One of the great, overlooked aspects of acoustics. The first thing everybody does when they build a new studio room is slap foam everywhere. Usually this goes very badly, because they use the wrong foam.
This is a basic, non-axial Sabin calculator for RT60, the length of time in seconds it takes for a signal to drop 60db. For a recording studio, this should be about .20-.40s at 1000Hz, for a vocal boot it should be < .20s. A home theater or music room has slightly looser tolerances.