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Join Date: Jul 1999
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Flex is a good thing. Flex is a bad thing. It depends on the amount of flex, where the flex occurs and the purpose of the flex (application dependent).
For example, flex in structural walls can be a bad thing...unless the flex was engineered in to protect the structure from impact (think earthquake, high wind loads).
In sound isolation applications, flex will convert acoustic (kinetic) energy into heat...but not all of the acoustic energy. A certain amount of flex is required in constrained layer damping (think green glue between two sheets of dryall). In a clips + channel application, the objective is to allow some degree of flex in the channel, drywall, green glue components. There is a second purpose to this type of construction ... to reduce the amount of flex (or vibration) transferred into the underlying structure (framing). Once you have flex in the framing, you have a few things happening that you'd prefer not occur. One is that flex/vibration is now carried through to the structure allowing sound transmission to occur throughout the structure. A second is if the whole wall is flexing you are then turning the opposite side of the wall into a speaker...somewhat contrary to the original purpose (it's not a perfect world, some of that flex/vibration is going to make it through in any case ... it's the amount that gets through which is key). A secondary issue with 'too much flex' in the wall is simply when the wall deforms under stress, it snaps back into its static state. When it returns, it doesn't necessarily return at the same rate at which the deformation occured. Depending upon the material, this could be a faster or slower rate. Thus the wall is returning a portion of the energy back into the room out of phase and at a differerent frequency. Is this a problem? Answer: "depends". "Depends" meaning just how much energy is returned.
Saying flex is good, is a truthful statement. Saying flex is bad is also a truthful statement. Grabbing onto a statement in a book (or the internet) like "flex is good" and running with it is not the best idea. Not all flex, or all amounts of flex is good. It depends on the amount of flex, where the flex occurs and the purpose of the flex (which is pretty much where I started.)
Dennis Erskine CFI, CFII, MEI
Subject Matter Expert
Certified Home Theater Designer