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
There are a number of whistles that possess the characteristics of both monopole and dipole sound sources. In several of the whistles described below, the driving source is dipole (generally an edge tone) and the responding source is a monopole (generally a tube or cavity in proximity to the dipole).