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I want to soundproof a room, where my wife can get some peace and
quiet from our son,
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The problem is not your wife, it is the noise. Don't isolate the wife.

If you are unable or unwilling to change the noise from the son, then
provide sound proofing of a place for the son to make his noise. How to do
that depends on the type of noise and home construction.
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Point source control (special absorption material) at the source of the
sound will also help.

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Point source control of the son!!!! That conjures up some interesting
images!!!