A steam whistle

Whistle

A whistle or call is a simple aerophone, an instrument which produces sound from a stream of forced air. It may be mouth-operated, or powered by air pressure, steam, or other means...

 is a device used to produce sound

Sound

Sound is a mechanical wave that is an oscillation of pressure transmitted through a solid, liquid, or gas, composed of frequencies within the range of hearing and of a level sufficiently strong to be heard, or the sensation stimulated in organs of hearing by such vibrations.-Propagation of...

 with the aid of live steam

Live steam

Live steam is steam under pressure, obtained by heating water in a boiler. The steam is used to operate stationary or moving equipment.A live steam machine or device is one powered by steam, but the term is usually reserved for those that are replicas, scale models, toys, or otherwise used for...

, which acts as a vibrating system (compare to train horn

Train horn

Train horns are audible warning devices found on most diesel and electric locomotives. Their primary purpose is to alert persons and animals to the presence of a train, especially when approaching a grade crossing. They are also used for acknowledging signals given by railroad employees Train horns...

).
The whistle consists of the following main parts, as seen on the drawing: the whistle bell (1), the steam orifice or aperture (2), and the valve (9).

When the lever (10) is pulled, the valve opens and lets the steam escape through the orifice. The steam will alternately compress and rarefy in the bell, creating the sound.