i have 2 layers of burber carpet on the walls of my vocal booth..
it sounds good.. if your looking to fix low-end its not going too. it'll help a little but nothing major...
if the booth is close to your control room, be sure you track low or track with headphones..
try carpet padding.. the thick green stuff you can get at home depot.. i used it in some vocal booths before and had great results for a project studio.. but like i said its not soundproof, but its better than bare walls and the sound bouncing