Remember that Acoustic Treatment is NOT the same as Soundproofing.
Acoustic Treatment will solve problems within a room. Soundproofing will solve problems outside of a room. Soundproofing is designed to stop sound from entering or existing the room. Acoustic Treatment simply makes the room sound "good".
Fixing a room of reflections can be quite costly. It is recommended that 80-90% of the room's surface area is treated.