To 2 or 3 leaf that is the ?

[matthewclay]

I'm reading a lot right now & spending hours on this forum which is highly addictive. Much like tv, but hear my brain isn't turning to mush! Well sort of. Anyway I am just not getting the 2 leaf 3 leaf wall idea. I guess I don't see when 2 becomes 3. I prolly should get made fun of but could someone point me to a post or explain so I can be enlightened? Thank You

[Soundman2020]

Nobody is going to make fun of you for a serious question! Don't worry. As one of my instructors used to tell me, many years ago: The only dumb question, is the one you DIDN'T ask.

OK, so leaves:

A "leaf" just means a massive surface (where "massive" implies heavy), next to an air gap. The leaf could be drywall nailed to a 2x4 frame, or plywood, or MDF, or anything else that is massive and creates a surface around the room. The leaf might actually be made up of more than one layer: For example, for someone who needed extreme isolation, their leaf might be a layer of plywood followed by two layers of drywall, all of them nailed to the same side of a stud frame.

So a two-leaf wall is simply one where you have a "leaf", then an air gap, then another leaf. A normal house wall is an example of a 2-leaf wall: There is a sheet of drywall on one side of the studs, then an air gap in the middle, then a layer of drywall on the other side.

A 3-leaf wall means just what it sounds like: somehow, another "leaf" appeared in the picture.

A typical case, for example, would be somebody who wants to build a vocal booth inside their living room. Since they already have a 2-leaf wall around the room (the normal house wall), if they then put up a stud frame inside the room and nail drywall to one side of that frame, then they have created a 3-leaf wall: the original 2-leaf, plus the new one. And if they put drywall on BOTH sides of that new frame (instead of just one side) then that would be even worse: it would be a 4-leaf wall.

Another typical case is someone who lives in a house or apartment where they share a common "party" wall with their neighbor, who is very noisy. The party wall is already a two leaf wall (studs with drywall on both sides). So someone who is not aware of the issue might decide that, in order to stop the noise coming from his neighbor's side of the wall, he is going to put some 2-inch "furring strips" (bits of wood, like studs) on the existing wall, then add a layer of drywall on top of that: he just made a 3-leaf wall! He added a third leaf to an existing 2-leaf, and the isolation will be WORSE than it was for low frequencies, but probably better for high frequencies.

There are many other possible ways of making a 3-leaf system, and sometimes it is unavoidable, but in general a 3-leaf wall will give you worse low-end isolation than the equivalent 2-leaf, so 3-leaf should be avoided, wherever possible.

- Stuart -

[matthewclay]

Thanks so much for that. I mostly understand now, so let's say you had sheet of drywall then a stud wall then an air gap of 3-4" then another stud wall then drywall that would be a 2 leaf system? Would what I described be the best kind of isolation for a normal studio? & why does doubling the layer of drywall increase the noise reduction in the case I'm describing? 1 more question. I assume you fill those 2 stud walls with some sort of rock wool or insulation?

[Soundman2020]

so let's say you had sheet of drywall then a stud wall then an air gap of 3-4" then another stud wall then drywall that would be a 2 leaf system?

If you change your "stud wall" to "stud frame" for clarity, then yes, that would be it! Like this: "so let's say you had sheet of drywall on stud frame then an air gap of 3-4" then another stud frame with drywall, that would be a 2 leaf system?" Yup!

Would what I described be the best kind of isolation for a normal studio?

Yes, provided that you build it around all sides of the studio (4 sides plus ceiling), and seal it air-tight. A fully decoupled 2-leaf MSM system is, indeed, the best way of getting really good isolation at relatively low cost.

why does doubling the layer of drywall increase the noise reduction in the case I'm describing?

In a word: Mass. To stop sound, you need mass. Remember that I defined a "leaf" as being a "massive surface"? There's a reason for that, based on physics. In order to stop something that is moving, you have to put something in the way that is either rigid enough to stop it, or massive enough to stop it. The bigger the "thing" that you want to stop, the more mass you need. If you want to stop a bicycle, then a simple wooden wall will do, but if you want to stop a car then you are going to need a brick wall. But if you want to stop an 18-wheeler then you are going to need a massive concrete wall. The only thing that changes is how much mass you are putting in the way. If you had enough wood, you could stop the truck by using the same total mass of wood (even though it would take up much more space).

In other words, sound waves react to mass. When a sound wave hits a wall, it makes the wall vibrate in sympathy. If the wall is light weight, then it vibrates a lot: the sound gets transmitted through to the other side. If the wall is heavier (more massive) then the sound cannot make it vibrate so much, so less sound gets though. And if the wall is REALLY heavy, so that the sound cannot make it vibrate at all, then no sound gets through.

So mass is your friend. The more mass you put in a wall, the better it will isolate.

However, the laws of physics that deal with mass and vibration are not very helpful: "Mass Law" says that each time you double the entire mass of the wall, then your isolation goes up by 6 dB. That isn't very much. Let's say that you have a single-leaf drywall wall, and it is giving you 20 dB of isolation, but you need 44 dB of isolation. Mass law says that you double the mass for 6 dB increase, so you add another layer of drywall. There are now 2 layers, and you are getting 26 dB. So you double again: add another two layers (total of four) now you get 32 dB. Now you double again: four more layers (total of 8 ) and you get 38 dB: Now you double again (8 more layers, total of 16), and you get another 6 dB, for 44 dB total. That's one hell of a wall you have there now! You had to multiply the mass by sixteen to get an increase of just 24 dB! Clearly, this "mass law" thing is not as much a friend as you thought it was.

That's where the two-leaf wall comes in: It is not restricted to mass law. A 2-leaf wall is a tuned system, that works on a different principle of physics: that principle is called "Mass-Spring-Mass" or MSM. It means that you have one chunk of mass attached to a spring that is attached to another chunk of mass. This is a whole different ball game, that gets a bit more complicated to explain, but it beats simple mass law, hands down. However, it still needs mass in order to work! It just multiplies the effect of the mass...

1 more question. I assume you fill those 2 stud walls with some sort of rock wool or insulation?

Exactly. Insulation is part of the MSM system: it acts as a damper on the resonance, which is a good thing!

- Stuart -

[matthewclay]

Thanks so much Stuart I learned more in 2 of your posts than the 2 books I read this week. Not knocking the books they were just very complex. Is there any posts on here explaining the asymetrical versus symmetrical ratio stuff? I don't understand how you determine how big a control should be or a live room should be? I've read that part in the book 3 times & still don't get it. I posted another post on here asking if there was someone (for not thousands of dollars) could help me design something. I am willing to pay cause I'm not for sure if I will ever understand all this completely or if I even want to. I guess I'm asking if you know anybody on here for $1000 or $1500 would help me with the design part of it? Thanks again so much appreciated!!!

[Soundman2020]

Is there any posts on here explaining the asymetrical versus symmetrical ratio stuff?

It probably is explained in several posts, but finding them might be the issue! The search feature on the forum works, but is very basic and might not be a lot of use to find that. But is is worth trying!

I don't understand how you determine how big a control should be or a live room should be?

The basic rule for a live room is: "As big as possible, with the highest ceiling possible". The basic rule for control rooms is to follow the ITU and EBU recommendations: For mono, 2.0 or 2.1, the floor area should be in the range 20 to 60 square meters, and for multichannel rooms, it should be 30 to 70 square meters. Height should, once again, be as high as possible, using one of the known good room ratios. There are a whole lot of other specs that they recommend for control rooms, but those are the basics The document that defines all this is ITU-R-BS1116-1, if you are interested.

However! This does NOT mean that you can't have a good room of you don't have 20 square meters of floor area! Their specs are just for optimum rooms, but smaller rooms are certainly still possible. In fact, John has designed and built studios inside shipping containers, so it absolutely is possible to get good sound in a smaller space. It just gets much harder to accomplish it.

I posted another post on here asking if there was someone (for not thousands of dollars) could help me design something.

I just PM'd you about that, with a list of some folks who offer that type of service, starting with John himself, of course!

- Stuart -