New music room extension – Hobart, Tasmania

Hi all, Happy New Year and congrats on a great forum. I am seeking some input on a music room extension, which hopefully will be starting soon. (This is my first post on any forum so please forgive any mishaps). At present we have a small (4.8 m x 3.7 X 2.8 m) ex-bedroom as a band practice and recording room. It is in the back corner of a single-story weatherboard house, with Tasmanian oak floor (on brick footings), lath and plaster ceiling and walls. The house is situated on a steeply sloping block and there is a major road on the hill behind the house. Traffic noise at night is generally < 65 dB although the largest trucks produce about 88 dB (max) at the back wall. Fortunately, most of the noise goes over the top of the house. We have a 5-piece band and playing folky rock music and generally try to keep it < 95 dB (electronic drums, plus 3 members on the wrong side of 55 who have learnt from (and paid the price for) youthful excesses in volume). We’ve never had any complaints from the neighbours. Mind you my closest neighbour said that from inside her brick house she could hardly hear me using a jackhammer recently! So we don’t have big problems with sound containment. The sound isolation problems come when we try to record acoustic music, when the trucks produce up to about 55 dB inside the present room. Also the room is too small for 5 people plus gear, especially since we totally lined it with heavy mattresses and cushions to contain the awful flutter echoes that made the room otherwise unusable. The big plan is to extend the room in two directions, 3.7 m in length and 1.3m in width, to produce a new room 8.5 x 5 m. We are going to retain the existing roof and ceiling by running two beams to the new walls. These will totally support the existing structure. My designer suggested that the room would appear boxy if we continued the existing flat 2.8m ceiling into the new bit, and suggested a shallow (22 deg pitch) cathedral ceiling instead. (We can’t do a cathedral for the old part of the room because it would interfere with supports for other parts of the roof). I figured the bigger volume would help the sound and also make it a more livable room. The new room will be multipurpose – primarily for band practice and recording, but also I will re-locate my hi-fi so that I can listen to it in a nice environment. My desk and mixing gear will be at the other end. This will be capped off with a table-tennis table (with sound treatment underside so that it can double as a gobo). The idea was to have the old (flat ceilinged) end of the room dead, with my KEF transmission line speakers built in to a soffit, and the other end more dispersive, with my desk, lots of bookshelves and a semi-cylindrical diffuser under the cathedral ceiling. Bass traps wherever possible of course. At the moment we have planning approval and we’ve excavated the site as part of general landscaping in preparation for the build. We were due to start this month but our builder suddenly became unavailable :( . This may be a blessing in disguise, as I may need to alter the design somewhat. I’ve been reading voraciously for about 6 months about sound treatment from Ethan Winer’s excellent site as well as some other forums. I think I understand that stuff pretty well. However I only discovered this site a month or so ago and realised that there was an awful lot more to learn about soundproofing. I bought Rod Gervais’ book (wow!) and I’ve been digging into it constantly for the last month as well as another complementary one by Jeff Cooper, not to mention countless hours on this site. So much to learn! My designer and I will be meeting with the engineer in a week or so to finalise the design in line with information I’ve learned from this site, and I have a new builder coming next week to talk about the job. It looks as if he’ll be available to start in late February - March. Budget: I had originally budgetted $25K for the extension. My builder said that was achievable. I’d be prepared to go another $20K if necessary to achieve a good result for music, but in the end it’s a hobby for fun and I don’t think I’ll be recording professionally. However I’d like to design as well as possible to get the maximum “bang for the buck”. Some specifics: The walls will be timber frame, not yet sure whether to go double studs or offset (it depends on how we do the roof (below)). Outside 2 layers cement sheet, inside 2 layers of 13mm plasterboard with green glue. I will put a new layer of plasterboard and green glue onto the existing lath and plaster ceiling and walls. The existing timber floor will remain. It is surrounded by brick strip footing, which will remain, and the new floor will use this for its inner support. This will be 2 x ply over bearers supported at 1.6m centres on concrete pads. I will put carpet on the new section and there will be some removable sections with cable troughs underneath so that I can run a snake and stage box to the centre of the room. Windows and doors as per Rod’s book. I am unclear on some issues that weren’t covered in Rod’s book which seemed more focussed on building in basements or shared accommodation rather than single story. Questions: Edit: I am no longer going for a wooden floor, following Stuart's feedback. I am going for an isolated slab and double stud walls. Help requested with design details for slab/footings 16 Jan (below). 1. Rod’s book was very critical of wooden floors, specifically with resonance. Is resonance still a problem when the floor was supported on concrete pads (1.6m centres) rather than a self supporting floor spanning a whole room? If so will green glue help this? 2. The builder suggested pad footings rather than strip footings. This is apparently easier and cheaper on a sloping rocky site. He recommended using (double treated) pine posts set in concrete to support the walls and floor. Using this system there is no peripheral wall. I was concerned about excluding external noise from under the floor and also about flanking noise travelling from the walls to the floor. I consulted with the designer and we came up with the idea of using two posts (into the same concrete pad) , the outer one supporting the wall and the inner supporting the floor. I would box in the section between the external posts using 2 layers of cement sheet, on both the outside and inside, to make it (almost) equivalent to the wall treatment. I haven’t run this past the engineer yet, but I’d like some feedback on whether you think it would be a good idea. 3. No-one seems to mention cathedral ceilings, even though I note that Ethan’s studio has one. The issue of course is how to ensure good isolation. One thing working in my favour is that the road noise will strike at a low angle. I thought that the best compromise would be to use a staggered stud approach to the roof structure: i.e. upper studs supporting steel roof, damped with fibreglass and cement sheet, then double plasterboard (+ green glue) on the lower studs. This obviously wouldn’t be as good as having a true double ceiling but I thought it would be OK. If I go this way, I would think there’s not much advantage in having double stud walls. Any comments or suggestions? Looking forward to any feedback Cheers Simon

Hi Simon, and welcome aboard! It's nice to have a new poster who already read the stickies, did his homework, an follows the rules! A refreshing breath of cool air! A couple of comments on the parts that I feel somewhat qualified to comment on (but I ain't no expert!):

1. Rod’s book was very critical of wooden floors, specifically with resonance. Is resonance still a problem when the floor was supported on concrete pads (1.6m centres) rather than a self supporting floor spanning a whole room?

Yep! You are building a drum head like that. The resonance is due to the lightweight floor (nowhere near enough mass to get down to low enough resonant frequencies where it doesn't matter) and the air-space underneath. It's a resonant cavity, basically, independent of how you support it. If you take a kick drum and lay it on the ground with the beater head facing up, does it still resonate when you beat it? Yep. Does it make any difference if you put it down on carpet, concrete, grass, or sand? Yep, slightly, but it still resonates, no matter WHAT you put it on. Just like a drum head, your floor will resonate at certain frequencies, which of course is not good. Resonance in drums is a good thing. But not in floors. It's also rather difficult to get the level of isolation you need with just a lightweight (relatively) single-leaf floor. Far better than a wooden floor, is a concrete slab on grade. In your case that might be difficult, due to the sloping terrain, but not impossible. It might be worth talking to your architect / builder, to see what they think about that. If you pour a slab on grade, that might even give you a lot of extra ceiling height for your room, which is GOOD! It might be more expensive, but it sure would be simpler, faster and better.

If so will green glue help this?

Yes and no. Green glue is designed for use between plasterboard panels on walls and ceilings, not plywood sheets on floors. My guess is that it would probably still help, slightly, but you would be in uncharted territory: As far as I'm aware, GG has not provided any data on how their product works in floors between plywood sheets, and they make no claims about it. So yeah, it might help fractionally, but it wont solve your basic problem which is lack of mass and an un-damped air gap. You'd have to make your wooden floor REALLY massive, and also seal and damp that air-gap, and do the math, and float it properly on springs or neoprene or something, in order to get decent isolation and resonance down at a low enough frequency. Once again, doable, but not easy and not cheap. Concrete slab is probably your best bet. Of course, as folks around here often say: You have to live with what you've got. If there are no other possibilities, then a wooden floor is not the end of the world, and will still work somewhat, if you build it properly, but you need to be aware of the limitations, and you have to decide if you can live with those.

upper studs supporting steel roof, damped with fibreglass and cement sheet, then double plasterboard (+ green glue) on the lower studs.

That sounds like a three-leaf roof to me! "Metal sheeting -- cement board -- plasterboard". If so, bad! You don't want three-leaf anywhere, and especially no up there on your roof, where it is right in the path of truck-rumble coming off the road! three-leaf gives you much worse isolation in the lower frequencies, which is exactly where "truck rumble" lives. How about doing a quick sketchup model of your idea for the roof, so we can check if it is 3-leaf, and offer suggestions on how to fix it? What other roofing materials are available in your area? For example, can you get what the Americans call "asphalt shingles"? If so, just lay that over thick tar paper on a thick plywood deck. More massive, easier to seal, and only one leaf. One point about your roof: don't forget the issue of ventilation. You do need to ventilate your "attic space", unless you specifically build an unvented attic, which is a different ball game. Search the forum for "attic" and "vent". By the way, about that "truck rumble" that you mention. 88 dB is fairly high level. Does the house shake when those trucks go past? Can you feel the vibration in the floor? Walls? Do windows rattle? Also, how did you do your SPL measurements? Did you use "A" weighting, or "C" weighting? Fast, medium, or slow response? - Stuart -

Hi Stuart, Thank you for your quick and detailed response. 1. I will talk to my architect about the cost and practicality of a concrete floor. I must admit I was loath to go that way because I have a beautiful existing wooden floor that cost weeks of my life to renovate. 2. The truck noise was measured with C-weighting, fast response. There was one very loud truck about every 10 min, but generally the noise is much less, especially at night when we play. However I thought I'd mention the worst case scenario. I haven't noticed any vibration or rattling. 3. Here is my idea for a roof structure. We planned to use corrugated iron to match the rest of the house, however that generates noise when it rains. My architect has had success in preventing that by putting a sandwich of fibreglass under the steel, usually just up against reflective film. I thought that a layer of cement sheet instead would increase the external mass and help isolation. The steel/fibreglass/cement sandwich would form the outer skin on one set of rafters. The inside would have 2 layers of plasterboard with green glue on the other set of rafters. By the way, I've drawn a vertically exploded view of the roof for clarity. The upper fibreglass layer will be sqashed into the corrugations of the iron. 4. I must admit I hadn't thought of ventilation between the two skins, if that's what you meant. I had thought of this as equivalent to a staggered stud wall, which as far as I know is not ventilated. Anyway, I'll search the forum as you suggest and consider the options. I've seen the general complexity of ventilation in Rod's book and I have some ideas, but I'm yet to consult the local suppliers to decide the best way to go. Best wishes Simon

Hey Simon. You are more then welcome! I'm just trying to give back a small percentage of what I got out of this forum. It's the least I can do. The wealth of solid information here is astounding.

1. I will talk to my architect about the cost and practicality of a concrete floor. I must admit I was loath to go that way because I have a beautiful existing wooden floor that cost weeks of my life to renovate.

Ouch! Maybe you can figure out a way to save that floor? Maybe take it it out in sections and re-use it? Sounds painful, though...

2. The truck noise was measured with C-weighting, fast response. There was one very loud truck about every 10 min, but generally the noise is much less, especially at night when we play. However I thought I'd mention the worst case scenario. I haven't noticed any vibration or rattling.

I guess that's good, but 88 dB(C) is fairly loud, and I do know that trucks put out a lot of low frequencies. Do you have any method of measuring the spectrum of those things? It would be interesting to see how much of the energy is in the lower end of the spectrum, but I'd guess that most of it is down there in the low triple and double digits. Murphy's law says that just as you hit the absolute peak of your best-ever, once-in-a-lifetime performance, it will be drowned out by the loudest-ever truck coming past, double de-clutching, and changing down... I think the low rumbles and roars are going to be your biggest issue. I assume that you got that 88 dB peak measured outside the room: what kind of level did you get inside the existing room for those loud guys? Was the the 55 db figure that you mentioned? If so, you are getting about 30 dB of transmission loss at present, which is about right for normal stud walls. But nowhere near what you need, or course. Any idea what your goal is here? How loud do you want it to be inside, when the trucks rumble past?

3. Here is my idea for a roof structure. We planned to use corrugated iron to match the rest of the house, however that generates noise when it rains. My architect has had success in preventing that by putting a sandwich of fibreglass under the steel, usually just up against reflective film. I thought that a layer of cement sheet instead would increase the external mass and help isolation. The steel/fibreglass/cement sandwich would form the outer skin on one set of rafters. The inside would have 2 layers of plasterboard with green glue on the other set of rafters. By the way, I've drawn a vertically exploded view of the roof for clarity. The upper fibreglass layer will be sqashed into the corrugations of the iron.

Hmmmm.... tough call! Maybe Andre can chip in on this one. It MIGHT be three leaf, since you do have an air gap in there, but it might also be coupled well enough that the metal and cement board act together as a single leaf, since you do say that the fiberglass is jammed in tightly.... Andre? Any comment on that? I'm guessing it's still a three-leaf, but not certain at all. It's beyond my ability to call. But either way, if that roof is exposed to the "truck rumble", then you are going to need more mass up there. Can the roof structure take more mass? On both leaves. The other issue is that in order to build the two-leaf system correctly (which you definitely do need in order to get that isolation), the roof outer leaf needs to rest on the outer wall, and the ceiling inner leaf needs to rest on the inner wall, with no mechanical connection between them. Room-within-a-room. Two totally separate structures. The roof trusses rest on the house walls, supporting the metal-and-cement board--sandwich, the ceiling joists rest on the inner walls, supporting the sheetrock-plus-green-glue sandwich. This is something that your builder will have a tough time getting his head around, but it is critical that there are no bridges, joints or connections at all, not even a single nail or screw, that link the inner leaf to the outer leaf. (Great drawing, by the way! Your SkethUp skills are pretty good already!)

4. I must admit I hadn't thought of ventilation between the two skins, if that's what you meant. I had thought of this as equivalent to a staggered stud wall, which as far as I know is not ventilated.

Right. Walls don't need ventilation, but roofs do. The difference is that walls don't have hot sunshine beating down on them all day, and cold night sky to radiate into all night!!! The problems in a sealed attic include heat build up and also condensation. It can be a big issue. Search the forum for key words like "attic" and "ventilation", and you'll find a few threads that have discussed the issue. Here are a couple that I found: viewtopic.php?f=2&t=3463&p=25364&hilit=attic+ventilation#p25364 viewtopic.php?f=2&t=1610&p=12068&hilit=attic+ventilation#p12068 viewtopic.php?f=2&t=3542&p=24992&hilit=attic+ventilation#p24992 viewtopic.php?f=2&t=6933&p=51376&hilit=attic+ventilation#p51376 viewtopic.php?f=2&t=11485&p=82976&hilit=attic+ventilation#p82976 Unvented attics are possible, but that's a different ballgame. read the threads, follow the links.

I've seen the general complexity of ventilation in Rod's book and I have some ideas, but I'm yet to consult the local suppliers to decide the best way to go.

Rod is speaking mainly about the ventilation of the rooms themselves, ie HVAC. A totally different animal, absoltuley unrelated to attic ventilation. Lives on a different planet. :) - Stuart -

Hi Stuart, Thanks again.

I assume that you got that 88 dB peak measured outside the room: what kind of level did you get inside the existing room for those loud guys? Was the the 55 db figure that you mentioned? If so, you are getting about 30 dB of transmission loss at present, which is about right for normal stud walls. But nowhere near what you need, or course. Any idea what your goal is here? How loud do you want it to be inside, when the trucks rumble past?

Yes 88 dB(C) outside, 55 inside. I am not sure of the absolute noise requirement inside. Jeff Cooper's book shows 10 dB @ 2000 Hz ramping up to 58 dB @ 31.5 Hz as max for a studio (= PNC 15). I measured my acoustic guitar at about 70 db(C) on the quiet bits and my partner's voice at about the same. Do you know a recommended minumum signal/noise ratio for recording? I don't recall seeing one.

Do you have any method of measuring the spectrum of those things?

My sound level meter only measures total noise - it doesn't split up the frequency bands. I guess I could run one of my good mics outside to see the spectrum of the noise, but that won't be calibrated. It would be interesting in any case. One thing working in my favour in terms of interfering truck rumble is that I routinely use a 75 Hz bass cut filter on vocal mics, and nearly everything else is either fairly loud or monitored electronically. But there seem to be some trucks about this afternoon, even though it's a Sunday, so I'll have a go at some measurements.

the roof outer leaf needs to rest on the outer wall, and the ceiling inner leaf needs to rest on the inner wall, with no mechanical connection between them.

This is one thing I was hoping to get from this forum. Whether I could get away with a staggered stud construction or whether I needed to go completely isolated. I assume that the inner and outer walls are still connected by some sort of isolation mount to prevent relative movement between them. The problem I can see is how the new roof bit joins onto the old. I will see if I can work out how it is supposed to happen so that I can do a SketchUp.

(Great drawing, by the way! Your SketchUp skills are pretty good already!)

Thanks :oops: I downloaded the corrugated iron, of course. I must say discovering SketchUp through this site was a huge bonus to the information learned. What a great program! Can't beat the price either. Also I discovered with the footings diagram that it lets you visualise and rehearse how you would actually build. Very useful. By the way, you'll notice I've learned to use quotes too :) and smilies :D . Tomorrow, the world!!

Right. Walls don't need ventilation, but roofs do.

Aaahh ! Yes this is an issue I'll have to consider. Thanks for the links to the threads. I'll check them out and get back. You've given me a lot of home-work, so to speak. It may take a while to address them all. Thanks again for your help. cheers Simon

I have the books you mention. I like the way Jeff Cooper writes, it is accessible to the lay person but does not give the detail that Rods work does. You may well see difference of opinion in areas between the two authors. My suggestion would be, on anything that hangs you up, do not compare one to the other. Get outside of both books to satisfy disparity. Both books are written by people qualified in the field so it can be confusing when dealing with experts.

Thanks xSpace. I'll bear that in mind. Stuart, I measured the outside noise for 20 min with a Behringer ECM8000 mic, which is supposed to have a 15 - 20 kHz flat response. Fortunately I didn't use one of my best mikes because it started raining and I had to get everything inside quickly. Here is the volume vs time. During that period there were only two loud trucks, about 88 and 83 dB, most of the trucks were closer to 70 dB. The spectrum for the louder truck shows big spikes at 70 and 110 Hz, with a bit at 40 Hz. The less noisy one shows more power in the bass freqs. Finally, I've been looking into the attic ventilation. Obviously a big issue. I searched for attic and ventilation as suggested and found quite a few, but I'm embarrassed to say I couldn't work out how to follow the links you sent. For instance: viewtopic.php?f=2&t=3463&p=25364&hilit=attic+ventilation#p25364. Obviously they are database numbers but I couldn't find any reference to them or any links on how to use them. :oops: How do you do that? I'm currently looking into the possibility of a concrete slab. I'll probably come back with some questions after a bit more thinking and researching cheers Simon

Hi Simon, concerning the link-question: just paste http://www.johnlsayers.com/phpBB2/ in front of the links Stuart gave to you. e.g. Stuart's link: viewtopic.php?f=2&t=3463&p=25364&hilit=attic+ventilation#p25364 -> http://www.johnlsayers.com/phpBB2/viewtopic.php?f=2&t=3463&p=25364&hilit=attic+ventilation#p25364 now the links will do their job. Regards, Carsten

Thanks Carsten, Much appreciated. Off to do some more research :) cheers Simon

Now that I go to use them it's so obvious. d'oh! :oops:

Thanks Carsten! I had cut-and-pasted those from another page, but forgot that that doesn't work too well here! So I edited the post, and fixed the links. They should work now, if you just click on them. Thanks!

Hi all, back again! Having taken on board and researched Stuart's suggestion for using a concrete slab, I met with the new builder (Leo) on Wednesday night. It was a quick meeting but I was really impressed with how quickly he grasped what I wanted and came up with solutions. So we are going for a concrete slab with 16mm cork tiles, suggested somewhere else on this forum. He suggested using a slab on ground with styrofoam waffle pods underneath. I didn't know about them but they reduce the use of concrete, form reinforcing bands (with steel) between them, and also insulate. Here's a photo I found. We need to build the perimeter footing before we pour the slab so that we can have something to support the existing structure while we build the new room. He came up with an instant plan which I realised is a version of the isolated slab favoured by many on this forum. It will have a double framed stud wall. However I have a few misgivings and I would appreciate some feedback on his idea and my alternative before we go to the engineer. Because it was a quick meeting, I am a little unclear on the idea He suggested building a concrete strip footing with two brick walls on top. (diagram below) The outer wall would support the outer stud wall. He was going to put some styrofoam on top and next to the inner wall, pour the slab, then remove the styrofoam by dissolving it with solvent (he suggested petrol). Inner wall then goes on the slab. This obviously makes a nicely isolated slab. But I had a few concerns: 1. I'm not that happy about sloshing petrol around the back of my house. I know I could use chloroform or something non-flammable but that would add to the ozone hole. 2. The wall sits on an overhang, albeit in reinforced concrete. 3. There is nothing stopping the sand from under the slab migrating out if there is vibration 4. The slab could potentially settle a little with respect to the footing. Point 4 is my biggest concern. Although the wall on the uphill side is in weathered rock so tough that I will get a jackhammer to dig the trenches, on the downhill side there is at least a metre of black clay before hitting anything so solid. I was going to dig the holes (which were previously intended as pad footings) deeper to make them as reinforced piers below the strip footings. But this means if the clay shrinks (further - as it has already done so due to a decade of diminishing rainfall in Tasmania), the slab could settle, which would tear apart the double framed wall. I've read everything I can about isolated slabs on this site and Rod's book and come with the following idea, which has the edge of the slab supported on the perimeter footing, but isolated from it by a layer of sand, and from the outer wall (which is now concrete blocks) by a layer of foam. I COULD remove the foam to create a void but then the sand could migrate again (I don't know whether this is a practical concern). Does anyone have any feedback on my concerns about plan A or any problems or suggestions with plan B ? I'll be meeting the builder again next week and I'd like some input from those knowledgable on acoustics before going to my own engineer with a plan. We have to move this along to get the engineering plan through council before the builder's availability window. cheers Simon