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
Whistles that generate sound through fluctuations of momentum or a force exerted on the surrounding medium are called dipole-like sources. The figure on the right is example of a small "rigid" sphere that is moving back and forth in a given direction. If the sphere is small with respect to the wavelength of the sound emitted, it can be called a point dipole. A force must be applied to the sphere in a specific direction to move it. The surrounding medium in the direction of motion is compressed to radiate sound, but the medium at right angles slides past the sphere and is not compressed.