Thought I'd add this in case anyone is interested in some of the technical side....
If you want to know at which frequency your room resonates:
f - 2l/c
where 'f' is the frequency
'2l' is twice the length (distance between parallel walls)
'c' is the speed of sound (say 320 metres per second - depending on altitude)
If use use this simple equation for each dimension - length, width and height - of your room, you can get a quick and simple guide to the troublesome resonant frequencies - a big issue in turning a domestic house-sized, cuboid room into a studio. This can help you choose acoustic materials that will be most effective.
I feel that I should mention that egg-boxes have no BS fire-retardant rating and are therefore considered by insurance companies to be a fire hazard!
Sorry if it sounds a bit science-lecture - it can be fun experimenting.