Wind Harp Physics

When you pluck the string of a regular harp, the string sounds its fundamental note. But when the wind blows across the string and you hear it, the note is much higher - it is a harmonic of the fundamental note.

Harmonics

The ancient Greek mathematician Pythagorus is credited with the first historical description of harmonics. He discovered that if a string was shortened to exactly half of its original size, the resulting tone would be an octave above the original note. If he took only half of the second string's length, the result would be another octave higher. Other divisions of a string will result in other interval relationships, like fifths and thirds. These related notes above the fundamental note are harmonic overtones.

When a string is plucked, we hear mainly the fundamental note, but in addition, the harmonic overtones are also present.

When the wind plays a string, it does not play the fundamental tone, but only the series of overtones. That is why the sounds are so high-pitched and fairy-like. On the giant wind harps, the sounds are lower and sound more mysterious than fairy-like because the strings are so long, but the tones are still harmonics of the fundamental pitch of the string.

Vortex-Shedding

Air and water are both considered fluids, and behave very similarly. (Of course, it's a lot easier to see the effects in water.) Wind does not always blow constantly or steadily. And when it bumps into objects, curls and eddies are created, just like water in a stream when it runs over or against rocks.

When the wind blows past a string, little spirals of air move to either side (the wind stream "sheds vortices"). These spirals or vortices form alternately and create an oscillation. The number of oscillations in a given time (a second is the amount of time generally used) is called the frequency.

When the wind is consistent over a long enough time and the vortex shedding occurs in a regularly repeating pattern, the string will sound IF the frequency of the oscillation matches the frequency of the string tuning. This depends on the speed of the wind.

Aeolian Harps and Wind Harps

An Aeolian harp is usually constructed as a rectangular sound box (the size of the box is determined by the width of the window in which it will be placed) with several strings of equal length, tuned to the same note. This means that only one wind speed will create vortices which match the frequency of the strings. The chances of the wind having exactly the right speed and coming from exactly the right direction to reach the window means that the Aeolian harp may not play very often.

However, Ron's wind harps are built like regular harps, with strings of varying lengths and notes, and the wind harps are placed outdoors in unobstructed places, so there is a wider range of wind directions, wind speeds and vortice frequencies that can cause strings to sound, and the chances are greater that these harps will play.