mam systems and airtightness etc

i have read posts saying you need mam systems to be airtight and others that say you don't. i'm on the sealed side of this. if you aren't sealed you have sound just pouring in and out through air. if you are sealed i can see having more resonance at the outer leaf's center frequency but across the entire spectrum there should be more attenuation. overall better. no? seems like something like green glue could help with the resonance. also there seems to be some confusion about 3 leaf systems. my take is it has something to do with the width of the air gap. if you could construct a 3 leaf system with 12" air gaps then the middle leaf wouldn't hurt you and possible could help. but if you are talking about dividing a 7" gap with an middle leaf (two 3" gaps) you are actually hurting your STC rating. seems you need a certain length of spring (air gap) for these systems to work well. thoughts?

I think everyone here is on the airtight side of this issue, but I wasn't aware it was opened for debate? It is common knowledge where air goes so will sound. As to a three leaf, it takes more space, requires more material and energy and in the end doesn't produce isolation for a musical environment near as well as a simple mass spring mass.

i have read posts saying you need mam systems to be airtight and others that say you don't.

If it's not sealed, then it's not really MAM. The more correct name for MAM is MSM, meaning "Mass-Spring-Mass", from the physics principle on which it is based. The "spring" is the air, which is ideally damped by insulation in the cavity. If the air can't be compressed well, then it doesn't act as a spring well. It will still work, since air has stiffness all by itself, regardless of whether or not it is sealed in, but it won't be as efficient as it could be. In any event, if you have even a tiny hole in a wall, it will act like a speaker, so the wall won't isolate well, no matter what else you do to it, regardless of how many leaves it has. If there is a pencil-sized hole that goes all the way through, then all bets are off on how well it isolates. Even a ten inch thick high density reinforced concrete wall won't isolate it if has a hole in it. That's teh main reason for the seal, not the "spring" reason. Having said that, I'm wondering if you might be referring to some posts here that clarify that a leaf does NOT need to be sealed in order to act like a leaf? If you hold a sheet of plywood up close to a sealed MSM wall, yes, it will act as a third leaf, and reduce the isolation of the wall: you will probably be able to the difference clearly. But it isn't actually a leaf: it just acts like one. The effect happens because air has stiffness, and that creates a spring in the air gap between the wall and the plywood. The plywood has mass, so you have created a MSMSM system, where the extra "S" is not sealed, so it is not 100% efficient. But it will still work like leaf, driving the resonant frequency up, and driving the low frequency isolation down.

if you are sealed i can see having more resonance at the outer leaf's center frequency

Actually, it is the resonant frequency of the entire MSM system that matters, not the individual parts. The entire point of an MSM wall is that it acts together as one single system, not as several separate things: the whole is greater than the sum of its parts. the system has a resonant frequency, which is entirely different from the resonant frequency of the leaves by themselves. It is the resonant frequency of the system that determines the transmission loss curve for the wall. That's why it should be designed so that the system has a resonant frequency as low as possible, ideally well below 20 Hz, which is the generally accepted lower threshold for human hearing. Since the wall is not only acoustically transparent at the resonant frequency, but in fact boosts transmission there, and doesn't start to isolate well until roughly twice the resonant frequency, ideally you want the resonant frequency of your wall to be around 10 Hz or so.

but across the entire spectrum there should be more attenuation.

Not necessarily. It depends on a number of factors. Adding a third or fourth, for example, skews the TL curve drastically, making for WORSE isolation at low frequencies, and better at high frequencies.

also there seems to be some confusion about 3 leaf systems. my take is it has something to do with the width of the air gap.

Well, the air gap is part of it, but so are the masses of the leaves, among other things. The issue with a 3-leaf wall is that, all other things being equal, by adding a third leaf you drive UP the resonant frequency of the wall, and you tilt the TL curve to be mush steeper. So you cause the wall to isolate WORSE at low frequencies: In order to compensate for that, you need to increase the air gap, or the mass, or both. That drives the frequency down again. But you need an awful lot more mass, or an awful lot more air gap, in order to compensate. It is much cheaper and more effective to NOT build a three leaf in the first place, and just put some extra mass on the two-leaf, or increase the gap slightly on the two leaf. You need less increase in mass, and less increase in gap to get the same net result at the low end.

if you could construct a 3 leaf system with 12" air gaps then the middle leaf wouldn't hurt you and possible could help.

Actually, it still would hurt you, and would NOT help you! And 12 inches is about the minimum thickness for a good MSM wall anyway. Much less than that, and your air gap is too small to get the resonant frequency down where it belongs, unless you add lots more mass. But if you add more mas, you make the wall thicker anyway. Same result. Unless you use something exotic like lead or steel plate for your mass, but then your cost goes WAY up. In order to get a 12 inch think 3-leaf wall to isolate as well as a 12 inch thick two-leaf wall, you'd need to more than double the entire mass of the wall. So if you had just oen layer of 5/8 drywall on each side of that wall (STC-57, roughly) adding a third leaf in the middle, consisting of a single layer of 5/8 drywall, would drop your STC to 50. You'd need to add and extra layer of 5/8 to EACH of those leaves to get back to STC-56 again, and you are still 1 dB short... Even then, the resonant frequency of that wall would be slightly higher than that of the equivalent 2-leaf.

but if you are talking about dividing a 7" gap with an middle leaf (two 3" gaps) you are actually hurting your STC rating.

And you get the exact same effect if you take a 12 inch gap and add a third leaf in the middle: it reduces your STC rating. Any time you add a third leaf to the wall, you reduce the overall isolation, unless you compensate by also increasing the mass considerably, or the air gaps, or both. And if you add a fourth leaf, then you damage the rating even MORE, since the resonant frequency moves even higher, and the TL curve gets even steeper. Of course, using STC ratings for studio wals is kind of pointless anyway, since STC doesn't even consider the bottom two and a half octaves of the spectrum at all, but it does give you a rough idea of how walls react to having extra leaves in them.

but I wasn't aware it was opened for debate?

Me neither!!!! :)

As to a three leaf, it takes more space, requires more material and energy and in the end doesn't produce isolation for a musical environment near as well as a simple mass spring mass.

Exactly! Very well summarized in three lines, where it took me a whole bunch more, to say the same thing! - Stuart -

thanks you guys for clearing up my non-controversy. :D

Having said that, I'm wondering if you might be referring to some posts here that clarify that a leaf does NOT need to be sealed in order to act like a leaf?

hmm. perhaps. i'll take another look. to me i read a leaf as a part of the msm/mam system. right? an outer leaf and inner leaf? so when i read it doesn't need to be sealed my spider sense tingled. http://www.johnlsayers.com/phpBB2/viewtopic.php?f=2&t=11316&p=82094&hilit=mam+airtight#p82094 search for this text.." ? Read this. Airtightness is not required for MAM effect." but who cares, it just made me do a double take. looks like i'm on the right track really? 12" for a decent mam? i was going to use about 7-8" in places so my room stayed large. and you'd lower the res freq with more of a gap because a longer spring can take more shock (longer wave lengths) before it vibrates the outer leaf. yeah? makes sense to me. thanks

to me i read a leaf as a part of the msm/mam system. right?

Strictly speaking, yes, but any panel that you close place in proximity to 2-leaf MSM wall, will ACT as though it is a third leaf, regardless of whether or not the air gap is sealed. In the link you mentioned, André is referring to exactly what I was talking about, originally documented here: http://forum.studiotips.com/viewtopic.p ... nce#p11048 The guy wasn't actually adding a leaf as such, just holding up a panel that suddenly started acting like a leaf, due to its proximity to the wall. Like Andre said, "Airtightness is not required for MAM effect." (My emphasis, not his). You get the effect, even if it isn't really a leaf. Like Ro said in another thread: "semantics..."

really? 12" for a decent mam? i was going to use about 7-8" in places so my room stayed large.

The more the merrier! In my case, I'm stuck with an air gap of no more than 4 inches, but I'm compensating for that with mass: One leaf will be a brick wall (high density, lots of mass) and the other leaf will be thick fiber-cement sheeting (high density, lots of mass). If I were to use drywall instead of bricks and fiber cement, then I'd have to increase the air gap to about 9 inches to get the same isolation. 9 inches of air, plus four layers of 5/8 is 11.5 inches... Yeah, you can go to a smaller gap, but you won't get such good isolation. I'm not sure how you'd get your wall down to be only 7 or 8 inches thick with studs and drywall: two sets of 2x4 studs with a one inch gap between them and 2 layers of 5/8 on each side, is already eleven inches thick. Even if you dropped that to 2x3 studs with a one inch gap, that would still be 8.5 inches thick, and in that case you would not be able to put any roof or ceiling on such thin, flimsy studs! How were you planning to build your wall in order to make it only seven inches thick? The more air gap you put in there, the better the isolation you get. You also have to live with what you have! If you can't do a 12 inch wall, then you just have to do what you CAN do, and live with the consequences. Not too many builders here have the luxury of being able to build 12 inch walls around their rooms! If all you can afford in terms of space is 9 inches, then go with nine. If you can only do eight inches, then that's OK too, but you'll have to do a staggered 2x4 stud wall on a 2x6 sole plate, and you'll just have to accept that your isolation will not be ideal.

and you'd lower the res freq with more of a gap because a longer spring can take more shock (longer wave lengths) before it vibrates the outer leaf.

No, not really. The entire SYSTEM vibrates at its resonant frequency, when excited by that frequency. If you want to think of it in terms of electronics, then the wall is a tuned circuit: it is a band-pass filter, which amplifies at its resonant frequency and blocks all other frequencies, to a greater or lesser extent. If you change one of the components in a tuned circuit, it does not make the electricity take longer to get to the other side, or make the wave take a longer path: all it does is to change the resonant frequency and the degree of attenuation. The key point is that components of the wall do NOT really act individually: they act together, as a single system. Change one thing, and you affect the entire system, not just that one thing. Take a very old radio with a fined variable capacitor in the tuner, and remove a couple of fins with a pair of pliers. You didn't just affect the capacitor by doing that: you affected the entire radio, and it is now tuned to a totally different band, and will probably sound different too, since you will probably have upset the hetrodyning too. In acoustics, as in electronics, all of the "components" work together as an entire system. changing one thing can have unexpected results, because it affects the system as a whole. I hope I'm not confusing you too much with this stuff! I know it sure confuses me the first time I heard it: it takes a while to get your head around it.... - Stuart -

yeah, i just got confused. mam effect? that meant to say you can wreck isolation, simulate a 3 leaf system, even if the 3rd leaf isn't sealed. such as a sheet of mdf held up.

How were you planning to build your wall in order to make it only seven inches thick?

arrrg! you caught me. i was using magic arithmatic. you're right. i was only thinking of the air gap not wall, but still 10" is more like it.

The entire SYSTEM vibrates at its resonant frequency, when excited by that frequency.

this is an excellent point you make about thinking in terms of a system. i was thinking of it as 3 parts. increasing the air gap should make the system less rigid and lower it's res freq. it reminds me of a large panel absorber. thanks