String players coax glassy, eerie harmonics by barely touching the string
Rest a fingertip lightly on a violin string at just the right spot, draw the bow, and a pure, silvery note floats out, higher than the open string and strangely hollow. Players call it a harmonic or flageolet, and the trick works by silencing every vibration that would have moved at that exact point.
In physics a harmonic is a sine wave whose frequency is a whole-number multiple of a basic one. The base itself counts as the first harmonic. Mains electricity at 50 Hz, for instance, has higher harmonics at 100, 150 and 200 Hz, and any blend of them still repeats 50 times a second. The same arithmetic governs a vibrating string, where each mode splits the string into equal segments separated by motionless nodes.
That is what makes the string player's technique possible. Touch the string gently at one third of its length and every mode that needs to move there is damped, leaving the third harmonic and its multiples to ring out. Harmonics made this way on an open string are called natural harmonics; an artificial harmonic uses two fingers, one stopping the string to set a new base and the other lightly touching the node above it. Natural harmonics beyond the fifth partial are rare except on the double bass, whose long strings yield more of them. Moving the bow towards the bridge can bring out a whole run of harmonics too, an effect called sul ponticello.
Wind players do something related when they overblow. By stopping the fundamental from sounding, a recorder player jumps up an octave, and brass and woodwinds routinely reach higher notes this way. Tubes closed at one end, such as clarinets and trumpets, in theory produce only the odd-numbered harmonics.
Not every instrument plays by these neat rules. Piano overtones run slightly sharp of true harmonics, and cymbals and drum heads throw off so many inharmonic partials that they suggest no definite pitch, which keeps them out of melodic work. Some composers treat harmonics as raw material: Arnold Dreyblatt draws different harmonics from the single string of a modified double bass with a bowing style between striking and stroking.
Source: Harmonic
