Very quick one! 1 Wall to built for soundproof reasons...

Hi, my studio room is in the basement with 2 walls lying directly in the soil. The door to the corridor is the weak point which leaks sound to the whole house (and therefore neighbours). I'm thinking of building 1 drywall in the back of the room but wonder if I should make it sloping or at right angle. Would it help to counteract standing waves and mid/high frequency reflection? What do you think? Also I'm not sure about the "mass air mass" principle in this case. Single wall would maybe be enough. Find attached 2 screens helping to explain. Thanks for any hints! M.