Don't confuse critical frequency (referring to coincidence) with Mass-Spring resonance.Ted White wrote: The curves are similar. Above the concrete (block or slab) critical frequency the added leaf provides advantage. Below that critical frequency the concrete (solid or block) alone is superior. As predicted by Mass Law.
Indeed concrete blocks and glass bricks behave as single leaf (hollow or not)
You call it a traditional double leaf, I call it a sick double leaf. That original curve is dominant defined by structural resonance, and the added panel by 2 things:Ted White wrote: So lastly I ask how does a standard coupled double leaf wood framed ceiling behave when a third leaf is added? Starting on page 70 of the NRC’s IR811 study I find some rough comparisons that might help me.
I see that adding a third leaf to the traditional framed double leaf does not de-rate the system, and does not appear to negatively affect the low frequencies, at least down to the available data limit of 50Hz
So I could infer that the addition of a third leaf to an existing double leaf may not reduce the low frequency transmission loss. I might further infer that due to the stiffness of the hollow block, it does not behave as a double leaf, but rather behaves more like a single leaf.
Clearly these triple leaves are a waste, but that wasn't the point.
1) far to the left of the resonance around 200 Hz (you have a cavity of only 13 mm) these 2 panels start moving in phase (restoring themselves to the mass law of both panels together, meaning that these panels further below the MSM start acting as a single panel as if that cavity hardly exists = standard behavior) and
2) the resonance itself is strongly damped to the extreme small cavity (visco-thermal acousto-elastic interaction - see Tom Basten ISBN 90-365-1597-1)
Wat I do notice is that on the same pages in this report you refer there are combinations wih more logical cavities where the phenomenon becomes more clearly. These you don't speak about.
As long as the others don't get confused here it's OK for me.
I also agree with xSpace: wat you see on that typical picture are STC values. A higher STC value is a higher STC value, nothing else. For years people are telling that STC isn't representative for low frequency content, or studio use.
For me that picture has no other value than showing the idea of MDOF systems (Mass-Spring systems with Multiple Degrees Of Freedom).
That's a typical picture once taken from a UK site (which is afterwards removed by them) and that went into the traditional copy/past net procedure, where giving sources are more of a sin than a virtue.
Mass-spring principles applying on STC values based on measurements where structural resonance is, or can be, a dominant factor is like comparing apples with oranges.
Ted, why do you think I'm so careful in expressing myself physically, if you try to get me to confirm simplified statements/conclusions?