Product called “Green Glue” is supposed to absorb sound energy:
The shop and neighbor’s apartment are separated by a concrete slab. The ceiling is low, so I don’t want to add a bunch of layers. I may not need to. Apartment above used to complain about radios/music. My guess is that major transmission was through windows into side yard, especially since side yard area is known to transmit sound up and down well between porches.