Now go and get some foam for seat cushions and put that in there. 1/2" will
do wonders, especially if you apply another sheet of plywood to that. The
noise level will not only go down but the frequency will do so also.
I'll note that 9 db really isn't that much and you can do a lot better. It
may be that the high frequencies have been attenuated more than that which
will make the attenuation of the noise better than what the number
indicates.

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Why isn't there an Ozone Hole at the NORTH Pole?