly.

how might we mitigate that it'll probably be asymmetrical? That part is a bit confusing to me, as it seems like adding a little reverb tank to a room.

Not if it is done correctly. Use thick duct liner inside all of your metal ducts, and use flexible round duct as much as possible, with in-line silencers, of course. Keep the grills large, the speeds very low, and the volume high. Shouldn't be a problem, if done right.

Please let me know if I've neglected any important information.

What I don't see there is the most basic of all information: Your design goals. What is this room going to be used for? Commercial, education, or hobby? What type of music? Is there an attached live room / iso booth? Is this for tracking or mixing or mastering, or some combination of those? How much isolation do you need (decibels)? What is your isolation plan? Those are the important things that seem to be missing. If it wasn't for that huge pillar in the rear of the room, your current plan could probably be modified to work, but with that "800 pound gorilla" in there, I'd really suggest that your best option is to rotate the layout and build your isolation walls to get rid of that thing, and get it out of the room completely. - Stuart -

Thanks very much for your reply; I will try to fill in some missing information and get on to re-thinking what is possible with the space we have. It's incredibly helpful to know that trying to make the best of that post shape-wise is not a good idea. I had been looking at the 2004 National Academy of Arts and Sciences recommendations (that's where the 6.5-7.5 foot distance from monitors came in), as well as the Dolby Laboratories 5.1 production guidelines, as well as consulting with the contractors, but have no practical experience and therefore almost no ability to set priorities for the different problems. Our design goals - it's intended to be a multi-user facility, and a bit difficult to pin down given that the users will be multi-media artists. Video post-production will play a significant role (editing, mixing and mastering). One of the main things we would like to facilitate is non-standard surround production for multi-media artworks. One of the adjoining spaces is a multi-purpose project room. We will run lines out from the mixing room to the project room, making an amplifier setup available there. The idea is that a person could build, say, a gallery installation with three speakers stacked vertically in a piece of sculpture, with an additional two speakers mounted inside two adjoining La-Z-Boy chairs, editing, processing, etc. all the audio in the mixing room, but able to reference the real-world elements in the project room. To make all this potential strangeness practical, though, the mixing room has to be a good standard reference space. The monitor speakers - We have a Blue Sky 5.1 system - the speakers are 5 SAT 5 MKIII plus a Sub 8 MKIII subwoofer. The speakers have adjustable stands. In terms of having a smaller room, I had been trying to go by the minimum Dolby recommendation of 320 square feet. I will get to measuring and see what is possible getting rid of the pillar. I will also try to figure out a ballpark for the amount of isolation we will need. It's not possible to actually measure levels, as the building is currently gutted, but it'd be good to have some perspective there. We may be able to measure a rush-hour traffic level at least. Thanks again - tom

Tom - here's an interesting article: The Recording Academy's Producers & Engineers Wing Recommendations For Surround Sound Production

Hi - thanks; that's the document I had gotten the monitor-distance measurements from.

Adding to John's NARA recommendation, here's a few more, Tom, just to keep you nicely confused! :) EBU recommended layout: ITU recommended layout: Genelec recommended layout: AES Recommended layout for HDTV production and post production studios: 5.1 plus 6.1 recommended layout (unknown source): There's a few other, too, but they are mostly similar to one or other of the above. That's one of the reasons why I asked about the purpose of the room, since there are different recommendations for different purposes. But it seems that yours is going to be a general purpose studio, and the NARA layout (the one John posted) covers pretty much all bases, so that's your best bet (unless you have something specific in mind). The main difference between all of those is where to put the rear surrounds. The early ITU recommendation was for 110 to 120°, but all the others put them much further back (even up to 150°). The general consensus is that around 135° is a good compromise. There are other recommendations that you might want to consider: For example, the floor area isn't really the most important indicator. Rather, room volume. Both ITU and EBU recommend at least 1500 cubic feet of room volume. But that doesn't mean you can't build a room with less volume! It just means that it will be harder to do, and won't be optimal. John has built studios inside shipping containers, so you certainly can get a great studio in a small space. - Stuart -

Thanks again for your replies; that post issue is a real problem, and I appreciate the perspective about it. We're going to head back into re-thinking mode, and I'll come back to the thread when there's a new plan. Best wishes for everyone in the meantime - tom