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
The number and variety of whistles created by humans is quite large, yet very little has been done to examine the physics of the process. There are three possible mechanisms: Helmholtz resonance, symmetric hole tone operation (monopole), or asymmetric edge tone operation (dipole).