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
An annular duct carries the fluid that creates the annular jet. The jet impinges on a sharp ended ring with two toroidal cavities on either side. In Levavasseur's patent, a structure is added downstream of the annular opening to act as a coupling horn to direct the sound. The sound generated is very intense. It appears that no scientific study has been done to elucidate the detailed feedback mechanisms of its operation, although it is clear that this whistle has Class I feedback mechanism, similar to the police whistle.