When you play middle-C on a piano, you are not just hearing the note middle-C. You’re also hearing E, G, another C an octave higher, the D and E above that, F#… You’re hearing several different notes sounding simultaneously, all part of a single note.
It’s called the Overtone Series (also referred to as the Harmonic Series). Its implications are far-reaching: In my list of notes above, notes that the first 3 overtones of the note C are C, E, and G. These three notes form a “C-chord.” C is the tonic note, E is the third, and G is the fifth. The overtone series forms the foundation of Western music’s concept of harmony and chord structures. In this example, the note C is referred to as the “fundamental” of the series.
Overtones are also part of a note’s “timbre” or tonal coloring. Timbre is an instrument’s sound. It’s why a D# on a clarinet sounds different from a D# on a guitar. (“formants,” the resonating aspects of an instrument, also play a part.)
All of this seems somewhat logical, but the fact remains that a single note played on a musical instrument is not a single note at all–it is many notes. Our ears resolve an entire overtone series into one, single tone. Not only that, if you were to play the entire overtone series of C simultaenously, but leave out the note C, our ears will still hear everything as “C.” It’s as if our ears fill in the gaps where they expect the fundamental to be. The notes E, G, D, G, F#… all imply “C” in the world of harmony.
Why? How? These are deceptively deep questions about the nature of sound and human evolution and, like all deep questions, don’t yet have an answer…