John Sayers' Recording Studio Design Forum

Hi All.

Heres my ceiling.

Should I two leaf.

So 4x2 joist, rock wool 100mm, rc channel and then drywall one or two skin.

This is a live room so its mainly for bands.
The first pic, you can see on the left how it was before which was just 100mm rock wool and 1 skin of drywall. Which I presume from all the info iv'e got from here isn't very good.
What does anyone suggest.

Thanks

Antony

Is that roof ventilated anywhere? In other words, gable vents at the ends, ridge vents at the top ridge line, vents under the eaves, etc. If so, then you are probably in one of those situations where your only real choice is 3-leaf. If not (no vents at all), then is that allowable by code where you live? Knowing the UK, humidity is pretty high so I'd expect that there is some sort of requirement for roof ventilation. But if that is approved by code, and is the normal way of doing things there, then that can indeed be your outer leaf, provided that you seal it totally airtight. What is the surface density of that roof? (Mass per unit area).

Exactly. That won't give you much isolation.

If you build the inner-leaf correctly, fully decoupled, then no RC is necessary. The purpose of RC is to decouple, but if the inner leaf walls and ceiling are on separate stud frame, not attached to the existing structure, then you are already decoupled: it makes no sense to decouple twice: that would be like taking a water can into the swimming pool with you, so you can get yourself extra wet! Wet is wet. Decoupled is decoupled. No need to do it twice.

However, if your inner leaf is still connect to the outer leaf in some manner, then yes, RC would be needed.

- Stuart -

_________________
I want this studio to amaze people. "That'll do" doesn't amaze people.

Thanks for your reply.

Answer to question one. No there arent any vents so the tile is a leaf.
Question 2 with all the money I've put into this place already I simply don't have the money to build an inner skin for the roof.

So I feel comfortable that if I rc channel and double drywall to the existing joist I should be ok then. There aren't any neighbours to near.

Thanks

Antony

I don't understand: First you say that you don't have money to build an inner skin, then you describe how you are going to build an inner skin with RC and drywall! So are you going to build an inner skin, or not?

When you say you "feel comfortable" with that, what calculations did you do to arrive at that conclusion? Acoustics is not base on feelings, but rather on math and science, so hopefully you used both math and science to arrive at the conclusion that RC plus drywall on the ceiling will gain you the amount of isolation that you need, at the frequencies where you need it!

How are you planning to tie that inner leaf ceiling to the inner-leaf walls? And more to the point, what is the plan for the inner leaf walls? You didn't mention that, but I guess you are going with ordinary 2x4 framing and drywall?

- Stuart -

_________________
I want this studio to amaze people. "That'll do" doesn't amaze people.

I prosumed when you when you mentioned an inner skin that you meant constructing another ceiling completely separate from the existing one. Which I don't have the money for. So I was suggesting that I would build onto the existing joists buy using 100 mm rock wool and then rc and drywall.

I will add even though my first attempt was obviously not good the room did sound great, so that I feel good about but I did want to cut more bottom end from leaking out. I don't have a huge problem with neighbours as the studio is set back.

But to be honest I have asked a lot of questions and had some great answers but I am feeling a bit overwhelmed by it all, I am basically looking for the best and most economical way to make the live room as leak free as possible.

I dont have a huge neighbour problem but I don't want to waste money on materials.

Once again thanks for your reply

Antony

I know that feeling only too well! Went through it myself several years back. Acoustics seems very complex and non-intuitive when you first start looking at it. I think a lot of the trouble is so many web sites, magazine articles, and well-meaning but ignorant people all over the place repeating myths, half-truths and downright nuttiness with no scientific background. It turns out the the basic scientific principles actually aren't too complicated, and are well documented, if you know where to look.

OK, so you mentioned two things that are important to you: stopping low frequencies leaking out, and doing it in the most economical way. Fortunately, the science behind that is easy to understand:

The basic way you stop sound is with mass, stiffness, decoupling, and damping. You put a barrier in its way that is so heavy that the sound wave cannot move it (make it vibrate): that's the "mass" part. Or you put a barrier in its way that is so rigid that the sound wave cannot move it: that's the "stiffness" part. Or you take away all paths that the sound can use to get from "here" to "there". That's the "decoupling" part. Or you put something in between that sucks all the energy out of the wave, kind of like if you were trying to walk through a vat of treacle: that's the "damping" part. There are no other passive ways to stop sound. Those are the only 4 simple options available.

The problem is that none of those fit your requirement of "best and economical". Mass by itself is fine, but you need a huge amount of mass to stop low frequencies (they are the hardest to stop), making it expensive. Stiffness by itself is really hard to achieve: there just aren't any affordable materials that are stiff enough to stop all sounds. Decoupling by itself is not achievable, as that implies that you'd have to suck out all the air around your room, since sound move though air very well. And damping is not very efficient: you need a very large amount of damping material to completely stop sound.

So what to do then?

It turns out that there is one other possibility: you can use the sound to stop itself, by building a wall that is an active filter: the entire wall can be tuned to a certain frequency, which it allows straight through really well, but then it also resists all other frequencies, just like a bandpass filter or a parametric equalizer allows one frequency through but blocks all others. It is the exact same concept.

This idea is based on the principle of resonance. Think of a weight attached to the bottom of a spring which is hung from the ceiling. If you pull the weight and then let it go, it will bounce up and down at a certain rate, which is its natural resonant frequency. No matter how small or large your pull is, the bounce will always be at the same rate. You cannot make it bounce faster or slower by anything you do to it, since it is tuned to that one frequency. If you want to make it bounce a bit faster or a bit slower, you have to physically force it to do so, by grabbing the weight with your hand and moving it at the rate you want, but it will not "want" to do that, and you'll feel that it is resisting you, because you are trying to make it move at a frequency other than its natural resonant frequency. This is called "Mass-Spring-Mass resonance" or MSM resonance, for obvious reasons: there is a spring here, attached to some mass on one end (the ceiling) and on the other end (the weight). The same basic principle applies to ALL tuned systems, including walls

And if you apply this concept to your wall, you can use this natural phenomena to block sound.

You have drywall (mass) then some air (spring) then more drywall (mass). Bingo! Tuned system! People don't normally think of air as being a "spring", but to a sound wave it is VERY springy.

It turns out that you can actually build a wall that has a very low natural resonant frequency, below the lowest sound that humans can hear, and will resist all of the frequencies that are important to you. Basically, you tune the wall the same way you tune a filter on your console or DAW: you adjust the parameters until it does what you want.

The way you do that with a wall is simple. The resonant frequency depends mainly on two things: how much mass you put on each leaf, and how big the air gap is between them. If you don't use enough mass, then the frequency will be too high, and the wall will allow some low frequency sounds to get through. Same thing if the air gap (spring) is not big enough: the tuning will be too high, and sound will get through.

That's what happened with your previous wall. You say that it let through low frequency sound, so clearly either there was not enough mass (drywall too thin, or not enough layers), or the air gap was too small (the drywall was too close to the outer leaf).

So the key here is to figure out what is the lowest frequency that you need to isolate, and tune the wall to a frequency that is even lower. For complex mathematical reasons, the wall only starts isolating at 1.4 times the resonant frequency, and isolates reasonably well at twice the resonant frequency, so the idea is to tune the wall to half your lowest frequency.

So once you have figured out how low you need to go, then you can figure out how much mass you need on each leaf, and how big an air gap you need. The equation is very simple.

The reason why your previous wall did not work the way you wanted, is because it was not designed to work that way. This time you have the opportunity to fix that, so that it DOES isolate the low end to the level that you want.

OK, so putting this into practice: in order to isolate your room (once you do the math to figure mass and air gap), you then need to build the walls and ceiling to the same level. You already have a complete outer leaf: the existing block walls and that roof above you. All you need to do is add the inner leaf, which means simple 2x4 framing built about 1/2" (1 to 2 cm) away from the existing wall, put a couple a layer of 16mm drywall on only ONE side of that frame, and you are done. You might need two or three layers of 16mm drywall, depending on your needs, but regardless of how many layers it is, they all go on the SAME side of the studs. So the studs have drywall on just one side, not both sides.

You already have the plan for the ceiling: RC across the joists, and drywall on the RC. So you just need to fill in the details of how you are going to address the place where the ceiling on the RC meets the new inner-leaf walls, and also how to support the tops of those inner-leaf walls, since you don't want to put new joists on top of them. Probably sway braces or isolation hangers would be your best bet.

So that's it. It isn't as complex or overwhelming as it sounds, once you understand the basic principle here: the wall is a tuned system, and must be tuned lower than the lowest tone that you need to isolate. You tune it with mass and air gap. That's all there is to it!

And that is, indeed, the best and most economical way of isolating your room. There are other alternatives, but they are either not as good, or more expensive, or both. There simply is no cheaper, better way to do it.

Hope that helps.


- Stuart -

_________________
I want this studio to amaze people. "That'll do" doesn't amaze people.

Wow, Thankyou Soundman, finally a reply that gives me some light on the matter.

If I can just mention. That one wall is a double skin 100mm block with a 4 inch air gap. We are starting to build this tomorrow. Should I, on that wall just add drywall straight to the inner skin (meaning stud,rc and rock wool on the inside would be a waste) and should I rock wool in the air gap.

Once again Thankyou for your very detailed reply.

Antony.

You mean as in "one wall of 100mm blocks, 4 inches of totally empty space, then another wall of 100mm blocks"? If so, then you already have a two-leaf MSM wall that should isolate well. However, if there are "bridges" across that air gap, then things are not so good. "Bridge" in this sense might mean metal ties that were deliberately built into the wall to aid in structural integrity, or just lumps of mortar that fell down inside due to careless workmanship, or even rubble that got thrown in there, such as pieces of broken brick, off-cuts of 2x4, a dropped hammer that could not be retrieved, etc. If there is a mechanical bridge like that, then you don't have the "decoupling" part of the equation that I mentioned in my previous post.

Also, is the joists, trusses, rafters or whatever happen to be attached to both parts of that wall, then they are tied together and don't isolate as well as they could. In other words, if the roof or ceiling rests on both of those walls, then you don't have good isolation. If the roof rests on ly on the outer wall, and the ceiling rests only on the inner wall, then that is great.

Likewise, at the corners: Is the inner wall a single shell that does not touch the outer wall at any point around the room, on all four sides? Or is the inner wall built as a pair of double walls that go right across, from outer wall to outer wall, then another pair that join those?

There are many ways that those walls could have been built, so it would be good to see the details in order to figure out if you actually have a proper two-leaf MSM wall.

You might not need to do much at all, if you already have a true two-leaf MSM system! So before building anything new, let's figure out what you already have. Post some photos and detailed diagrams of what you have.

- Stuart -

_________________
I want this studio to amaze people. "That'll do" doesn't amaze people.

That is exactly how were are doing it. I have some resillent ties for the wall and we are going to connect the outer roof to the outside skin and the inside roof to the inside skin so nothing will be attached to the outside skin.
It is a 4" air gap so figured that just drywall on the inside would be enough. I would also add that we have two types os block both are 100mm but some are dense block and the others are lighter blocks. So I was going to put the dense block as the inside skin and the lighter block on the outside skin.
Please see attached pic it's just the engineering brick so far.

Thanks

Antony.

As you lay those bricks, you can increase your mass by filling the holes with sand or grout as you go.

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Brian
As you slide down the bannister of life, may the splinters never point the wrong way...

Great! If you do it like that, and do it right, then you don't need anything else: those are your two leaves right there, so you don't need any more drywall, frames, or anything else: The surface of that inner brick wall facing the room, is your final inner-leaf surface. You could plaster it (render, stucco), paint it, seal it, etc. But you don't need any more surfaces next to that.

The only caveat here is to be very, very careful about keeping that cavity spotlessly clean: no rubble, dripped mortar, etc.

- Stuart -

_________________
I want this studio to amaze people. "That'll do" doesn't amaze people.

That's great. Thankyou lots for your well expressed knowledge.
Just finally if I choose to use some drywall on the inner wall for aesthetics, would that be ok or detrimental to the two leaf system.

Once again many thanks

Antony

If it is attached directly to the brick, then that is fine: no problem at all. But don't put it on a framework over the brick, as that could create a third leaf.

- Stuart -

_________________
I want this studio to amaze people. "That'll do" doesn't amaze people.

Cheers, thanks a lot for all your help.

Antony.