A question about soffit mount wall

Hello everyone and many thanks for all the information provided on this site. I have been reading exhaustively over the last half year or so but still have much to learn. So, onward. The background info: My background is in Audio Post for the last 20 years and through that I have participated in the building of several rooms. I am currently working on a Post facility in Shanghai that wishes to build a Score stage and I've been asked to help design the control room. Quite a different project than a dubbing stage. Room Dimensions Length: 7050 mm Width: 5700 mm Height: 3100 mm Outer Leaf: 200mm Cement wall on 3 sides, 25mm or 32mm Drywall on wood studs on fourth side outer surface. Inner Leaf: 25mm Drywall on wood studs inner surface. The false wall is available for acoustic treatment(s) and covered in acoustic cloth. I can build the 'baffle' wall from any material. Any suggestions? All four corners will be bass traps, most likely floor to ceiling hangers. To the rear is a seperate booth/producer's room with the same inner leaf construction. 100mm gap. My Question (of the moment) Is there a point to flush mounting speakers if the 'baffle' wall does not extend to the inner leaf, but terminates at another false wall? I have seen in John's design that the bottom foot or so is ported to allow low frequency to reach the hangers. This suggest to me that I could leave the edges of the 'baffle' wall open to allow the same thing. Would this retain the room measurements or simply confuse the matter further? My sketchup file is not quite complete. I'll clean it up and upload in a short while. Thanks in advance.

Hi there "Scorpiain", and welcome" :)

Length: 7050 mm Width: 5700 mm Height: 3100 mm

That's a really nice size. You should be able to build a very nice studio in that space.

Outer Leaf: 200mm Cement wall on 3 sides, 25mm or 32mm Drywall on wood studs on fourth side outer surface. Inner Leaf: 25mm Drywall on wood studs inner surface.

That's a good plan. Assuming that you have a properly sized air gap between the leaves, and that both are properly sealed, then you should be able to get excellent isolation.

The false wall is available for acoustic treatment(s) and covered in acoustic cloth.

There seems to be a conceptual problem with that "false wall": it is not needed! The acoustic treatment should be attached to the inner-leaf wall, not hung in mid air a long distance away. It will not be effective like that, it wastes space, and it will make the room look strange.

I can build the 'baffle' wall from any material. Any suggestions?

There's a conceptual problem with that too. It seems you are trying to build an RFZ design with flush-mounted ("soffited") speakers, and that is an excellent idea, if done right. RFZ is one of the best design concepts available currently, and my personal favorite. But it has to be done right, not the way your drawing shows. The entire purpose of soffit-mounting your speakers is to eliminate all of the artifacts caused by having the speaker inside the room. The basic concept is that the speaker is mounted flush with the surface wall (hence the name "flush mount"), so that from the acoustical point of view it is NOT in the room, but rather in the wall. The front baffle of the soffit becomes the new wall of the room. If you have your baffle sitting out in the middle of the room, not touching the side walls and ceiling, with air all around it, then you have not built a soffit. You have only moved the baffle step response point down to a lower frequency, but you have not eliminated SBIR, front wall reflections, phase cancellations, comb filtering, edge diffraction, or any of the other artifacts that a properly constructed soffit does eliminate. So I'd suggest that you re-design the front end of your room as a proper soffit-mount system, with the speakers fully embedded in the wall, flush mounted with the surface, and the baffles forming the new front wall of the room. Take a look at some of John's designs, to see how to do soffit-mounting correctly, and also look at some of the many threads by forum members, which show in detail how they have built their soffits.

All four corners will be bass traps, most likely floor to ceiling hangers.

With a properly built soffit, there is not front corner! There is space below the speaker shelf, inside the soffit, where you can have hangers, yes, but that is a ported cavity, not a true room corner. Hangers would be great in there, and that's the way that John usually does it, but they only fill the bottom part of the soffit: the top part is filled by the speaker itself, and the ventilation "chimney", and the damping insulation. Hangers would be great in the rear corners of the room, and you might also want to consider putting them across the entire rear wall, the way John does, to get the deep low frequency damping that your room will need.

To the rear is a seperate booth/producer's room with the same inner leaf construction. 100mm gap.

Maybe it would be better to turn the control room around, and put a window in there, so you have good visibility into that booth? Sight lines are important when tracking.

Is there a point to flush mounting speakers if the 'baffle' wall does not extend to the inner leaf, but terminates at another false wall?

No, none at all. Or rather, there is a slight benefit, since it does help to compensate for the power imbalance issue, and pushes the baffle step response curve down to a lower frequency, but it does not give all the other benefits that a true soffit would give you.

I have seen in John's design that the bottom foot or so is ported to allow low frequency to reach the hangers.

Right, but that's a partly sealed cavity inside there: it does not extend up and around the sides of the soffit, or behind the speakers. It's just a "box" with two holes in it: one is the opening to the room, and the other is the ventilation port under the speaker. Apart from that, it is an enclosed cavity. No chance of having edge diffraction or SBIR issues.

This suggest to me that I could leave the edges of the 'baffle' wall open to allow the same thing.

If you leave the edges open, then edge diffraction can still occur, as can SBIR and all the other artifacts associated with speakers that are inside the room.

My sketchup file is not quite complete.

There's one more issue that needs fixing on there: you show the "outer leaf" on the other side of some type of hallway, then the inner leaf bordering the room. That implies that your hallway is inside the "spring" of your MSM isoaltion system, which is not a good idea. You should complete the outer-leaf just behind the inner-leaf, and have the hallway outside the outer-leaf. - Stuart -

Stuart, Thanks for clearing up all the misconceptions and ' I don't knows '. I will do as you say in redesigning the front wall and post a new drawing. On further reflection I see that I've angled both side walls 12 degrees. I will reduce that to 6 degrees each and make that the inner leaf; gaining a little more real estate. In bringing the front wall forward, I should then extend the back to retain the room ratio? I know that angling the walls alters the calculations but I'd like to keep it in the same ballpark. To further complicate matters, the room is to be equipped 7.1 for mixing film scores. Again, many thanks.

On further reflection I see that I've angled both side walls 12 degrees. I will reduce that to 6 degrees each and make that the inner leaf; gaining a little more real estate.

For a true RFZ design, you'll very probably need to angle the front part of the side walls by MORE than 12°, not less! Much more... The only way to determine the correct angle, is by "ray-tracing" manually: projecting lines out from the acoustic center of the speaker at various angles, seeing what surfaces of the room they hit, then "bouncing" the line off that surface as though it was a mirror, and seeing where that reflected line goes: if it gets too close to the mix position (the engineer's head), then you need to increase the angle more until it goes well clear of his head.

I know that angling the walls alters the calculations but I'd like to keep it in the same ballpark.

You can use the average dimensions of the room to get a reasonable idea of how the modal response will be. For example, if the room length is 5m at the edges of the front wall and 6m at the middle, then use a value of 5.5m. But do also check for 5m and 6m, just to make sure there are no surprises... :)

To further complicate matters, the room is to be equipped 7.1 for mixing film scores.

:shock: :!: That makes it a whole lot more complicated, if you are doing RFZ! MUCH more complicated. You need to ray-trace all speakers in all directions, to very wide angles (60° or more off-axis), then provide suitable treatment in the room to deal with the reflections and overall room response. Mixing for film also implies that your subs will be going down all the way to the bottom end of the spectrum, and your isolation will need to be able to handle that: Designing a studio that isolates earthquake noises and canon fire, as well as machinery rumble and other very low frequency sounds, is not easy at all. That's what film work implies. That also implies that this is a high-budget studio, not just a typical home studio or project studio, and you'll also need to take into account things like the large video screen on the front wall, which must extend to Academy aspect ratios (and even wider) while still allowing or correct speaker placement and neutral room acoustics. You should probably consider hiring a professional designer to do that. It's way more complex than designing a typical home studio for 2.0. or 2.1 sound. I'd suggest that you send a Private Message (PM) to John, and ask him to quote for designing your room. I'm not saying that you can't do it yourself! You sure can, especially with the help of the forum. But it will take a long time to learn all of the intricate details of the isolation and acoustic aspects that you'll need to know in order to do this. A project studio is one thing, but a 7.1 RFZ room for film scores is a very, very different thing. Take a look at ITU-R BS1116.1 and EBU-Tech-3276-E to get an idea of how complex this is. Also check out AESTD1001.1.01-10, and even Dolby S05/14926/15996. All of those will give you some insight into multi-channel rooms, how to set them up, and what acoustic response is required. - Stuart -

Soundman2020 wrote:

There's one more issue that needs fixing on there: you show the "outer leaf" on the other side of some type of hallway, then the inner leaf bordering the room. That implies that your hallway is inside the "spring" of your MSM isoaltion system, which is not a good idea. You should complete the outer-leaf just behind the inner-leaf, and have the hallway outside the outer-leaf. - Stuart -

Glad I spotted this, because I'm planning to have a small vestibule in between inner and outer skins. Partly so the two doors aren't close together and parallel, and partly to house a step down into the studio. The vestibule walls will be very absorbent and the ceiling will be a deep bass trap. I'm thinking I might also be able to use the room as an isolation booth for guide vocals, the occasional guitar amp etc. I realise the attenuation between this room and the outside world won't be great, nor will the attenuation between the room and the studio. But why will this adversely affect the MSM performance of the whole? Isn't it just like having a part of the spring tuned lower than the bulk of it (mostly a 100mm cavity for the walls, more like 250mm for the ceiling), and I understand a wider cavity is preferable to a shallow one as it lowes the resonant frequency? Many thanks, Chris

Anyone? Still seems reasonable to me, its just a generous (1.6 x 1 x 2.4 metre) airlock really...have I missed something?

Glad I spotted this, because I'm planning to have a small vestibule in between inner and outer skins.

It's not a good idea to do that, basically because the air gap between the two leaves is part of the isolation system. It is a tuned system, so anything you do inside there changes the tuning.

Partly so the two doors aren't close together and parallel,

There is no problem with having the doors close to each other, nor with having them parallel. They should be just far enough apart to create the correct MSM resonant frequency, the same as for the rest of the wall. It is a complete system: the surface density of the two leaves and the air gap between them must be kept in a consistent relationship throughout.

The vestibule walls will be very absorbent and the ceiling will be a deep bass trap.

Actually, for optimum isolation the entire air gap between the two leaves should be filled with porous insulation. It is part of the damping mechanism, that acts on any resonance that happens to occur in the cavity. It's a key part of the system.

I'm thinking I might also be able to use the room as an isolation booth for guide vocals, the occasional guitar amp etc.

It would not be isolated from either! You only have a single leaf between the cavity and the CR, and also between the cavity and the outside. So the total isolation in each direction would be limited by the Mass Law equations, not by the MSM equations ... Not very useful!

But why will this adversely affect the MSM performance of the whole?

Think of it as not just changing the frequency, but also as moving the entire TL curve over to the left... including the coincidence dip, which no falls at a lower frequency too. You also now have two different resonant frequencies for the wall, so there are two different curves, and two different isolation factors, plus there is potentially interaction between the two, if they happen to be related mathematically (musically). You are also changing the point o transition between the various types of isolation for that section of the wall. It MIGHT work out to your advantage, but it would be rather complicated to figure out if that would be the case or not.

Still seems reasonable to me, its just a generous (1.6 x 1 x 2.4 metre) airlock really...have I missed something?

Except that it isn't a sound lock at all! It's just a large cavity in the wall... .) You COULD build a proper isolation booth in there, by adding another leaf around it and adding two more doors, or you could use the space for something else, such as making the control room bigger, or perhaps for storage, or as a machine room. I'm looking forward to seeing the updated design. It would also be good if you could post the SketchUp model somewhere, do we can download it and look at it in more details, as well as posting some photos of the place where the studio will be going. - Stuart -

Soundman2020 wrote:

Glad I spotted this, because I'm planning to have a small vestibule in between inner and outer skins.

It's not a good idea to do that, basically because the air gap between the two leaves is part of the isolation system. It is a tuned system, so anything you do inside there changes the tuning.

OK, but the tuning hasn't been chosen to be a particular f in the first place, it's just as low as I can get it as constrained by the available space...it's a 100mm gap in the walls with more in the ceiling. I'm thinking the Q of the system is low and therefore broadband. Is that muddled?

the surface density of the two leaves and the air gap between them must be kept in a consistent relationship throughout.

Really? Why?

The vestibule walls will be very absorbent and the ceiling will be a deep bass trap.

Actually, for optimum isolation the entire air gap between the two leaves should be filled with porous insulation. It is part of the damping mechanism, that acts on any resonance that happens to occur in the cavity. It's a key part of the system.

I'm thinking I might also be able to use the room as an isolation booth for guide vocals, the occasional guitar amp etc.

It would not be isolated from either! You only have a single leaf between the cavity and the CR, and also between the cavity and the outside. So the total isolation in each direction would be limited by the Mass Law equations, not by the MSM equations ... Not very useful!

I'm aware that it'll be half-arsed ;-) I'm not expecting more than mass law for this bit of the design.

But why will this adversely affect the MSM performance of the whole?

Think of it as not just changing the frequency, but also as moving the entire TL curve over to the left... including the coincidence dip, which no falls at a lower frequency too. You also now have two different resonant frequencies for the wall, so there are two different curves, and two different isolation factors, plus there is potentially interaction between the two, if they happen to be related mathematically (musically). You are also changing the point o transition between the various types of isolation for that section of the wall. It MIGHT work out to your advantage, but it would be rather complicated to figure out if that would be the case or not.

Still seems reasonable to me, its just a generous (1.6 x 1 x 2.4 metre) airlock really...have I missed something?

Except that it isn't a sound lock at all! It's just a large cavity in the wall... .)

So what would I have to change to make it an airlock?

You COULD build a proper isolation booth in there, by adding another leaf around it and adding two more doors, or you could use the space for something else, such as making the control room bigger, or perhaps for storage, or as a machine room.

These are all possibilities. I'm unconvinced, though, that the existing design is any more than a 'bulge' in the cavity, and as such will have any ill effects. Are you saying that a MSM system must have equal M and gap throughout? In my case I have 30mm drywall inner skin throughout, then a variable gap, then 100mm concrete on three sides, 200mm concrete on the fourth and a roof made of 3 sheets of 18mm MDF + roofing felt. I'd have thought the roof would be the weak spot, it being less massive than the walls, but you suggest that the fact that one of the walls is more massive than the others might be an issue? Actually, it's even more variable than that. Stay with me, folks...the building is partly underground so the M of the outer skin varies between the relatively flimsy roof, the 100mm concrete walls on three sides, the 200mm wall on the fourth and massively massive walls throughout for the lowest 800mm of the structure. So "keeping the [surface] density of the two leaves and the air gap between them... in a consistent relationship throughout" isn't really going to be a possibility! Surely adding mass to any one skin can only have beneficial effects?

I'm looking forward to seeing the updated design. It would also be good if you could post the SketchUp model somewhere, do we can download it and look at it in more details, as well as posting some photos of the place where the studio will be going. - Stuart -

I shall post Sketchups as soon as I work out how to use it meaningfully...initial efforts have been time-consuming and confusing, I'm handier with a trowel than a mouse ;-) But, grateful thanks for your replies, much appreciated.