If you Google you can find many technical specs for isolation, but basically, use your ears. IMHO, if you can hear any outside noise in the studio it’s not isolated enough. In 1978 we built a studio with double-wall construction and floating ceiling & floor. We were a hundred feet away from a railroad line and never heard a whisper. But you pay for that. Our construction cost was six-figures. Also, don’t forget ventilation noise can be an issue.
John
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