This is the standard barbershop version of Happy Birthday. You can listen to any combination of the four parts by clicking the numbered circles to turn parts on and off. Part 1 is the lead, or melody. Part 2 is the tenor, which is typically above the melody. Part 3 is the baritone, just below the melody, and Part 4 is the bass, which adds the bottom notes.
The trick with this implementation is that you can speed it up or slow it down and the four parts always stay in synch because all four parts use the timer to determine when each note is played. Another interesting feature is that the notes and rhythms are stored in lists, with the notes specified as offsets from the previous note.
