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
It is the ratio of the steady speed to the speed of sound. The number was named in honor of Ernst Mach who first studied (among other things) supersonic flow and shock waves.