Whistle

An aerodynamic whistle (or call) is a simple aerophone, 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. There are many ways to create pure tones, but we restrict the descriptions here to what are called aerodynamic whistles. Strictly speaking, they are fluid mechanical whistles since they occur in gases, such as air or steam, as well as in liquids, such as water. The only difference between them is the fluid density and the sound speed.

The word aerodynamic whistle is used here since it is in common use. 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 (galloping Gertie). Other examples are circular disks set into vibration.

The whistles described in this article are in a subclass where only the fluid is in motion and there is no significant dependent motion of the interacting solid. 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.

Joseph Hudson set up J Hudson & Co in Birmingham, UK in 1870. With his younger brother James, he designed the 'Acme City' brass whistle. This became the first referee whistle used at association football matches during the 1878–79 Football Association Cup match between Nottingham Forest and Sheffield. Prior to the introduction of the whistle, handkerchiefs were used by the umpires to signal to the players.

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