What is a “rigid surface”?

Can someone explain as detailed as possible what, exactly, a rigid surface is? The context in which I am inquiring is in regards to Green Glue. It requires application between two rigid surfaces. I am sure this is a global term that doesn't apply to just Green Glue.

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This might be a better fit on English Language and Usage. This might be helpful. – Tester101 Nov 12 '12 at 18:05
ooohhh schnap.. – oscilatingcretin Nov 12 '12 at 18:24
Sorry, I forgot to add the smiley at the end of the last comment. ;p Sensitivity training has failed me. – Tester101 Nov 12 '12 at 18:29

Rigid means something that will hold its shape on its own. Wood (except veneer), drywall, backer-board are all rigid.

A piece of thin paneling made from plastic/fiberglass would not probably count.

However, the definition is somewhat subjective. In the context of Green Glue, I presume they want to ensure no movement between the layers.

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Rigidity is important for glues that become brittle when they dry. – Wayfaring Stranger Nov 12 '12 at 18:10
Actually, I think the context of Green Glue matches your definition. Rigidity would probably make sure the two surfaces stay apart. For example, if you have subfloor, put Green Glue down, then MLV (mass-loaded vinyl) on top of that, things like table legs would probably mash down hard and cause the vinyl to come in direct contact with the subfloor. It seems to make sense now. – oscilatingcretin Nov 12 '12 at 18:56
@WayfaringStranger Green Glue is supposed to be flexible when dry. It absorbs the vibrational energy from the first layer due to its elasticity and prevents a good part of that from being transmitted to the second layer. Hence its need to be used between two rigid layers. Rigidity so you don't have a large speaker cone due to thin, flexible material. – Fiasco Labs Nov 18 '13 at 16:32

Formally, a Rigid Body is one that doesn't deform under stress. (i.e. It has a Young's Modulus of infinity).

Practically, it means that it's a body that doesn't bend or flex under normal loading.

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"Rigid" just means the opposite of "flexible" and its meaning is subjective and context dependent. It's really more of a relative term: "drywall is more rigid than house wrap", "CMU walls are more rigid than drywall".

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