A steam whistle is a device used to produce sound in the form of a whistle using pressurized steam, which creates, projects, and amplifies its sound by acting as a vibrating system.
Operation
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 how far the operator has opened the valve. Some locomotive engineers invented their own distinctive style of whistling.
Uses of steam whistles Steam whistles are often used on buildings such as factories, universities, 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.
Gallery
Railway whistles
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. It is said that George Stephenson invented his trumpet after an accident on the Leicester and Swannington Railway where a train hit either a cart, or a herd of cows, on a level crossing and there were calls for a better way of giving a warning. Although no-one was injured, the accident was deemed serious enough to warrant Stephenson's personal intervention. One account states that [driver] Weatherburn had 'mouthblown his horn' at the crossing in an attempt to prevent the accident, but that no attention had been paid to this audible warning, perhaps because it had not been heard.
Stephenson subsequently called a meeting of directors and accepted the suggestion of the company manager, Ashlin Bagster, that a horn or whistle which could be activated by steam should be constructed and fixed to the locomotives. Stephenson later visited a musical instrument maker on Duke Street in Leicester, who on Stephenson's instructions constructed a 'Steam Trumpet' which was tried out in the presence of the board of Directors ten days later.
Stephenson mounted the trumpet on the top of the boiler's steam dome, which delivers dry steam to the cylinders. The company went on to mount the device on its other locomotives
Locomotive steam trumpets were soon replaced by steam whistles. Air whistles were used on some diesel and electric locomotives, but these mostly employ air horns.
Music An array of steam whistles arranged to play music is referred to as a calliope.
In York, Pennsylvania, a variable pitch steam whistle at the New York Wire Company has been played annually on Christmas Eve since 1925 (except in 1986 and 2005) in what has come to be known as "York's Annual Steam Whistle Christmas Concert". On windy nights, area residents report hearing the concert as far as 12 to 15 miles away. The whistle, which is in the Guinness Book of World Records, was powered by an air compressor during the 2010 concert due to the costs of maintaining and running the boiler.
Lighthouse fog signals
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, until they were later replaced by other compressed air diaphragm or diaphone horns.
Types of whistles Plain whistle – an inverted cup mounted on a stem, as in the illustration above. In Europe, railway steam whistles were typically loud, shrill, single-note plain whistles. In the UK, locomotives were usually fitted with only one or two of these whistles, the latter having different tones and being controlled individually to allow more complex signalling. On railroads in Finland, two single-note whistles were used on every engine; one shrill, one of a lower tone. They were used for different signaling purposes. The Deutsche Reichsbahn of Germany introduced another whistle design in the 1920s called "Einheitspfeife", conceived as a single-note plain whistle which already had a very deep-pitched and loud sound, but if the whistle trigger is just pulled down half of its way an even lower tone like from a chime-whistle could also be caused. This whistle is the reason for the typical "long high - short low - short high" signal sound of steam locomotives in Germany. 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. In Scotland, the Caledonian Railway was noteworthy for its usage of the "hooter" whistle.
Whistle acoustics
Resonant frequency A whistle has a characteristic natural resonant frequency that can be detected by gently blowing human breath across the whistle rim, much as one might blow over the mouth of a bottle. The active sounding frequency (when the whistle is blown on steam) may differ from the natural frequency as discussed below. These comments apply to whistles with a mouth area at least equal to the cross-sectional area of the whistle.
Whistle length – The natural resonant frequency decreases as the length of the whistle is increased. Doubling the effective length of a whistle reduces the frequency by one half, assuming that the whistle cross-sectional area is uniform. A whistle is a quarter-wave generator, which means that a sound wave generated by a whistle is about four times the whistle length. If the speed of sound in the steam supplied to a whistle were 15936 inches per second, a pipe with a 15-inch effective length blowing its natural frequency would sound near middle C: 15936/(4 x 15) = 266 Hz. When a whistle is sounding its natural frequency, the effective length referred to here is somewhat longer than the physical length above the mouth if the whistle is of uniform cross-sectional area. That is, the vibrating length of the whistle includes some portion of the mouth. This effect (the “end correction”) is caused by the vibrating steam inside the whistle engaging vibration of some steam outside the enclosed pipe, where there is a transition from plane waves to spherical waves.Tohyama, M. (2011) Sound and Signals. Berlin: Springer-Verlag, 389 pp. Formulas are available to estimate the effective length of a whistle, but an accurate formula to predict sounding frequency would have to incorporate whistle length, scale, gas flow rate, mouth height, and mouth wall area (see below).