Step # 1 - Establish Your Base TempoThis is, for all intents and purposes, the first tempo your listener is exposed to. The first one they really remember. Their brain will become accustomed to the groove of that tempo. Picture a drum pattern written as quarter notes: Kick, snare, kick, snare, with the hi-hat hitting on all four notes. For realism's sake, this groove would probably be written as sixteenth notes, voiced as quarter notes via three rests in each note grouping; but let's just focus on the 1-2-3-4 feeling of quarter notes for now. Your listener is likely to zero in on that repeating feeling. With slightly more complex grooves, probably with more interesting kick patterns, the hi-hat or ride cymbal at least tend to retain the same 1-2-3-4 groove, providing an anchor of listener-friendliness among the complexity. This is beneficial for the reason that your listener has something to grab onto. It is this predictable pattern that really gets people headbanging with the groove. The downside to this stability, though, is restriction. Even with a proper transition, again detailed in my previous lesson, we still might fail to make the listener forget the frequency at which their brain expects the 1-2-3-4 pulse. To remedy this, we need to do a bit of math.
Step # 2 - Pull A Factor From Your Base TempoThis is the real key. Pros do this, some by intuition, others by conscious awareness, but proper tempo changes always seem to follow this rule. Let's say your base tempo is 120. Cutting it in half will yield 60. Cutting it in fourths will yield 30, 60, and 90. Taking your base tempo and adding or subtracting any one of these numbers is viable, but there's one more factor that often sounds even better: 15. Since the real source of our problem comes from that pulse that hits at the rhythm of 1-2-3-4, we need a faster pulsation in order to alter it in a rhythmically pleasing manner. Since it's 4, let's try doubling it: 8! Cutting 120 by 8 results in the factor of 15. Think about that for a second. How many tabs of professional prog songs have you seen on this site that seem to feature tempos like 105, 135, 150, 90, and 60? All of these are reached by repeatedly adding or subtracting 15 to/from 120. Basically, the number yielded by dividing the base tempo by 8, subtracted from the base tempo, causes a curious mental effect in the listener's brain. They perceive that there's been a shift, certainly, but it's a more musical shift, one seemingly mandated by the song, rather than forced. This is due to a subconscious, unvoiced interplay of mathematics that the listener processes without even realizing what it is, similar to how a tritone tends to sound scary even to those who don't know what a tritone is. They may not know what they're hearing, or why they process it the they do; they simply feel it. For some audio examples and a deeper explanation of this concept, check out the video below: