Whistle - Whistle Physics

Whistle Physics

The whistle works by causing the smooth flow of air to be split by a narrow blade, sometimes called a labium or windcutter, creating a turbulent vortex which causes the air to vibrate. By attaching a resonant chamber to the basic whistle, it may be tuned to a particular note and made louder. The length of the chamber typically defines the resonance frequency. A whistle may also contain a small light ball, usually called the pea, which rattles around inside, creating a chaotic vibrato effect that intensifies the sound. Japanese bird whistles use several small balls and are half filled with water in order to reproduce the sound of a bird song.

A steam whistle works the same way, but using steam as a source of pressure: such whistles can produce extremely high sound intensities.

Sometimes, unintentional whistles can be set up. A common one is the opened sunroof of a car: air passing over the top of the vehicle can, at certain speeds, strike the back edge of the sunroof, creating a very low frequency whistle which is resonated by the closed interior of the car. Since the sound frequency is infrasonic, around 4 Hz, the effect is very uncomfortable for occupants, who feel the vibration rather than hear it. Such low frequencies can induce nausea, headache, disorientation and dizziness. The effect can be prevented by opening a side window a few inches. Subsonic whistles have also been developed for use as non-lethal area-denial weapons for crowd control purposes, or to deliberately create a sense of uneasiness in an enemy.