thismanysounds - Basement Studio in London UK

[Johnnie]

If I may ask a question related to this thread, but it's for my own edification. Earlier I'd advised thismanysounds that different materials were OK as long as the isolations were comparable. Stuart you'd recommended that it would be beneficial to have those extra walls built the same way so that consistency would be achieved. (thismanysounds... always go with Stuart if there's ever an inconsistency between his advice and mine... I'm an amateur early in the learning process, he's the expert with real practical experience.) I just wanted to ask whether same materials are really important, or is it comparable isolation that's important regardless of material? Or maybe it's just a practical issue and getting sufficient mass to match a sand filled brick wall is just impractical with wood built construction...

[thismanysounds]

Johnnie,
I really appreciate that this has stayed in your mind, despite my silence for the last week. The way I read it, it was more that the mass was consistent rather than the absolute material.

So here is where I am at the moment:
- We now have a much larger area in the basement we can use (~130m2).
- There still needs to be access across the basement (see sketchup model).
- The extra space should add function, and not just room size. As such a second control/edit room and lounge is desired.
- Part of the basement is not submerged, so has thin single pane windows.
- The absolute slab to ceiling height is 3.15m, so height efficient construction is desired.

Attached is my current design. Here is my reasoning:
- Two control rooms share a larger live room. Larger live room makes for better recordings.
- One of the control rooms (A) is larger (with better spec). The second (B) is more of an edit room.
- Studio A is now a Non-Environment design. I have heard similar rooms before, and enjoyed them. It also makes listening across the room more enjoyable (less of a sweet spot).
- Studio B can double up as booth for large sessions, or for overdubs (using the live room) if Studio A is mixing.
- This is a project studio, so this seems like a better use of space rather than two isolated live/control studios.
- The existing windows/daylight/poor isolation walls are used for corridors and lounge space, rather than blocking them up.

Let me know your thoughts!

Colm

[thismanysounds]

Actually, scrap that. I have a few questions:

HVAC system (see attached photo)
- Seeing as I need ducting with silencers for ventilation anyway, it seems logical to go for a ducted air handler system rather than 3x mini splits. Good idea / bad idea?
- With silencer boxes, do I need them internally in the rooms (as per the diagram)? I guess the advantage of doing so means that the sound is attenuated before it reaches the shared cavity.
- For Control B, will it be better isolation wise to have a separate cool air supply run from the boiler room along the corridor?
- What size silencers are needed to achieve the isolation desired?
- I have put in a HRV unit to handle the fresh air and to 'push' the air. Is this sufficient, or will I need extra fans for air flow?
- The common convention seems to be cool air supply is at floor level (front of CR) and air exhaust is at ceiling level (back of CR). Is there any reason why I would go against this?

Flush mounted monitors
- Now that the room is a bit bigger, and Stuarts persuasion, I am going for flush mounted speakers. I know that this wall needs to have high mass to work. Does the inner leaf continue behind the speaker creating a box, or does the new massive wall now act as the inner leaf?

Low frequency absorption
- At what size does it start to make sense to use hangers over other designs?
- It seems like: Internal Wall -> VPR absorber (125mm) -> 100mm Rockwool -->Fabric would perform well at LF as well as broadband at a small depth (<250mm). The downside, is that the materials are expensive. If I don't have the space for hangers, what is the best way of achieving Non-Environment absorption?

[Soundman2020]

- Seeing as I need ducting with silencers for ventilation anyway, it seems logical to go for a ducted air handler system rather than 3x mini splits. Good idea / bad idea?

Good idea! For three rooms, yes, definitely worth doing that.

- With silencer boxes, do I need them internally in the rooms (as per the diagram)? I guess the advantage of doing so means that the sound is attenuated before it reaches the shared cavity.

As long as you put them right at the room boundary (inner-leaf), then it doesn't really matter if you put them in the room or in the cavity between the leaves. The advantage of putting them in the wall cavity is that those things are BIG, and take up a lot of space inside the room, so putting them outside makes the room less cluttered and more spacious.

- For Control B, will it be better isolation wise to have a separate cool air supply run from the boiler room along the corridor

?I prefer to always use one duct per room, rather than trying to share a duct between two rooms. The reason is simply that it is very hard to balance the air flow properly between the rooms when you have a shared duct: As you close down the damper for one room, you automatically force higher flow into the other room, like it or not. And the converse also: as you open up the damper to get more flow in one room, you automatically rob the other room of some of its flow. So it's better to run a large central duct plenum with branch ducts and silencers for each room, or to just split them right at the air handler into individual ducts that go to each room, in which case it is easy to have all of the dampers right there, at the air handler, where you can reach them easily for maintenance, repair or replacement.

- What size silencers are needed to achieve the isolation desired?

The cross-sectional area inside the silencer needs to be at least twice that of the duct, and the registers need to be at least as large as the silencer cross-section, or even larger. This is to keep the air flow velocity very low, to reduce air noise, and also to create two "impedance mismatches", where the cross section changes suddenly, which greatly attenuates the sound. Then the number of baffles inside the silencer and the total length of the air path are factors in providing isolation: I have not yet found any way of calculating how much attenuation you get from each baffle (a lot), but there are tables that show how much you get from air path length for different cross-sections (not much). So as a general rule of thumb, I would do one or two baffles for low to medium isolation requirements, three for medium, and four or more for high isolation requirements. The final point is that, since the HVAC duct is basically a huge hole in your isolation wall, the silencer has to be built the same way as the wall: Keep the same surface density on the silencer box at the place where it goes through the wall. Obviously, at the other end of the box you don't need to do that, since you are just connecting to a thin metal thing, but it's easier to just build the box with the same surface density all over.

- I have put in a HRV unit to handle the fresh air and to 'push' the air. Is this sufficient, or will I need extra fans for air flow?

Assuming that your rooms are properly sealed (ie, absolutely and totally air-tight), and that the final static pressure of your complete ducting system is within the specs of the air handler unit, then you probably don't need extra fans. You only have problems when the static pressure is too high for the AHU to handle, which leads to overloading of the fans, inefficient operation, and early failure of the fan motor. Not to mention increased noise...

- The common convention seems to be cool air supply is at floor level (front of CR) and air exhaust is at ceiling level (back of CR). Is there any reason why I would go against this?

Personally, I prefer to do it the other way around: cool air in the back, and exhaust air out the front. Since most of your equipment is usually at the front of the room, it makes sense to have the exhaust above that, extracting it directly, instead of forcing the warm air to flow across the rest of the room to get out. And yes, the general HVAC contractor recommendation is to have the supply coming in down low on the wall, but in most studios that is hard to do, since the silencers are so big that they take up a lot of space, either behind the wall or inside the room. So I generally put the supply registers in the ceiling, and direct the air flow down or sideways from their. Cool air will descend in the room naturally, especially considering that the air flow speeds are very low: the registers are not forcing strong jets of air across the room, but rather gently diffusing air into the room, where it descends towards the floor and is drawn across the room, rising again past the equipment to the return registers, at the top front. At least, that's the way that seems to work for me.

- Now that the room is a bit bigger, and Stuarts persuasion, I am going for flush mounted speakers.

I know that this wall needs to have high mass to work. Does the inner leaf continue behind the speaker creating a box, or does the new massive wall now act as the inner leaf?

The inner leaf is actually both! Yes, it dies continue behind the speaker. If it did not, then the speaker would be inside the MSM cavity, which would mean that it only had a single leaf isolating it from the other rooms or the outside world. But the front surface of the soffit (technically, the "baffle") also becomes the inner leaf, in the sense that it is now the hard boundary of the room, so that's what you use for your room calculations.

- At what size does it start to make sense to use hangers over other designs?

That's probably a question for John, since he's the expert on hangers, but they have to be pretty big to be effective (like all low-frequency treatment. You should allow for at least a couple of feet depth at the rear of the room.

If I don't have the space for hangers, what is the best way of achieving Non-Environment absorption?

NE and RFZ are very similar in that aspect, so if you don't want to use hangers then I would use large Superchunks in the rear corners plus at least 20cm of absorption across the rest of the rear wall, with air space behind it, and possibly angled forward (larger gap at the top than the bottom). Not sure how you would combine VPR and superchunk though, as they both need to go in the same place, roughly.

You might also want to use diffusion in front of the superchunks: perhaps pyramids of various sizes, or polys, or random slats. Or if the room is large enough, maybe even tuned diffusers, such as Skylines, QRDs, or something like that. But the room has to be big enough for that, especially if you want to have a couch at the back: you have to leave enough space between the diffuser and the closest seating position, to avoid hearing the artifacts.

- Stuart -

[thismanysounds]

Thanks Stuart, slowly but surely I am getting a grip of this. A/C is not common domestically here, so my starting knowledge is almost none.

Silencers
I didn't ask this question very clearly, so I am still a little confused. The silencer can be attach on the inside or outside of a leaf, as long as it doesn't couple them. Do I need a silencer for each leaf of the wall? My intuition tells me this is yes, down the same concepts as the drawing I posted earlier in the thread.

Rear Wall Bass Traps
My GOAL is to achieve good LF control without loosing too much space from back wall. As feared, hangers will need >1m to work, so I am looking at membrane, VPR and helmholtz options. These work well with smaller depths (especially VPRs), but being solid plates will reflect highs back into the room, denying the NE criteria. I would like to have a velocity based trap on front to counter this, hence the VPR - Rockwool idea. The real question is 'Does it work to have a porous absorber in front of a pressure (e.g. membrane) trap?'

Side Wall Trapping
There is considerable area in the side walls for trapping, but it is shallow. Is this a good spot for hangers? The maximum depth is 700mm, so a porous absorber (with air gap) should get pretty low.

Bass Trapping Concepts
In the room, there is modal support from about 40Hz, below which it is pressure dominated. As the room will effectively have the same pressure everywhere, does this mean that I cannot do any effective trapping? Does it even matter if the room modes aren't ringing?

Flush Wall
So that box is going to take up the space I had planned for silencers, which is a pain. Also, the idea of creating a small box (even heavily damped) seems like asking for trouble!
How big does it need to be?
If if boxed in behind the speaker (with the same mass as the wall), then the speaker would be in the inner leaf. Using passive speakers to removing the cooling requirement, would this work?

[thismanysounds]

Okay, so I had a chat with a distributor of acoustic products which has slightly dented my confidence in my isolation plan.

The current plan is to create a dense outer leaf (100mm thick dense concrete block) around the studio area, and then construct stud walls for the studios (100x50mm wood stud, 19mm plasterboard, 19mm plasterboard). A minimum air gap of 250mm between leaves will be maintained. The inner leaf ceiling will be built on the stud walls, with 2x19mm plasterboard, the joist size is to be determined, but will maintain the 250mm air gap.

This gives me a MSM frequency of 33Hz between stud walls, and 25Hz between stud and blockwork. This was using Eric Desart's calculator, which I don't think incorporates this lowering due to insulation. Does this need to be less than 20Hz, or lower than 10Hz like a floating floor?

The distributor proposed:
Block wall - 200mm air - wooden stud - resilient channel - 19mm plasterboard - quiet/green glue - 19mm plasterboard - quiet/green glue - 12.5mm plasterboard.

This seems excessive, and very material heavy. Also, I was under the impression that resilient channel was unnecessary with separate stud walls, and would reduce LF performance due to increased stiffness. Is he on to something, or just trying his luck?

Also, I am contemplating green glue between the 19mm plasterboard layers. Will this be beneficial with my wall plan?

[Soundman2020]

Do I need a silencer for each leaf of the wall? My intuition tells me this is yes, down the same concepts as the drawing I posted earlier in the thread.

For high isolation, yes, that's the best way to do it.

My GOAL is to achieve good LF control without loosing too much space from back wall.

This might sound a bit trite, but "good luck with that"! Not meant to be insulting: just a comment on how hard it is to put a large amount of bass trapping in a small space. You are fighting against the laws of physics there. Yes, VPR's are rumored to accomplish that to a certain extent, but as you point out, the are also reflective for highs and mids. And I'd still like to see some independent laboratory test results on VPRs, to see how they perform under suitably controlled conditions, before committing to them. I guess there must be something out there already on that: I haven't really looked.

Also, I'm not sure if you have decided to implement a true RFZ design, but if so you'll still need sufficient absorption on the rear wall to attenuate ALL frequencies enough to meet the criteria. That's hard to do without at least a few inches of absorption.

As feared, hangers will need >1m to work

Not necessarily: you can angle them, and fit them into a smaller space, just not quite as effective. Perhaps 60 cm or so.

The real question is 'Does it work to have a porous absorber in front of a pressure (e.g. membrane) trap?'

I can't think of any reason why that would not work.

There is considerable area in the side walls for trapping, but it is shallow. Is this a good spot for hangers? The maximum depth is 700mm, so a porous absorber (with air gap) should get pretty low.

How much absorption does the room need (total sabins), and what frequencies are you predicting you'll need to cover? Side walls are good candidates for slot walls, except for the first reflection points, of course, where you need thick absorption.

In the room, there is modal support from about 40Hz, below which it is pressure dominated. As the room will effectively have the same pressure everywhere, does this mean that I cannot do any effective trapping?

Not really. The pressure doesn't really remain constant everywhere; it just varies constantly everywhere, like blowing and sucking on a paper bag. So there are still pressure variations in the room at lower frequencies, just that they vary consistently everywhere. Think of it like the speaker cone acting sort of like a piston in a large cylinder, where the room is the cylinder. And since there's no such thing as a perfectly rigid building material, the the entire room also "breathes" to a certain extent along with those pressure changes. Not much, of course, and it depends on the construction, but there can be some movement there.

In other words, pressure based traps can still work (eg, panel traps, membrane traps, and even Helmholtz resonators) , and so can velocity based traps; they just aren't very effective.

Does it even matter if the room modes aren't ringing?

How do you prevent a mode from ringing? Modes are a fact of life, always present. A mode might be triggered to a greater or lesser extent, by higher or lower levels of sound, but it is always there, potentially. The only way to prevent a mode from ringing is to not play any note at that frequency loud enough to trigger it. Since that's not really an option, all modes can potentially ring. You can't stop them. But what you CAN do, is to damp them, so that the ringing us under control, and consistent with the rest of the room decay.

So that box is going to take up the space I had planned for silencers, which is a pain. Also, the idea of creating a small box (even heavily damped) seems like asking for trouble!

I think you are taking about the speaker soffits here, right? If so, the volume of the box behind the front baffle is irrelevant, acoustically: it isn't even "inside" the room, technically, so it has no effect on the room acoustics. The only thing that matters, is the size of the front baffle, and the angle that it is set to the room. What happens behind that baffle might as well be in a different universe, since it has little effect on the room.

However, it does have an effect on the speaker itself, of course: speakers need cooling, as you mention, so that's the key issue here: as long as you have enough ventilation for your speaker, then you'll be fine. The insulation inside the soffit cavity is there for one reason only: damping. It damps any resonance that might be going on inside the cavity. So the size of that cavity isn't too important, as long as it is well damped and provides a suitable ventilation path.

How big does it need to be?

The cavity size doesn't really matter: It's the size of the front baffle that mattes (width and height, margin around speaker, etc.).

If if boxed in behind the speaker (with the same mass as the wall), then the speaker would be in the inner leaf.

That's not an "if": It is a "must"! The speaker MUST be boxed in behind with the same mass as the wall: If you don't do that, then the speaker is emitting serious sound levels into the MSM cavity...

Using passive speakers to removing the cooling requirement,

Passive speaker still need cooling when played at high levels. Maybe not as much as active speakers, but still need some.

The current plan is to create a dense outer leaf (100mm thick dense concrete block) around the studio area, and then construct stud walls for the studios (100x50mm wood stud, 19mm plasterboard, 19mm plasterboard). A minimum air gap of 250mm between leaves will be maintained. The inner leaf ceiling will be built on the stud walls, with 2x19mm plasterboard, the joist size is to be determined, but will maintain the 250mm air gap.

Good plan. Theoretically, that will tune your wall to about 16 Hz, so it will start isolating at around 23 Hz, isolate reasonably from around 32 Hz, and isolate decently above 48 Hz. Isolation should be around 52 dB.

That's all theoretical, of course: Assuming you build everything perfectly. Reality might be different, but you should be able to get close to that.

This gives me a MSM frequency of 33Hz between stud walls, and 25Hz between stud and blockwork.

Not sure where you got that! The simplified for of the MSM equation for very massive outer leaves is F0 = 60 / SQRT(m * d) with no internal damping, and F0 = 43 / SQRT(m * d) for fully damped. "m" is around 29 kg/m2 in your case (two layers of 19mm drywall), and "d" is 0.25 meters. With those parameters, F0 = 15.97 Hz.

Even if you go with the full version of the equation ( F0 = 43[(m1 + m2)]^.5 / [(m1 x m2 x d)]^.5 ), using more realistic values for the surface density of the concrete, it only changes things by a couple of Hz.

Are you sure you plugged in the correct values?

Does this need to be less than 20Hz, or lower than 10Hz like a floating floor?

What is the lowest frequency that you need to isolate? That's the key. Not many people need to isolate all the way down to 20 Hz... You'd only need to do that if your work in the studio involves serious effects work for movies, or something like that, where you are doing earthquakes, canon fire, heavy rumbling, and that type of thing. You'd also need some pretty decent subs to get down to 20 Hz cleanly. Do you need that? If you are just doing typical music, then there's no need to go that low.

The distributor proposed: Block wall - 200mm air - wooden stud - resilient channel - 19mm plasterboard - quiet/green glue - 19mm plasterboard - quiet/green glue - 12.5mm plasterboard.

Either he doesn't understand the purpose of resilient channel, or he does understand it but wants to sell it to you anyway! If your inner-leaf is already decoupled (which it is, due to the second stud frame), then there is no point in decoupling it a second time.

The 12.5mm plasterboard is also questionable... Why? For what purpose? If he thinks your wall needs that much mass, then why not put at least 16mm drywall on there. 12.5 is not much use.

Also, I was under the impression that resilient channel was unnecessary with separate stud walls, and would reduce LF performance due to increased stiffness. Is he on to something, or just trying his luck?

I vote for "trying his luck"...

Also, I am contemplating green glue between the 19mm plasterboard layers. Will this be beneficial with my wall plan?

Yes. It's a proven product, well tested in independent labs, with published results. It works. It will be beneficial, especially for low frequencies, since that is where it helps most.

- Stuart -

[thismanysounds]

Okay, lets start with the rough modal calculators (slightly changed dimensions due to similar L & W):

Room Dimensions: Length=4.5 m, Width=4.8 m, Height=2.95 m
Room Ratio: 1 : 1.52 : 1.62
R. Walker BBC 1996:
- 1.1w / h < l / h < ((4.5w / h) - 4): Fail
- l < 3h & w < 3h: Pass
- no integer multiple within 5%: Pass
Nearest Known Ratio:
- "21) Origin unknown: meant for small room" 1 : 1.5 : 1.6
RT60 (IEC/AEC N 12-A standard): 246 ms
- ±50ms from 200Hz to 3.5kHz = 196 to 296ms
- ±100ms above 3.5kHz = 146 to 346ms
- <+300ms at 63hz = 546ms
- 300<RT60<600ms
RT60 (ITU/EBU Control Room Recommended): 214 ms
- ±50ms from 200Hz to 4kHz = 164 to 264ms
- <+300ms at 63hz = 514ms
- 200<RT60<400ms
Absorbtion to achieve ITU RT60: 514 sabins
Volume: 63 m^3
Surface Area Total: 96 m^2
(sabins - front wall - carpet) / Left+Right+Rear wall: 31 %
(sabins - front wall) / Left+Right+Rear wall: 84 %
Schroeder Fc: 109hz
Frequency Regions:
- No modal boost: 1hz to 38hz
- Room Modes dominate: 38hz to 109hz
- Diffraction and Diffusion dominate: 109hz to 436hz
- Specular reflections and ray accoustics prevail: 436hz to 20000hz
Count (35.8-195hz) : Axials=13, Tangentials=55, Obliques=75
Count (35.8-100hz) : Axials=5, Tangentials=7, Obliques=1
Critical Distance (direct = reverberant field): 4.50m

So that is 514 sabins. I hope that is in imperial. So does that mean I need 14.6m3 of perfect absorption? How does that translate to treatment?

Seeing as I have a fair bit of width / want to minimise the complexity of the build, I plan on simply absorbing the first reflections (NE style) rather than having a nice termination. The smaller control room will probably in an RFZ vein.

In regards to the independence of the test results, Fraunhofer are a very respected in research, and any 'field' results I have seen show good results. The thinner (1mm steel - 50mm basotect) VPR shows 0.8 absorption up to 250 Hz where a porous trap (with >150mm remaining depth) should come into effect. The other option is a membrane absorber, but as they have a higher Q and trickier construction, I am still leaning on the VPR side (despite the cost).

Thank you for your response about the modal region, however I was asking about something different (sorry for the poor phrasing on my behalf). My main questions are:
-What happens in the time domain below modal reinforcement?
- Does the room still resonate or does the replay system limit the transient response?
- Do I need to worry about absorption below the modal region (38Hz in my case)?

I have attached a screenshot of the MSM calculations.

[thismanysounds]

The current plan is to create a dense outer leaf (100mm thick dense concrete block) around the studio area, and then construct stud walls for the studios (100x50mm wood stud, 19mm plasterboard, 19mm plasterboard). A minimum air gap of 250mm between leaves will be maintained. The inner leaf ceiling will be built on the stud walls, with 2x19mm plasterboard, the joist size is to be determined, but will maintain the 250mm air gap.

Good plan. Theoretically, that will tune your wall to about 16 Hz, so it will start isolating at around 23 Hz, isolate reasonably from around 32 Hz, and isolate decently above 48 Hz. Isolation should be around 52 dB.

That's all theoretical, of course: Assuming you build everything perfectly. Reality might be different, but you should be able to get close to that.

Stuart, can I question you on that number? Are you talking STC numbers (a dirty word around here!) or what I could expect once isolating decently (>48Hz)?

[Soundman2020]

Stuart, can I question you on that number? Are you talking STC numbers...

NEVER!!!! Blasphemy! Swearing on the forum! Profanity! Bad bad bad!!!!



That's actually a number produced by a spreadsheet I'm working on that does a lot of things to predict isolation. Assuming my spreadsheet is working OK, that's the number you would see on a Sound Level Meter set to "C" and "Slow". So if your meter reads 102 inside, then it should read 50 outside. I chose C weighting, since it is reasonably representative of how we perceive sound.

My spreadsheet should eventually produce an actual TL curve as well, that roughly predicts the overall isolation. It's producing a curve already.... just not very believable.... When I have spare time, I do work on that, to see if I can get it to produce more realistic results. As it stands now, the "C" weighting isolation prediction and frequency predictions are fairly reliable, but I wouldn't trust the curve yet (which is why I didn't post it.... ).


- Stuart -

[thismanysounds]

Ooh, that sounds intriguing.

So that means we are talking 50dB at 63Hz (where it is more or less linear)? Presumably more higher up?

I can't really tell if that is great or whether I need to go further. A 115dB SPL(C) drummer would be 63dB SPL(C) outside. Equal loudness contours may save me at low frequencies, but further up may be insufficient.

Working with your same model, what does it take to get that to 60 or 65? Does that factor in any benefit from green glue?

[Soundman2020]

So that means we are talking 50dB at 63Hz (where it is more or less linear)? Presumably more higher up?

Not really: 50 dB overall. The curve (future) will tell you what to expect at each frequency, but the overall number is just what you'd see on a meter, covering the entire spectrum. The curve generally takes a steep dive in the low end, unless you have extremely massive walls or cavernous air gaps.

A 115dB SPL(C) drummer would be 63dB SPL(C) outside.

Right.... roughly! Drums are always a problem...

Working with your same model, what does it take to get that to 60 or 65?

Heavier walls, and/or a larger air gap, and/or more insulation in the cavity. Doubling the total mass should give you about 10 to 15 dB extra isolation, all other factors being equal. (Theoretically 18 dB, but that assumes perfect mass, perfect seals, perfect damping, etc. More realistically on this planet, is about 12 dB). In other words, if you are getting 50 dB of isolation and you want 62, then double the mass on your walls.

Does that factor in any benefit from green glue?

The curve will take that into account, with a very rough estimate. That's one of the parts that is lying right now: I can't figure out how to model that, mathematically! I wish I had more time to work on it...

- Stuart -

[thismanysounds]

Stuart, is it the overall mass of the wall which matters, or does the mismatch in mass come into effect? Adding two more 19mm plasterboard only increases the mass by 15%. It will take 12 more sheets (and 230mm) to double it. The only logical way to achieve double the mass is to have a double block wall, which really complicates construction.

Am I being greedy with isolation? Is the proposed wall sufficient?

I plan on setting up a PA speaker in the room to measure the existing TL to the floor above (the real problem area), and to see what a drum recording at 63dB SPL(C slow) through the floor would sound like. Are there any other tests I should do whilst I have a SPL meter?

[Soundman2020]

Stuart, is it the overall mass of the wall which matters, or does the mismatch in mass come into effect?

For low frequencies, below the MSM resonant frequency of the wall, it's mass and rigidity that matter most. The equations don't even consider mismatch in mass: they just take into account the overall mass. Mismatch only helps once you get up to the region of the coincidence dip, where having two different densities means that the coincidence dips do not coincide. However, coincidence normally occurs at frequencies where the wall is isolating very well in any case, so there's not much to be gained there.

Adding two more 19mm plasterboard only increases the mass by 15%. It will take 12 more sheets (and 230mm) to double it. The only logical way to achieve double the mass is to have a double block wall, which really complicates construction.

Are you planning to fill the cavity with insulation? That has a huge effect. A wall with no insulation in the cavity is not all that good: putting 4" (10 cm) of insulation in the wall increases isolation by at least 5 dB, and filling the cavity can increase it by as much as 16 dB (provided that you don't over-fill it: don't force it in. Just enough to fill it, and no more). However, there might be fire code restrictions on how much of the cavity you are allowed to fill, so check that before you decide.

The other option for increasing isolation, is to increase the air gap. That lowers the resonant frequency and increases attenuation across the board, so adding a few more cm of cavity depth can add another few dB.

One other question: when you say "concrete block", are you talking about the usual hollow concrete "bricks" that have spaces inside? If so, then filling those internal spaces with sand can make a huge difference: Sand adds a large amount of mass to the wall, and also damps resonance to a certain extent.

Am I being greedy with isolation? Is the proposed wall sufficient?

to be honest, it's probably more than enough! At 40 Hz, there really isn't that much energy anyway, and the Phon curves are also on your side: the human ear isn't that sensitive to low level sound at very low frequencies. Concrete block, a decent air cavity with damping, and a couple of layers of decent mass with GG is going to give you excellent isolation, any way you look at it.

I really wouldn't worry too much about the wall: the weak points are going to be elsewhere, and your overall isolation is only as good as the weakest part. I would be thinking about things like ceilings, doors, windows and HVAC. Once you get your walls isolating in the region of 60 dB or better, it's pretty hard to improve on that. Doing windows to 60 dB is a challenge, and doors even more so. 60 dB insertion loss for HVAC silencers is also not easy to accomplish.

So I wouldn't sweat your walls too much more: you seem to be OK there!

- Stuart -

[thismanysounds]

Stuart, was your calculation without any insulation? I thought insulation was a given!

Two more question before I stop worrying about walls:
- Between the rooms there is 2x19mm PB - >250mm air (including studs) - 2x19mm PB. Is it worthwhile putting extra plasterboard in one of the rooms (perhaps the 2nd control room, which will be use independently) and/or increasing the gap, so that it is more independent?
- Similarly, should I increase the air gap between the CR front wall and the studio?

Okay, now on to other matters...

Windows
As with a lot of studio build stuff, it is often more cost effective to go with less specialised products. This got me to thinking about other places where thick glass is used. I came up with glass flooring, for example (http://www.diometonline.co.uk/product/2 ... loor-glass). I called up and they said it was 12-12-1.5 construction. They also have a 15mm which would be of similar density to 2x19mm plasterboard (assuming the actual glass is 14mm). Would this work? Also in terms of size, is there a good ratio (square looks odd) and or a minimum size to go for? Does it make sense to use a larger pane in the live room for a bigger viewing angle (possibly not worth the extra complexity)?

Doors
It seems to be a pain to get blank hardwood doors around here. Would it be possible to layer MDF to make 50mm doors (possibly with another smaller layer for an additional seal)? This should be plenty in terms of density. What about seals? What is everyone using (especially curious for European sources)? I will need 6 doors, so really would like to keep these costs reasonable.

Ceiling
I plan on have an independent joist ceiling built on the stud walls, with the same 19mm-GG-19mm sandwich. This will require something like a 220mm deep joist for the span. If I used a resilient hanger instead, then I could use the space for duct runs. Would that be a weak link in my design?

Bass!
Also, I am struggling to find answers to these questions in books/this forum. Please can you point me in the right direction.

Thank you for your response about the modal region, however I was asking about something different (sorry for the poor phrasing on my behalf). My main questions are:
-What happens in the time domain below modal reinforcement?
- Does the room still resonate or does the replay system limit the transient response?
- Do I need to worry about absorption below the modal region (38Hz in my case)?

p.s. Thanks again for your help Stuart, it is people like you that make this forum such a great resource.