Phantom Tempo

"My song is MIDI-based. I have my tempo set at 120 beats per minute with no tempo changes. As the song is played and proceeds into the chorus, I could swear it is slowing down and then returning to its correct tempo when entering the next verse. I know this is not really possible - why then, am I sensing these tempo changes that are not really happening?"

You have just stumbled upon an interesting phenomenon, the relationship between song structure and tempo. In contemporary pop music, it is the conscious (or unconscious) affect of the drummer pushing the tempo (not more than 2 to 4 beats per minute) when entering the chorus. The drummer "leans into the beat" to help add extra energy to the chorus. But in this case, it is the absents of this "leaning-in" that is leading you to believe the song is slowing-down - when in actuality, it should be speeding-up. Because of these phantom-tempo variations, one can begin to understand the fundamental reason why tempo maps are so important with respect to MIDI-based contemporary music genres. Dance music, for example - may need very little tempo adjustment (maybe none at all).