Often we think of a tone as a “continuous” sound, but when you get down to it, a tone is any periodic oscillation of any kind: it could be a ruler slapping rapidly against an elementary-school desk (and as it slows down, you start hearing the separate slaps). But when any sound recurs quickly enough to pass the Flicker-Fusion Threshold, it stops being perceived as individual sounds and becomes a tone. This is the same reason a series of still images displayed quickly enough appear to move.
When you hear two tones simultaneously, you usually perceive them as a kind of new tone: an interval. If you take a tone of one frequency and combine it with a tone whose frequency is 3/2 times the first, you get what’s called a “perfect fifth.”
In the video above, we’re hearing nothing but a drum beat: a snare and what sounds like a rimshot or cowbell (the characteristics of those sounds, oddly enough, won’t matter too much for this experiment). What’s important is actually the rhythm: the ‘cowbell’ is recurring 3/2 times as frequently as the snare. Sound familiar?
As the drum beat speeds up fast enough to ‘become’ a tone (at around 40 seconds), we hear the perfect fifth interval just as though you were holding two keys on a synthesizer—even though it started out as ordinary drums.