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A whistle is an instrument that sounds from an air stream, usually air. It can be operated by mouth or by air pressure, steam or other means to provide power. Whistles vary in size, from small sleek whistles or snuffs to large, multi barrel church organs.

Whistles have been used since early humans first carved gourds or branches and found that they can make sounds. In prehistoric Egypt, small shells were used as whistles. Many wind instruments today are heirs to these early whistles. With the development of greater mechanical power, other forms of whistles have been developed.

A characteristic of whistles is that they emit pure or near pure tones. The conversion of flow energy to sound comes from the interaction between solid materials and fluid flow. The force in some whistles is enough to move the solid material. Typical examples are the sandstorm tone or the Tacoma channel bridge (the so-called "gallop Getty" in popular media) that causes the power cord to gallop. Other examples are disks that vibrate.

Depending on the geometry, there are two basic types of whistles: one that generates sound by oscillating the mass flow of the fluid, and the other that generates sound by oscillating the force applied to the surrounding medium.

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

The whistle was used by the ancient Greeks to keep the kitchen slaves caressed. During the Crusades, the British whistled orders to the archers. In the era of naval ships, ship age management was also used to issue orders and pay homage to politicians

In addition to many other purposes, human whistles, which are not available on any musical instrument, can be used for musical entertainment, and can also be used as Whistler language for long-distance communication to avoid clear voice. Instruments include nose whistle or flute, tin whistle and sliding whistle. Whistles are useful for signaling because they make a lot of noise and travel a long distance. On board, the captain's phone is used to remind the crew. Dog whistle can be used to train dogs in hunting, herding or other occupations. Industrial factories often use sirens to signal changes in shifts or to send out emergency alarms. Steam locomotives are equipped with sirens for warning and signaling. A small-scale whistle was found on the whistle to remind the user that the water is boiling. The tank may be equipped with a whistle vent that will continue to sound when the tank is filled. When the tank level covers the exhaust pipe, the whistle stops and the tank is full.

They also appear as unexpected by-products of fluid flow, such as supersonic jets, cavity resonances, whistled telephone lines, and idling circular saws.