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My theater is on the second floor of our new house. I read about people using two layers of plywood on stages and risers, why is this? I understand that two layers is stronger than a single but if two layers are necessary why is the entire second floor just one layer?
 

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It's not strength, it's mass. It's an attempt to minimize the sound transmitted through the floor.


It's only useful if part of an integrated acoustic isolation plan (good door, double drywall on the walls and ceiling, no shared air ducts, etc.) If you don't care if they hear your movie downstairs or if you hear the dishwasher running during a quiet part of your movied, skip it.
 

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Your case is a little different given your HT is on the 2nd floor.

You are probably weight limited and a second sheet of plywood might be better than sand in your stage.

I don't recall seeing much discussion on a second layer for the riser, and unless you have buttkickers or such in chairs on the riser, I wonder if a second sheet there would really do anything from a sound deadening standpoint. If your buttkickers were in the riser itself, then the second sheet would do nothing to prevent sound from going through the floor beneath the riser.


I question the concept of the mass added by a second sheet of plywood added to the stage when the stage is typically filled (mostly) with sand (for basement application anyways). The mass of the sand typically used seems to exceed the mass of an additional sheet of plywood. Plywood weight varies by type and source, but you could consider a 4 x 8 sheet of 1/2" to come in around 45 lbs. Even if a stage has sand only under the outer ends (typical sub locations) the sand used is several times the mass of the 2nd sheet of plywood.


I'm sure one of the acoustic experts will weigh in and clarify.

Paul
 
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