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
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.