The harp simply consists of a number of strings stretched over a sound box. From what I can tell, there are as many theories on the best configuration as there are people who make them. Some sources suggest the use of strings of different diameter tuned to the same note, the second A below middle C, the fundamental, because this allows the chords to stretch to infinity, any fundamental would do. Others say use the same string tuned diatonically and every combination you can think of.

The problem of course is, that the sounds produced are not those of the natural vibration of the strings. It is quite different to the notes produced by plucking the strings. Material I have suggests that the sounds can span 5 octaves. Alas, I have not been able to achieve that. Certainly a couple of octaves are discernible though. So if one does not hear the vibration of the string, then it is clear that something strange is happening.

The method of sound generation is still not clearly understood. Kirchner, noticing that several notes may be heard from the one string, suggested that the string acted like a prism. Since then, there have been many theories propounded. Current thought by Etkin and others in the USA in 1956 suggests that there is an eddy system set up in the air stream as it passes the string. Beyond each string there is a system of eddies turning clock wise and anti-clockwise with a sinuous airflow between them. For a given air velocity the eddy system depends on the diameter of, and the distance between each string. The closer together, the more complex the relationship.
The distance between the strings is important. The shorter the vibrating length, the closer together the strings should be. The tension on the strings also has an effect.

So what does it sound like? The harp sings, hums and pulsates with the tones rising and falling with the natural variation in the strength and duration of each puff of wind. Because of this, the tones vary from one moment to the next. A fan will obviously make it work, but the sounds produced are monotonous. The fan however is very handy when it comes to tuning the instrument for best effect. One wonders how people ever managed without one. Because you need a few seconds of breeze to allow the sound to develop, an oscillating fan does not work.

How much wind does it need? 15 KPH over the strings has been suggested. In the absence of an anemometer I reckon this is a reasonable estimate. The harp is best mounted in a window, either vertically or horizontally and the window is then closed onto it. This causes a choke effect and the breeze pressing on the side of the building, speeds up as it passes through the narrowed opening. So even a fairly gentle breeze will work. The best way to test your window is to open it about 4 cm and hold your hand behind the opening when a breeze is blowing. If you can feel the air-stream jetting on your hand, it will probably work. I made a recording of the sounds produced by the natural wind, which I use at displays. People find it intriguing.

On the subject of materials, because the instrument is essentially a toy, more in the nature of a fancy wind chime, cost is an inhibiting factor. I use furniture grade veneered plywood for the sound box, also for the cowling, which directs the wind over the strings.
Aerodynamics do play a part in the operation of the instrument. Guitar strings are expensive when you use 16 of them, so fishing line is used with good results, the wind can't tell the difference. It looks and sounds quite good. You need a mono-filament string. The originators used catgut or wire. There is evidence to suggest that wound strings won't work as the winding apparently effects the airflow.

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