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 diameter of the orifice and cavity is "d", the distance between orifice and cavity is "h", and the orifice pressure "P" was given in kilograms per meter squared. At the lower limit of "h" the second term disappears. In this case, the equation could have been reformatted in terms of the acoustical Strouhal number as shown in the second equation above. The characteristic speed "U" at the nozzle is the sound speed "c". It is interesting that the number is very close to that found by Strouhal for flow over a cylinder. There are two characteristic length scales. Nozzle diameter "d" characterizes the sound power while the separation distance "h" characterizes the frequency.