Shepherd's whistle
They are also produced commercially. Simple whistles may cost less than 1 USD in bulk; expensive ones may be quite ornate. They are commonly made of plastic, aluminum, stainless steel, silver, brass, titanium, Corian, jade, buffalo horn and other materials.
Physics of whistles
In a whistle, the instability starts at some point in the three dimensional region and then moves along some path in that region as the local variables change. This makes the comprehensive understanding of whistle instability mechanisms very difficult.