Yay! We've made it to the Harmonic Series!

Okay.  Let's get physical.  No, Olivia, not the song.  Let's talk string theory.  And I don't mean the tiny vibrating ones that many scientists believe make up the fabric of space-time.  I mean sound!*  Strings, like on a guitar or a cello, vibrate to make sound.  Sound is produced by moving the air molecules around the vibration.  These vibrations are picked up by the apparatus in our ears and vibrate at exactly the same frequency as whatever it is that's being heard.  These vibrations are relayed to the part of the brain that interprets vibration signals and our cortex allows us to decode what the signals mean.  Are we hearing music or just noise?  A Bach Cantata or a blender?

Before we get to the violin, though, visualize a jump rope.  One end is attached to something, or in the hand of a friend, and the other end is in your hand.  You begin swinging the rope.  The large arc that is created is a fundamental.  A fundamental is the lowest frequency that can be made, and referred to as the first harmonic.  Remember that a frequency is the number of sound waves that passes a given point within a given period of time.  It is possible to create a second harmonic of this fundamental by swinging one end of the rope faster.  This will create two smaller arcs that will rotate opposite each other.  We can theoretically take this further by increasing the speed of one end again, creating another (third) harmonic.

This isn't my image, but here is what it looks like:

Sort of, anyway.  You can imagine the harmonic as the section labelled "one oscillation".  But they are moving opposite each other.

Now the cool part.  Let's put away the rope and get out a stringed instrument.  Pluck the string of the violin, sitar, banjo, whatever, and you have produced not one, but several harmonics simultaneously.  That means all at once!  This is the essence of the harmonic series.  You know how I mentioned that instrument shape is only a part of the reason why instruments have unique sounds?  The bigger reason is the harmonic series.  Each instrument has its own harmonic series.  Certain harmonics are stronger or weaker, giving very characteristic sounds.  This is why a trumpet sounds like a trumpet and not like a flute.  The harmonic series is also a big part of how a brass instrument works, but I'll leave that for another time.


*With some help from the very interesting For the Love of Physics by Walter Lewin.  New York: Free Press.  2011.