I'm not a soundproofing expert but was a home improvement contractor for 45 years. I had a job once remodeling a funeral home that had offices on the second floor. I hired a professional to soundproof the second floor thinking he was only going to do the ceiling of the first floor. Well the mechanic said he would have to come down the walls of the first floor about 2 ft. as well otherwise the soundproofing wouldn't work. As others have mentioned above, the sound does transfer from the floor through the walls below.
