Pulse

Opening section of "Music for 18 Musicians" by Steve Reich (he can be seen playing the piano about 2:50 in, wearing the baseball cap). Good way to introduce the idea of pulse. This is one of the most profound ways that music affects us. The repeating beat of music is directly related to our heartbeat, hence, "pulse". In fact, during the Renaissance, the human pulse or tactus was the basic reference for finding a tempo in music. The metronome wasn't invented until the early 1800s. Beethoven was initially enthused at the idea, but quickly became disenchanted and said something like the metronome marking is good for the first four bars, after that you're on your own. Steve Reich began by working with tape loops and varying them mechanically to create interesting effects. But he soon tired of this and tried re-creating the same effects with human musicians. This music may seem at first to be rather mechanical, but it is the slight human variations that make it interesting. Here is a different kind of pulse:

Also, fairly strict pulse, but it is the variations that make it. Ringo has been highly praised for his drumming on this one--the precursor to today's music video. There is a lot of drumming and the 'fills', those short breaks where he fills in a lot of quick subdivisions, are very original. But notice a couple of things: as is usual in rock and roll the strong beats are 2 and 4, called the 'backbeat'. They are very, very slightly early, making for extra tension. Also notice that right around the 1:00 mark, the band slows down slightly to set up the chorus. George Martin once remarked that Ringo was not the most even drummer, but he always seemed to vary the tempo in exactly the right place. That's what a musical drummer does, George!

Here, chosen pretty much at random, is a recent pop song:

The pulse here is provided by a drum machine, that is, a mechanically synthesized rigid pulse. Like Auto-Tune, it takes the trial and error out of the pulse, but at a price. Auto-Tune removes all imperfections in the pitch for singers just as a drum machine takes out the imperfections in drumming. Of course, to my mind, it is the imperfections where the interest lies, because in the hands of good musicians, they are not imperfections at all, but aesthetic inflections that give direction and shape to the music. No wonder that all songs with drum machines sound static. They are. In a lot of pop music, considerable effort is made to conceal the mechanical nature of the beat, such as in this song by Beyoncé:

Notice how the beat is so highly syncopated that you barely notice its rigidity. Syncopation is a technique where you put a stress in an unexpected place.

In some music the pulse is layered: there are quicker pulses, like running water, and slow ones, like deep wells:

And some music has no regular pulse at all:

So, to quote a Duke Ellington song title, it isn't quite true that "It Don't Mean a Thing (If It Ain't Got That Swing)".