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Audible warning device powered by steam
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A steam whistle is a device used to produce sound in the form of a whistle using live steam, which creates, projects, and amplifies its sound by acting as a vibrating system.

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 actuated (usually via a pull cord), the valve opens and lets the steam escape through the orifice. The steam will alternately compress and rarefy in the bell, creating the sound. The pitch, or tone, is dependent on the length of the bell; and also how far the operator has opened the valve. Some locomotive engineers invented their own distinctive style of whistling.

Steam whistles were often used in factories and similar places to signal the start or end of a work shift, etc. Steam railway locomotives, traction engines, and steam ships have traditionally been fitted with a steam whistle for warning and communication purposes. Large diameter, low pitched steam whistles were used on light houses, likely beginning in the 1850s. The earliest use of steam whistles was as boiler low water alarms in the 18th century and early 19th century. During the 1830s, whistles were adopted by railroads and steamship companies.

Steam warning devices have been used on trains since 1833, when George Stephenson invented and patented a steam trumpet for use on the Leicester and Swannington Railway. Period literature makes a distinction between a steam trumpet and a steam whistle. A copy of the trumpet drawing signed May 1833 shows a device about eighteen inches high with an ever widening trumpet shape with a six inch diameter at its top or mouth.

Beginning in 1869, steam whistles began being installed at lighthouse stations as a way of warning mariners in periods of fog, when the lighthouse is not visible. 10" diameter whistles were used as fog signals throughout the United States for many years,
Chime whistle – two or more resonant bells or chambers that sound simultaneously. In America, railway steam whistles were typically compact chime whistles with more than one whistle contained within, creating a chord. In Australia the New South Wales Government Railways after the 1924 re classification many steam locomotives either had 5 chimes whistles that sound similar to the Star Brass 5 chime fitted (this include many locomotives from the pre 1924 re classification, or were built new with 5 chime whistles. 3 chimes (3 compact whistles within one) were very popular, as well as 5 chimes, and 6 chimes. In some cases chime whistles were used in Europe. Ships such as the Titanic were equipped with chimes consisting of three separate whistles (in the case of the Titanic the whistles measured 9, 12, and 15 inches diameter). The Japanese National Railways used a chime whistle that sounds like a very deep single note plain whistle, because the chords where just accessed in a simple parallel circuit if the whistle trigger is pulled down. Organ whistle – a whistle with mouths cut in the side, usually a long whistle in relation to diameter, hence the name. These whistle were very common on steamships, especially those manufactured in the UK. Gong – two whistles facing in opposite directions on a common axis. These were popular as factory whistles. Some were composed of three whistle chimes. Variable pitch whistle – a whistle containing an internal piston available for changing pitch. This whistle type could be made to sound like a siren or to play a melody. Often called a fire alarm whistle, wildcat whistle, or mocking bird whistle. Toroidal or Levavasseur whistle – a whistle with a torus shaped (doughnut shaped) resonant cavity paralleling the annular gas orifice, named after Robert Levavasseur, its inventor. Unlike a conventional whistle, the diameter (and sound level) of a ring shaped whistle can be increased without altering resonance chamber cross sectional area (preserving frequency), allowing construction of a very large diameter high frequency whistle. The frequency of a conventional whistle declines as diameter is increased. Other ring shaped whistles include the Hall Teichmann whistle, Graber whistle, Ultrawhistle, and Dynawhistle. Helmholtz whistle – a whistle with a cross sectional area exceeding that of the whistle bell opening, often shaped like a bottle or incandescent light bulb. The frequency of this whistle relative to its size is lower than that of a conventional whistle and therefore these whistles have found application in small gauge steam locomotives. Also termed a Bangham whistle. Hooter whistle a single note whistle of greater diameter with a longer bell, resulting in a deeper “hoot” sound when blown. These found use in rail, marine, and industrial applications. In the United States, the Norfolk and Western Railway made extensive use of these kinds of whistles and were noted for the squeaks and chirps produced when blown in addition to their low pitch.

Acoustic length or effective length is the quarter wavelength generated by the whistle. It is calculated as one quarter the ratio of speed of sound to the whistle's frequency. Acoustic length may differ from the whistle's physical length, also termed geometric length. depending upon mouth configuration, etc. but recently has been used for physical length including the mouth.

Loudness is a subjective perception that is influenced by sound pressure level, sound duration, and sound frequency. who tested whistles as large as 1.5 meter (59 inch) diameter (37 Hz). A 20 inch diameter ring shaped whistle (“Ultrawhistle”) patented and produced by Richard Weisenberger sounded 124 decibels at 100 feet. The variable pitch steam whistle at the New York Wire Company in York, Pennsylvania, was entered in the Guinness Book of World Records in 2002 as the loudest steam whistle on record at 124.1dBA from used by Guinness. The York whistle was also measured at 134.1 decibels from a distance of 23 feet. Other records of large whistles include an 1893 account of U. S. President Grover Cleveland activating the “largest steam whistle in the world,” said to be “five feet” at the Chicago World's Fair. The sounding chamber of a whistle installed at the 1924 Long Bell Lumber Company, Longview, Washington measured 16 inches diameter x 49 inches in length. The whistle bells of multi bell chimes used on ocean liners such as the RMS Titanic measured 9, 12, and 15 inches diameter. The whistle bells of the Canadian Pacific steamships Assiniboia and Keewatin measured 12 inches in diameter and that of the Keewatin measured 60 inches in length. A multi bell chime whistle installed at the Standard Sanitary Manufacturing Company in 1926 was composed of five separate whistle bells measuring 5 x15, 7 x 21, 8x 24, 10 x 30, and 12 x36 inches, all plumbed to a five inch steam pipe. The Union Water Meter Company of Worcester Massachusetts produced a composed of three bells, 8 x 9 3/4, 12 x 15, and 12 x 25 inches. Twelve inch diameter steam whistles were commonly used at light houses in the 19th century. It has been claimed that the sound level of an Ultrawhistle would be significantly greater than that of a conventional whistle, but comparative tests of large whistles have not been undertaken. Tests of small Ultrawhistles have not shown higher sound levels compared to conventional whistles of the same diameter.