Soundproofing Wood Floors

Has anyone ever put down soundproofing insulation to decrease the sounds for the apartments below. My neighboors are complaining about noise. I just bought this apartment and just had the floors sanded and stained. I want to keep the floors as is, but add insulation. There must be major issues in the subfllor because I walk around in socks and they still say that it wakes them up in the middle of the night. But there is just one creaky area and it's not where they say the noise is coming from. The bedroom has a large carpet and pad and the floors don't make sound. But they say they are awoken at night when I got to bed. There must be some major issue. But I didn't want new floors. Can the insulation be laid downa dn the existing floors used?