Did they agree to pay for the soundboard? At $15-25 per 4x8 that stuff's pretty pricey. If you're paying, I'd say two layers of sheetrock with something thinnish in between would make much more sense financially. Felt, foam or sheet cork would work well.
I mean, sheetrock is only $6 per 4x8 5/8 up here. You could even use 1/2" for the second layer to eliminate sympathetic vibration, and it's cheaper too.
I like homasote's environmental approach, but that's a pretty huge difference in cost compared to sheetrock.
Harold
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meh
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