Triple Leaf Effect

There is an ongoing question about triple leafs and what they do. This post is to try and answer that question with regards to music isolation. The attached jpg is taken from NRC IR-586. There are three lines on the graph showing the Transmission Loss (TL) of three similar construction walls. The gray line is for a bare 190 mm (8") concrete block wall. The STC is 50 and the TL at 80 Hz is 34 dB. The open circle line is for a 190 mm block wall with drywall on studs attached to one side. The STC is 59. However, the TL at 80 Hz is 29 dB. A decrease of 5 dB. The filled square line is for a 190 mm block wall with drywall on studs attached to both sides. The STC has to 64, a very respectable number. But the TL at 80 Hz has dropped to 22 dB! The isolation of the triple leaf wall for bass frequencies is up to 12 dB LESS than than the bare block wall, despite having added mass and an increase in STC by 14 points! I hope this helps people see the effect of triple leaf partitions. Andre

Excellent! Thanks, André! When you say "studs attached", I guess you mean directly attached to the wall and therefore coupled to it? So we are not talking about a second, separate decoupled stud wall, Correct? - Stuart -

Another good explanation about leaves... http://www.greengluecompany.com/underst ... leLeaf.php

Soundman2020 wrote:

Excellent! Thanks, André! When you say "studs attached", I guess you mean directly attached to the wall and therefore coupled to it? So we are not talking about a second, separate decoupled stud wall, Correct?

Correct. The reduction is all due MAM and triple leaf effect. Nothing to do the stud connection. The greenglue link you posted deals only with STC in their examples. I showed TL values also. Imagine a good hearted person has a concrete block wall in their condominium basement. Their neighbour covers the wall with drywall on studs and all of a sudden complains about the booming coming through the wall. The person not knowing the physics involved thinks the neighbour is a little crazybecause the STC has increased by nine points, but says "No problem, I will build a wall on my side to reduce the sound." The wall is built and the neoghbour says "the booming is louder!." The person is now convinced that his neighbour is crazy, because the STC was increased by another 4 points. . In reality it was explained by physics. As shown by graph, STC is suitable for music isolation purposes. Andre

Thanks again, André! I think I'm getting the idea, finally.

The greenglue link you posted deals only with STC in their examples.

You are right about that for most of their web site, but actually that particular page that I linked is almost all about TL. That's why I posted the link, since their graphs seem to agree (to a certain extent) with yours, even though they are not mean for the same purpose. See the graphs below, which are TL graphs from that link (I'm sure they won't mind me reproducing their graphs here!). But I sure do appreciate your taking the time to explain in more detail! I'm already totally convinced about the "2-leaf good, 3-leaf bad" mantra (and I'll probably be chanting that in my sleep for the next few months!) So don't worry about me trying to build a 3-leaf wall, or anything. I'm already sold on that. It's just that I have a very curious mind, and I like to to try to understand the physics behind things, why they work the way they do. You sure have been a great help in that area! I came here as a total acoustic dunce a few months back, but you guys have made it easy to learn, and I sure do thank you. Now I now WHY I need to build things the way you said I should, and believe that knowledge will help me to avoid stupid mistakes during the build. - Stuart -

OK, André, here's a tough one for ya, that I'm still having trouble getting my head around: Can you explain "coincidence" in simple terms? Is that where the panel's resonances due to stiffness and due to thickness coincide, or am I totally off base with that?

Thanks for the clarification of the link to the greenglue page.

Soundman2020 wrote:

Can you explain "coincidence" in simple terms? Is that where the panel's resonances due to stiffness and due to thickness coincide, or am I totally off base with that?

Fairly close. The "coincidence effect" is when the TL of a material is reduced because the length of the bending wave in the material equals the wavelength of the frequency in air. The good news is that because of the predominance of low frequencies, which are more difficult to contain, a partition that is good enough for low frequency TL will have more than ample TL at the frequency of the coincidence effect. This can be seen in the graph I posted at the beginning of the thread. The coincidence effect reduces the TL at 2500 Hz by about 6 dB in the worst case, to about 65 dB. With the single drywall partition the dip is around 3 dB. The limiting factor in both cases is the low frequency isolation. Not at the coincidence effect frequency. I hope this helps. Andre

Thanks Andre! Now I get it! I just came a cross a great link that explains coincidence visually, with an animation that shows what is happening to the panel. http://squ1.com/archive/index.php?sound ... ssion.html Scroll about half way down for the animation.... ( BTW that entire site is pretty cool! Great stuff there....) - Stuart -

Thanks for the great post, Andre! And Soundman, yer really working the Google engine, right?! Good stuff.

And Soundman, yer really working the Google engine, right?! Good stuff.

Yeah, you bet! I want to make certain that I understand this stuff as much as possible before I actually start building! I reckon I've already absorbed a large chunk of the great advice you guys have spent years accumulating on this forum, and you bet I'm going to follow every word of it. But I also want to be sure that I also understand WHY you give that advice, they principles and theory behind it, so that I'm building from a position of full understanding, not just from a position of "following the crowd blindly". On other forums I've seen advice given that sounds good on the surface, but when you check under the hood you find there's nothing really there: if you ask, you often gets comments like "somebody told me once it was a good idea" or "I saw it on the internet" or even worse "That's the way they always do it". But the advice given here is solid and sound (no pun intended!), always with good theoretical backing, and that's what I love about this place!

Jup, that's the same reason why I love this forum, and it's crowd!, so much. Good solid comments. And great projects ofcourse! (AND I like the "friendly" admosphere which is not very common on every internet fora... or so it seems. )