Long time ago I had the opportunity to jam with a band who used a strip mall store front for their HQ. They rehearsed in the same place and had sound a sound proof room the size of a small warehouse. The room had two layers of insulation; one of acoustic foam tiles in the walls, acoustic tile on the celling and carpeted floor with rugs for each members. The second layer was a thick theater like curtain that draped all around the room. Once you were inside it was death silent and echo less, outside it sounded like the radio was on because the AC vents were not cover or sound proof. The curtain trick may work for you, it may be something to consider.