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A whistle is an instrument which produces sound from a stream of gas, most commonly air. It may be mouth-operated, or powered by air pressure, steam, or other means. Whistles vary in size from a small slide whistle or nose flute type to a large multi-piped church organ.

Whistles have been around since early humans first carved out a gourd or branch and found they could make sound with it. In prehistoric Egypt, small shells were used as whistles. Many present day wind instruments are inheritors of these early whistles. With the rise of more mechanical power, other forms of whistles have been developed.

One characteristic of a whistle is that it creates a pure, or nearly pure, tone. The conversion of flow energy to sound comes from an interaction between a solid material and a fluid stream. The forces in some whistles are sufficient to set the solid material in motion. Classic examples are Aeolian tones that result in galloping power lines, or the Tacoma Narrows Bridge (the so-called "Galloping Gertie" of popular media). Other examples are circular disks set into vibration.

Depending on the geometry, there are two basic types of whistles: those that generate sound through oscillations of fluid mass flow, and those that generate sound through oscillations of the force applied to the surrounding medium.

Whistles made of bone or wood have been used for thousands of years.

Whistles were used by the Ancient Greeks to keep the stroke of galley slaves. The English used whistles during the Crusades to signal orders to archers. Boatswain pipes were also used in the age of sail aboard naval vessels to issue commands and salute dignitaries

Human whistling unaided by any instrument can be used for musical recreation or as a whistled language for communication over distances too great for articulate speech, among many other purposes. Musical instruments include the nose whistle or nose flute, the tin whistle and the slide whistle. Since a whistle produces a loud sound that carries over a great distance, whistles are useful for signalling. On ships, the boatswain's call is used to alert members of the crew. A dog whistle can be used to train a dog for hunting, herding, or other occupations. Industrial plants often use a steam whistle to signal shift changes or to give alarms of emergencies; steam locomotives were equipped with train whistles for warning and signalling. A small-scaled steam whistle is found on a whistling kettle, to alert the user that the water is boiling. Storage tanks may be equipped with a whistle vent which sounds continually as the tank is being filled; when the tank level covers the vent pipe, the whistle stops and the tank is full.

They also occur as accidental byproducts of fluid flow such as supersonic jets, cavity resonances, whistling telephone wires, and idling circular saws.

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