Audition Online is a rhythm based game where you need to complete (by pressing) a sequence of arrow buttons followed by a space button every predefined interval. The interval is calculated based on the bpm of song used for the match/session. An interval is equal to the length/duration of four beats, which means higher bpm produce shorter interval, thus harder difficulty.
In between input interval, there's a resting interval where player does not need to press anything.
start > sequence (4 beats) > rest (4 beats) > sequence (4 beats) > rest (4 beats) > ... > finish
So for a song with 120 bpm, the sequence/resting interval is
intervalInMs = 60 * 1000 / 120 * 4 = 2000