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
This whistle has dozens of popular names. Pipes with sinusoidal variations of radius are often created to permit bending. Steady flow through the pipe at low Reynolds numbers results in a fluctuating volumetric flow rate that generates a monopole-like sound field at the pipe exit. Examples of such pipes are shown in figure on the right.