I've built a special room for my piano teaching/rehearsing studio. It's not completely soundproof, but the walls are 6" with off set studs and covered with 2 layers of 5/8" sheetrock with green glue between them. For fulfilling my main purpose, which was to separate the studio from my house so I can work in peace and my wife isn't bothered by the sound, it's good enough. I'm unhappy, however, with the door to the outside. It's a steel, solid core door, but it lets enough of the A/C compressor hum in to bother me (it's really not much, but it always grabs my attention when I'm composing and the compressor kicks on). The sound definitely is getting in through the door rather than the wall, as is apparent if you walk alongside the wall past the door. The sound always comes from the direction of the door. So I'd like to improve on that.
Adding a second door is definitely an option, but I know that could get expensive. I would not want to try to install one myself and it would have to be made to work with the existing frame. So I'm looking into adding something like this 1" foam mat on the inside http://www.supersoundproofingsales.com/SSP-Foam-Mat-1-thick-by-48-wide-w_PSA-per-foot/productinfo/09-42730-PSA/
. It looks easy to do, except I'm not sure how to deal with the door knob. If I cut out a big enough hole to accommodate someone's hand when they turn the knob, won't I be defeating the purpose somewhat? Could I just get a knob for a thicker door and make a hardwood spacer for the inside, cutting the foam around that? Any other thoughts? Thanks in advance!