Falling Whistles
Falling Whistles sells whistles for $34 to $104 to raise money for education, advocacy, and the rehabilitation of war-affected people in Congo. The organization got its name from a journal written by its founder, sharing his experiences during a trip to eastern Congo in 2008. During that trip, Falling Whistles' founder met a group of five former child soldiers who told him that some children who were too young to carry weapons were sent into battle armed only with whistles. By wearing whistles, the campaign aims to use this weapon as a tool of protest and to raise awareness of the situation in Congo. As of September 2010, $500,000 has been raised.
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.