A well treated acoustic space is one in which the sound source will be audible and clear without obvious coloration, or interference caused by sound reflections from walls and other surfaces. The control and manipulation of direct and indirect sound waves reaching the listener is what determines this clarity and audibility. This is done by arranging acoustic absorbers, reflectors and panels around the space in very specific patterns, places, and angles, and covering a very specific amount of surface area.
The number of panels needed, their optimal sizes, and their placement, are all calculated using acoustical measurement tools, mathematical models and knowledge of the acoustic qualities of certain materials in the room before and after it is treated.
Once a room is treated, these materials all act together to delay, absorb or trap particular frequencies that have been determined to be problematic.
The key concept in acoustics is the control of frequencies, not their elimination, so acoustic treatment is often confused with soundproofing.
A well treated acoustic room is not necessarily soundproof, and a conversely a soundproof room is not necessarily acoustically balanced.
Another misconception about acoustic treatment is that it is used to remove problematic frequencies entirely. Many amateurs will over treat a room and “deaden” it completely, ironically introducing acoustical issues rather than solving them. The reason is because such an approach affects only a certain range of frequencies (often from 1kHz and up) while completely failing to control the more troublesome and more difficult to absorb low end frequencies (250Hz and below), potentially leading to an even more unbalanced frequency response in the room than when it was untreated.
Of course in some studios a near completely dead room may be desirable for voice over work or dry foley recordings because they are recordings that will have artificial spatial information added later with reverbs and delays in the sound design or mixing stage.
The purpose of good acoustic treatment is to smooth out frequencies so that no one frequency dominates the listening experience. This is known as a “flat response” across all frequencies, and a room that has been manipulated to behave in such a way is considered to have good acoustics.