Garage conversion to Home Theater (Perth Australia)

Hi All, New to forum. Talk about striking gold in finding this forum; so much information available here, and a great level of details as well. Thank you to the mods and all other that volunteer their time and provide such great assistance. Ok, onto my project: Background I live in Australia (Perth) and my project is to convert my two car garage into a home theatre. At the moment, I’m in the planning stage of my project. Hopefully, I can move to the build stage by early/mid November. By starting this thread, I’m mainly looking feedback on how to achieve my sound proofing/reduction goals. On that note, my goal is to achieve a reduction in the order of 67dB. Given the room will be used for a home theatre, low frequencies will be a problem (down to 20Hz, and maybe even lower). As such, I’m looking to put in place everything I can do maximize sound reduction. I’m looking to spend around $6 -7,000, but will go up to $10,000 to implement the sound reduction strategies (this will include HVAC). From my research, I know my goal will be challenging. This will not stop be proceeding with the work, and I’ll do everything I can within my budget to achieve this. Existing room The room I’m looking to convert is my existing two car garage. Room dimension is: 5.45m x 5.24m x 3m Construction of the garage is as follows: • Floor – concrete slab • Walls – Brick; 190mm thick for external walls (facing outside if house), 90mm thick for internal walls (facing inside of house) • Ceiling - 172mm suspended concrete slab Layout as follows: Images: Some of the potential issues I’ve identified (as noted on above floor plan) that will hamper meeting my sound proofing goals are as follows: 1. Structural beam running across ceiling protrudes into room - 360mm high and 400mm wide. Structural support on two walls, also protrude into room - 200mm deep by 390mm wide and 100mm deep by 460mm wide 2. Upstairs plumbing (PVC pipes) running across ceiling in part of room. 3. Penetration (6cm in diameter) in existing ceiling slab for existing lighting/electrical cable 4. Small gap (4cm wide) between two of the external walls, currently sealed with piece of timber Proposed design To achieve my sound reduction goals, I’m planning to build a room within a room. Sketch and details as follows: • New room will be 5m x 4.5m x 2.7m • New internal stud wall for each side of room. Each stud wall will be as follows: - Freestanding and isolated from existing walls – only secured to floor and new detached ceiling - New internal walls will be offset from existing external walls. Offset will vary for each wall (minimum will be 100 mm – this is total cavity between exiting brick wall and plasterboard, and ignores studs) - Each wall will be clad in 3 layers of 16mm Firestop plasterboard (link: http://www.gohardware.com.au/buy/16mm-f ... rd/PPBX016) - Green glue will be utilized between 2 of the layers of Firestop (not all 3) - Cavity between internal and external walls will be filled with insulation. Proposing to use glass wool insulation - 600mm center for studs • New ceiling - Secured to new internal stud walls only (will not touch existing ceiling) - Ceiling will be clad in 3 layers of 16mm Firestop plasterboard - Green glue will be utilized between 2 of the layers of Firestop - Offset from exiting ceiling by 250 mm - Cavity between internal and external ceiling will be filled with standard insulation • HVAC - New mini split system for cooling - 150mm (6 inch) air inlet and outlet, located on opposite sides of room. Each will be provided a silencer box located between the new and existing walls (i.e. inner and outer leaf) - Inlet will draw air from directly outside the house. There will be no fan on the inlet. - Outlet will be equipped with a fan and vent air into separate dedicated space housing Hometheatre equipment. This dedicated space will have another penetration to the outside of the house, to allow air to flow out. No intention at this stage to put another silencer on this penetration. • For the silencer box - Constructed from 15mm plywood, and enclosed in 2 layers of 16mm Firestop plasteboard (with green glue between the 2 layers of plasterboard). - For 150mm inlet, will maintain internal cross section of 100mm x 360mm - Internal lined with 50mm (2”) duct liner - Designed for 4 internal baffles - Based on above, overall dimension of box will be 300mm x 1200mm x 2400mm • Floor - Probably just end up with underlay and carpet - Haven’t really considered anything more in much detail, as I understand soundproofing the floor is an expensive exercise, and money is better spent in addressing walls and ceiling • Doors - Each door will be a pair of back to back doors (a door each on internal and external wall) - Will likely buy a solid core door, and cover with green glue and layer of firestop plasterboard on each side for additional mass and damping. - Will only have fixed handle on door, with no locking mechanism - Gap around door will be sealed • Garage Door - Current plans is to remove garage door, and seal cavity with stud wall as follows (material in order moving in to out): - 2 layers of 16mm Firestop plasterboard - 1 layer of fibre cement sheeting to provide weather protection - An additional layer of cladding (Perspex or aluminum), to retain external appearance of garage door • Cable penetration and lighting - Will only have one penetration between main room and hometheatre equipment room for cables. All cables between the two rooms will be routed through here. Will implement based on the following recommendation: viewtopic.php?f=2&t=15447&p=108608&hilit=pipe#p108608 - For lighting, planning to use remote control battery operated lights, to avoid any penetrations. This is also to minimize cost, given this room may one day be converted back to garage. Also, given this is a hometheatre, lighting not a key consideration. Questions The questions I’m looking answered to assist me meet by sound proofing/reduction goals are as follows: 1. I’m removing the garage door to avoid creating a triple leaf effect (to achieve MAM barrier while keeping the garage door, I would have to put a second wall inside the garage). However ideally, to enable me to more easily convert the space back into a garage in the future, retaining the garage door would be a better option. Is the triple leaf effect such a significant issue that it will seriously hamper meeting my sound reduction goals? 2. For my current garage door replacement plan, do I need a moisture barrier between the plasterboard and fibre cement sheeting? Is there anything else I need to provide for weather protection? Given the garage door replacement will be a weak link in the external leaf of my MAM barrier, will my current approach be adequate to seal this void and achieve my sound reduction goals? 3. For the plumbing running across the ceiling, I’m planning to box the PVC pipes in with firestop plasterboard and fill with insulation – essentially creating a soffit in my ceiling air gap. The soffit will be secured to the existing ceiling. Do I need to make this barrier thicker (i.e. multiple layers of plasterboard and maybe even green glue) or implement anything further to achieve my sound reduction goals (e.g. wrap pipes in MLV)? Also, regarding the soffit, I’m considering 3 options as follows. Which option will provide the best solution to reduce sound flanking through? 1. Use the existing ceiling and external leaf/wall to form sides of the soffit 2. Dedicated plasterboard on all sides, and installed flush against existing ceiling and external leaf/wall 3. Dedicated plasterboard on all sides, and installed offset from existing ceiling 4. For the existing penetration in the ceiling slab for the existing lighting/electrical cable, I’ll be filling the cavity with two layers of plasterboard and then sealing around this with acoustic sealant (the existing cabling will still need to penetrate through though). Will I need to do any more than this (e.g. put the lighting cabling running across the ceiling in a conduit)? 5. One of the walls will only have a separation of 100mm between internal and external leaf. Will this be sufficient? I can increase another 50mm if required, but to retain the room size, I would be reducing the opposite wall offset by 50mm (currently 250mm). 6. For the structural beam in the ceiling and structural support on the wall protrude into the room, I plan to just frame the internal leaf around both these obstacles, with a very minimum offset (around 50mm) to avoid stepping into the room too much. Will this minimal offset have a significant impact on my sound reduction goals? The structural beam is 360mm high and 400mm wide. The structural supports on the wall are 200mm deep by 390mm wide and 100mm deep by 460mm wide. 7. There’s a gap between two of the external walls (approx 4cm) that’s sealed only by a piece of timber. Do I need to remove this timber and do something more significant to seal this gap, or can I leave as is? 8. For the minisplit unit, do the pipes need any treatment to avoid sound passing through or flanking? Also, is it ok to run the pipes directly through to the external wall (option A), or will deviated through the equipment room enable sound leakage to be reduced (option B)? 9. To address room ventilation penetrations, I’m proposing to locate the silencer boxes between the internal and external walls. I understand this is more effective at sound reduction than placing the silencer on the inside or the outside of the room. But how does it compare to having a silencer on both sides of the room (i.e. one on the inside of the inside leaf combined with another on the outside of the outside leaf)? Will I need to go down this path to reach my sound reduction goals? 10. Is 4 baffles sufficient for the silencer boxes to meet my sound reduction goals, or should I have more? 11. Do I need to take into consideration how the new stud walls are secured to the floor, to minimize sound transmission? Is there anything that can be done to minimize this contact (e.g. purpose built spring bolt), that won’t require a structural engineer to undertake an assessment? 12. Can I do anything to my floors to reduce sound flanking through to other areas of the house, or more importantly, outside? 13. Do I need to do anything to the existing brick wall to maximize its effectiveness in sound proofing? 14. The firestop plasterboard I’m using weighs 13 kg/m2 for a 16mm sheet (which is about 800 kg/m3). At the moment I’m proposing to use 3 sheets for walls and ceiling. Can I get away with only 2 sheets, and still meet my sound reduction goal? If there any other issues that I haven’t identified that would prevent meeting my sound reduction goals, I would appreciate these being pointed out. Thanks in advance for anyone's time and input.

Hi there "superaaaaa", and welcome to the forum! :)

Construction of the garage is as follows: • Floor – concrete slab • Walls – Brick; 190mm thick for external walls (facing outside if house), 90mm thick for internal walls (facing inside of house) • Ceiling - 172mm suspended concrete slab

That's a pretty good start for your isolation system. You should be able to get quite decent isolation with that. However, you do note that there will be an equipment room set off from that, and it will only be isolated with a simple stud wall partition: that could potentially be a major weak link, depending on how you plan to do that. Is that going to just be a separate machine room but still located within the brick and concrete outer leaf, or is it going to be a separate room, on its own, outside of the outer leaf? It's not clear from your drawings what you are planning to do here, as part of it seems to show 2-leaf construction while part shows single-leaf.

Some of the potential issues I’ve identified (as noted on above floor plan) that will hamper meeting my sound proofing goals are as follows: 1. Structural beam running across ceiling protrudes into room

There's not much you can do about those. You can either build your inner-leaf ceiling flat below all of those, or build it a bit higher and soffit around each one.

2. Upstairs plumbing (PVC pipes) running across ceiling in part of room.

Will you need access to that piping after the room is finished? You made a note that those pipes will end up in the cavity between the inner leaf and outer leaf: they cannot. They must be fully relegated to the outer leaf. If you leave them inside the cavity, they will become a major flanking path for sound to the rest of the house. You'll have to build hefty soffits around those, to make them part of the outer leaf.

3. Penetration (6cm in diameter) in existing ceiling slab for existing lighting/electrical cable

Same issue there, but it can be fixed a lot easier than the pipes. If that is going to be your single electrical feed to the room, then just treat it in the same way as for any cable penetration.

New room will be 5m x 4.5m x 2.7m

Nice! That's a pretty good ratio, and should work well.

Offset will vary for each wall

Why? Your total overall isolation will only be as good as the part with the thinnest air gap.

- Each wall will be clad in 3 layers of 16mm Firestop plasterboard

You could consider substituting plywood or OSB for the first layer, as that has the advantage of giving you much greater structural integrity in the wall (in sheer), and also a nailing surface around the entire room, which could come in hand for mounting acoustic treatment later.

Ceiling ... Offset from exiting ceiling by 250 mm

Are you measuring that from the concrete surface above you, or from the bottom of the structural beams?

- For 150mm inlet, will maintain internal cross section of 100mm x 360mm

Is that the cross section before putting the duct liner in, or after? If that is the before measurement, then you'll only have 260mm left after lining. For 6" duct (150mm) the cross sectional area is about 170 cm2, and you should have about twice that for the cross section within the silencer box, meaning around 340 cm2. With 260mm x 100mm, you'd only get 260 cm2.

- Internal lined with 50mm (2”) duct liner

You should probably still be fine with 25mm (1") duct liner. Overall, it seems like your HVAC silencer boxes are under designed, compared with the rest of the isolation plan. Everything else points to a very high level of isolation, but the silencer plan isn't up to the same level, I suspect. You say you are aiming for 67 dB TL, which is high (and attainable) with everything else you are doing, so you might want to consider beefing up the HVAC silencing system a bit. For example, put silencers on BOTH leaves, not just one, over-size them even more, and put in at least one extra baffle.

• Floor - Probably just end up with underlay and carpet - Haven’t really considered anything more in much detail, as I understand soundproofing the floor is an expensive exercise, and money is better spent in addressing walls and ceiling

If your concrete floor is sitting directly on the ground, then you already have it isolated about as well as it possibly can be: Concrete is high density, lots of mass, and very rigid, plus it is damped by the entire planet.... hard to beat that! :)

• Garage Door - Current plans is to remove garage door, and seal cavity with stud wall as follows (material in order moving in to out): - 2 layers of 16mm Firestop plasterboard

:shock: :!: That's a MAJOR weak point! Your other walls are concrete block, your floor is concrete, your ceiling is concrete, and all of it is very thick. Concrete weighs in at about 2300 kg/m3, drywall is about 700 kg/m3. In order to match the mass of your walls/floor/ceiling, you'd need enough layers of drywall to make it about three times thicker... For example, assuming your ceiling is 4" (10cm) of concrete, you'd need about 12" (30cm) of drywall to match that mass, meaning about 19 layers of 16mm drywall. I'd suggest that you need to re-think your plan for blocking off the garage door opening. I would suggest brick or concrete block, to match the other walls.

- For lighting, planning to use remote control battery operated lights, to avoid any penetrations.

Probably not a good option. The cost of batteries over time is going to be much higher than the cost of simply running some surface mount ducts and wiring through those.

Also, given this is a hometheatre, lighting not a key consideration.

That probably depends on what your aesthetic goals are. Some people invest hugely in lighting, seating, and decor for their home theaters, attempting to impress visually as well as sonically. Others are more "minimalist". But it doesn't need too much money to make the lighting look aesthetically appealing.

1. I’m removing the garage door to avoid creating a triple leaf effect (to achieve MAM barrier while keeping the garage door, I would have to put a second wall inside the garage). However ideally, to enable me to more easily convert the space back into a garage in the future, retaining the garage door would be a better option. Is the triple leaf effect such a significant issue that it will seriously hamper meeting my sound reduction goals?

Your problem with the garage door isn't really so much of a triple-leaf issue, as a "mass continuity" issue. The total isolation of your entire room is only as good as the weakest link, whatever that might be (doors, windows, HVAC, electrical, etc). In your case, it is the garage door, by a mile. ALL of the rest of your theater is isolated very well, with massive, thick walls, ceiling and floor, which will provide excellent isolation. If you don't do the same on that one final side of the room, then all the rest is wasted.

2. For my current garage door replacement plan, do I need a moisture barrier between the plasterboard and fibre cement sheeting? Is there anything else I need to provide for weather protection?

Check your local building code to find out what is required in your area.

Given the garage door replacement will be a weak link in the external leaf of my MAM barrier, will my current approach be adequate to seal this void and achieve my sound reduction goals?

Simple one-word answer? NO. Detailed answer: Everything else you are doing can theoretically get you to around where you want to go, in the mid 60's of isolation. That wall as you proposed will give you high 40's to low 50's at best. But you say that you want good isolation in the low frequencies too, which that wall won't give you.

3. For the plumbing running across the ceiling, I’m planning to box the PVC pipes in with firestop plasterboard and fill with insulation – essentially creating a soffit in my ceiling air gap.

Right. That's the best way to do it, unless you could find a way to re-route the pipes completely outside the garage.

Do I need to make this barrier thicker

Probably, yes.

1. Use the existing ceiling and external leaf/wall to form sides of the soffit

That's the best way to do it! Easiest too, and also the cheapest.

2. Dedicated plasterboard on all sides, and installed flush against existing ceiling and external leaf/wall 3. Dedicated plasterboard on all sides, and installed offset from existing ceiling

Neither of those is the right way. Your option #1 is correct.

5. One of the walls will only have a separation of 100mm between internal and external leaf. Will this be sufficient?

100mm (4") is the minimum that is realistically useful for isolation. More is better, as it forces the MSM resonance down to a lower frequency, which is especially important in your case, due to your need for very good sub-bass isolation. So if you can get a bigger gap then I sure would go for it. If not, then you can compensate somewhat by adding an extra layer of drywall at that location.

I can increase another 50mm if required, but to retain the room size, I would be reducing the opposite wall offset by 50mm (currently 250mm).

If you have a choice between one side at 100 with the other at 250, vs one side with 150 and the other with 200, I would most definitely go with the second option. Even better would be to have both sides at 175 mm, so the isolation is the same on all sides. Try to keep your mass and air gaps about the same size all around. Any place where the air gap is smaller is a weak link, unless you also beef it up with more mass. Likewise, any place that has less mass than the rest of the wall is a weak link, unless you beef it up with a bigger air gap. The idea is to keep the MSM resonant frequency roughly the same all around the room.

7. There’s a gap between two of the external walls (approx 4cm) that’s sealed only by a piece of timber. Do I need to remove this timber and do something more significant to seal this gap, or can I leave as is?

How thick is the timber? You probably do need to beef that up.

8. For the minisplit unit, do the pipes need any treatment to avoid sound passing through or flanking?

The need to be de-coupled as well as possible. One option is to separate the inner-leaf and outer-leaf penetrations by as much distance as possible, both vertically and horizontally, perhaps also supporting the pipes with isolation hangers if the distance is very large. Another is to bend the pipes around in a large vertical circle, sort of like a spring, between the two penetrations. (OF course, you can't do that with the condensate drain pipe! Only the two copper coolant pipes).

9. To address room ventilation penetrations, I’m proposing to locate the silencer boxes between the internal and external walls. I understand this is more effective at sound reduction than placing the silencer on the inside or the outside of the room. But how does it compare to having a silencer on both sides of the room (i.e. one on the inside of the inside leaf combined with another on the outside of the outside leaf)? Will I need to go down this path to reach my sound reduction goals?

For the high levels you are looking for, I would go with two silencers, one on each wall penetration. It doesn't matter if those two silencers go inside the wall cavity or inside the rooms, but you can make them a lot bigger if they go in the rooms.

11. Do I need to take into consideration how the new stud walls are secured to the floor, to minimize sound transmission? Is there anything that can be done to minimize this contact (e.g. purpose built spring bolt), that won’t require a structural engineer to undertake an assessment?

One option is the IsoSill system:

12. Can I do anything to my floors to reduce sound flanking through to other areas of the house, or more importantly, outside?

Well, it is possible to cut the slab all around the edge (to separate it from the outer-leaf, pour new foundations under the now "floating" edge, and fill the gap with a resilient isolation compound, but you probably don't need to go to that extreme. With slab-on-grade, you already have pretty good isolation, so as log as you can keep impact noise and vibration out of the slab, you should be fine. That implies using some type of isolation pads to keep your sub-woofers and main speakers off the floor.

13. Do I need to do anything to the existing brick wall to maximize its effectiveness in sound proofing?

Yes: it needs to be sealed air-tight. Brick and concrete are porous, so the surface needs to be sealed with a good quality masonry sealant, or even just with a couple of layers of good paint. But before doing that, check it all over very carefully for any cracks, holes, chips or poor quality joints, and patch those first. Then seal.

14. The firestop plasterboard I’m using weighs 13 kg/m2 for a 16mm sheet (which is about 800 kg/m3). At the moment I’m proposing to use 3 sheets for walls and ceiling. Can I get away with only 2 sheets, and still meet my sound reduction goal?

If you increase the size of the air cavity inside the wall, then yes.

If there any other issues that I haven’t identified that would prevent meeting my sound reduction goals,

There's probably a bunch more that you'll run into as your build progresses, but at this early planning stage, I think you have pretty much covered all of the important points! - Stuart -

Hi Stuart, Thank you for the very detailed and insightful response. Clearly my biggest issue is how I deal with the garage door. I'll look into the practicality of bricking up this gap. But clearly it seems my options are limited if I want to achieve my sound reduction goals. Just to answer some of your queries: - the new ceiling will be 250mm from the existing ceiling, not the beam (otherwise ceiling will be too low). I'll just frame around the beam at that point. - my offsets for the new stud wall vary, to be able to achieve a reasonable ratio for my room. If I made the offset uniform on all walls, the room will be close to a square. Also the thickness of the existing brick wall varies, so I've modified my offsets to try and compensate. - the cross section of 100 x 360mm is after I put the duct lining in, so should achieve the requirement of atleast double the cross sectional area of the duct. To silencers are already quite tall/wide due to be able to fit into the cavity between the inner and outer wall. I'll look into using two silencers to provide more effective sound reduction. Again, a big thanks for your input Stuart! Regards Radhaa

So I've investigated using a brick wall to seal up where the garage door was. Its going to require breaking sections of the current drive way and putting a new foundation in to support the new wall - I haven't received the quote yet, but given the work involved, its going to be very expensive. As such, I'm looking at other options. I went back and researched wall construction techniques to achieve my sound reduction goals. Based on the information in this thread: viewtopic.php?f=2&t=17604&view=next ; I noted its possible to achieve ST69 by using "two layers of 16mm type X drywall, separated by a 205 mm cavity filled with two layers of 90mm fiberglass insulation, using 2x4 studs 24" OC". This is less than what I was originally intending to implement to replace the garage door - my cavity is slightly larger, and I will be using 2 layers of drywall + 1 layer of plywood (or particle board). As such, I'm quite confused why my original plan was going to be an issue - what's the issue in my particular situation? Stuart, I may have not conveyed my plan properly and it may have been misinterpreted. I've tried to make things clearer in a new sketch: Note: The bottom of the diagram is the side of the garage where the garage door will be replaced from. Based on the above, can I use a stud wall to fill the garage door cavity and achieve my sound reduction goals?

I noted its possible to achieve ST69 by using "two layers of 16mm type X drywall, separated by a 205 mm cavity filled with two layers of 90mm fiberglass insulation, using 2x4 studs 24" OC".

Perhaps, but I'd take that number with a pinch of salt, and please note that it is STC rating, which is pretty useless for studios and home theaters. STC does not consider (at all) low frequencies. It considers mainly speech frequencies, and typical office / school / apartment noise. It just does not consider the bottom two and a half octaves of the scale, which is where you find things like drums, bass guitar, the bottom end of keyboards and electric guitars, etc., plus the even lower end of movie sound tracks, such as rumbling earthquakes, canon fire, thunder, heavy machinery pounding, spooky thuds and rumbls, etc. The typical "shake your seat" type effects that movie makers and home theater buffs love. So even though if it did actually give you STC-69 (debatable!) it would very likely be really lousy for isolating a home theater.

As such, I'm quite confused why my original plan was going to be an issue - what's the issue in my particular situation?

"Home Theater"! :) That's the issue in your situation. Pretty much every home theater I've seen has some amazingly deep sub-woofers pumping out plenty of low frequency energy (and even sub-sonic energy, in some cases). "two layers of 16mm type X drywall, separated by a 205 mm cavity filled with two layers of 90mm fiberglass insulation, using 2x4 studs 24" OC" will not get you there. The isolation you will actually get from that, you'll find on page 272 of IR-761, which is a very carefully compiled set of laboratory tests of literally hundreds of different types of wall construction techniques and materials. It's one of the best reference papers available anywhere, at any price. You can download it here, for free: http://archive.nrc-cnrc.gc.ca/obj/irc/d ... /ir761.pdf Page 272 and up to page 296 show the type of construction you mention. As you can see, not one of them is anywhere near STC-69. They are all around STC-40-something to STC-50-something. The pink line on each graphs shows you the frequencies that are actually taken into account when calculating STC. As you can see, there¿s a rather important chunk of he spectrum extending off to the left, below the end of the STC band. And as you can see, for all of those frequencies, and all of the wall types similar to what you are talking about, the isolation is somewhere between "lousy" and "non-existent". Later on, starting on page 316 and for the next 40 pages or so, is a set of tests on more robust walls, with more encouraging STC ratings, and here there is one that actually does get STC-69 (page 350). But outside of the STC range, even that one shows a rather poor 26.7 dB of isolation at 50 Hz, which is as low as they were able to test, accurately. For a home theater, 50 Hz is not really "low" at all. There's still another entire octave below that, even on the musical scale, and if you are into infra-sonic subs, then there's another octave below that.... So what makes your situation different is the "home theater" thing. In your very first post, you yourself said: " ... low frequencies will be a problem (down to 20Hz, and maybe even lower). " Yes! Absolutely they will be a problem! And as you can see from the graphs, STC does not even come close to measuring such low frequencies. Neither does the type of construction you mentioned able to do much in that range.

my goal is to achieve a reduction in the order of 67dB.

Right. That's a good goal. But "STC-67" is not the same thing as "reduction of 67 dB". They are two totally different things.

Based on the above, can I use a stud wall to fill the garage door cavity and achieve my sound reduction goals?

Not if you want to get good isolation below 20 Hz. The type of wall construction you are talking about is a resonant system, often referred to as "MSM", which stands for "Mass-Spring-Mass", named after the principle of physics that makes it work. It's the same basic concept as a pendulum, a car shock absorber, a child on a swing, or in the simplest representation, a weight bobbing up and down on a spring. Take the weight as the example: It bobs up and down at one specific frequency, and refuses to bob up and down at any other frequency. You can FORCE it to bob up and down at a different rate, but doing so requires a lot of energy, since you are fighting against the natural resonance of that system. If you want it to bob up and down slower (lower frequency), then you either have to make the weight heavier, or you have to make the spring more resilient. Your wall works on the same principle: you have mass on each side (the drywall) and a spring in between. The MSM system resonates at a natural frequency set by the amount of mass (surface density, in terms of kg/m2) and the resilience of the spring (basically, how much air gap there is between the two masses). At its resonant frequency the wall does not isolate at all. In fact, it can even amplify the passage of that specific frequency, making it louder on the other side. However, at all other frequencies, the wall "fights back", increasingly better as the frequency goes up. In fact, from 1.414 times the resonant frequency, all the way up the scale, it isolates. At 1.414 times it is "invisible" (neither amplifies nor isolates), and below that it amplifies. So the idea of an MSM wall is to "tune" it such that the resonant frequency is much lower than the lowest frequency you want to isolate. In your case, you need to tune it so that the resonant frequency is much lower than 20 Hz, since you say you have substantial energy at that frequency. The general rule of thumb is you should tune the wall no higher than half of the lowest frequency, and one third is even better. So, for 20 Hz isolation, you should tune it to 10 Hz, or ideally 7 Hz. With drywall as the material on both sides, that's a really tall order! The equation for calculating MSM resonance is as follows: Fc=c * [ (m1 + m2) / (m1 * m2 * d) ]^.5 Where: c=constant (60 for empty, 43 if cavity is filled with insulation) m1=mass of first leaf (kg/m^2) m2 mass of second leaf (kg/m^2) d=distance between the leaves (meters) Do the math. For what you propose: with 2 layers of 16mm drywall on each side of a 20 cm cavity, filled with insulation, the resonant frequency is 27.7 Hz. The wall does not isolate until 38.7 Hz, isolates reasonably at 55.3 Hz, and isolates well at 82.9 Hz and above. To get the resonance down to 10 Hz, you'd need 4 layers of 16mm drywall on each side of a 76 cm cavity, filled with insulation. That would get you a resonant frequency is 10.03 Hz, the wall would not isolate until 14.05 Hz, isolate reasonably at 20.7 Hz, and isolate well at 30.1 Hz and above. If you wanted to aim for 7 Hz: you'd need 6 layers of 16mm drywall on each side of a 104 cm cavity, filled with insulation. That would get you a resonant frequency of exactly 7 Hz, the wall would not isolate until 9.81 Hz, isolate reasonably at 14 Hz, and isolate well at 21 Hz and above. The math doesn't lie, and the principles are well established in physics. Low frequencies are very hard to isolate, need huge amounts of mass, and very large air gaps. On the other hand, for a wall where one leaf is 10cm thick brick: If the second leaf has three layers of 16mm drywall and the air gap is 25cm, then the resonant frequency is 15.4 Hz, isolation starts at 21.5 Hz, is good at 30.7 Hz, and really good at 46.1 Hz. Add an extra layer of drywall to the inner-leaf and increase the gap to 40 cm, then you get: resonant frequency is 9.6 Hz, isolation starts at 13.4 Hz, is good at 19.2 Hz, and really good at 28.8 Hz. So I guess it all boils down to defining how much you need to isolate those very low frequencies: If you really do need to do that, then you don't have a lot of options: You can get there with ridiculous thicknesses of drywall and enormous air gaps, or you can get there with brick, a much smaller air gap, and fewer layers of drywall. And if you render that brick with a couple of cm of plaster, you are doing yourself a huge favor: you could reduce the size of the air gap still further, and/or maybe even drop one layer of drywall. - Stuart -