A steam whistle is a device used to produce sound with the aid of live steam. Unlike a horn, the sounding mechanism of a whistle contains no moving parts (compare to train horn).
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. 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 style of whistling.
Uses of Steam Whistles
Steam whistles were often used in factories, and similar places to signal the start or end of a shift, etc. Railway locomotives, traction engines, and steam ships have traditionally been fitted with a whistle for warning and communication purposes.
The earliest use of steam whistles was as boiler low-water alarms in the 1700s and early 1800s. During the 1830s, whistles were adopted by railroads and steamship companies.
Steam whistles for use on locomotives have since been replaced by air horns.
An array of steam whistles arranged to play music is referred to as a calliope.
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.
- 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. 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).
- 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, Ultrawhistle, and Dynawhistle.
A whistle has a characteristic 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. Several factors that determine frequency are discussed below. These comments apply to whistles with a mouth area at least equal to the cross-sectional area of the whistle.
- Whistle Length – Frequency decreases as the length of the whistle is increased. Doubling of the effective length of a whistle reduces the frequency by about 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. The speed of sound in steam is 15936 inches per second,so a whistle of 15-inch length would have a resonant frequency near Middle-C: 15936/(4 x 15) = 266 Hz. Formulas are available to estimate the passive effective length of a whistle.
- Blowing Pressure – Frequency increases with blowing pressure, allowing a locomotive engineer to play a whistle like a musical instrument, using the valve to vary the flow of steam. The term for this was “quilling.” Industrial steam whistles typically were operated in the range of 100 to 300 pounds per square inch gauge pressure (psig) (0.7 - 2.1 megapascals, MPa), although some were constructed for use on pressures as high as 600 psig (4.1 MPa). All of these pressures are within the choked flow regime, where mass flow scales with upstream absolute pressure. Excessive pressure for a given whistle design will drive the whistle into an overblown mode, that is the whistle will sound a frequency that is three times the fundamental frequency.
- Whistle Scale – The more squat the whistle, the greater is the change in pitch with blowing pressure due to a lower Q factor. The pitch of a very squat whistle may rise several semitones as pressure is raised. Therefore a set of whistles may fail to track a musical chord as blowing pressure changes if each whistle is of a different scale. This is true of many antique whistles divided into a series of compartments of the same diameter but of different lengths. Some whistle designers minimized this problem by building resonant chambers of a similar scale.
- Mouth Vertical Length (“cut-up”) – Frequency of a plain whistle declines as the whistle bell is raised away from the steam source. If the cut-up of an organ whistle or single bell chime is raised (without raising the whistle ceiling), the effective chamber length is shortened. Shortening the chamber drives frequency up, but raising the cut-up drives frequency down. The resulting frequency (higher, lower, or unchanged) will be determined by whistle scale and by competition between the two drivers.
- Mouth Angle – The natural frequency of a whistle with a 360-degree mouth (that extends completely around the whistle circumference) is lower than that of a whistle of the same length and same mouth area but with a partially walled mouth, resembling an organ pipe. The walled mouth whistle is said to have a lesser effective length.
- Steam Aperture Width – Frequency rises as steam aperture width declines.
- Gas Composition – A whistle blown on steam has a frequency about 1.5 semitones higher than when blown on compressed air due to the greater density of the latter.
Sound Pressure Level
Whistle sound level varies with several factors:
- Blowing Pressure – Sound level increases as blowing pressure is raised.
- Whistle Scale – Sound level increases as whistle length/width ratio decreases. A halving of effective length may enable the whistle to tolerate a doubling of absolute pressure resulting in a quadrupling of sound level. Variable pitch whistles vary in both frequency and sound level as scale is changed. The sound level of a very squat 587 Hz whistle recorded at the Boot Hill annual whistle blow in 1994 measured 126 C-weighted decibels at 30 meters. A short six-inch diameter plain whistle sounded 113 dbC at 100 feet whereas a six-inch diameter “organ-pipe” design (much lower in frequency) tested under the same conditions sounded 110 dBC at 100 feet.
- Whistle Diameter – Sound level increases with whistle diameter, as the sound radiating area increases with diameter. Tests of a sample of 13 single-note whistles ranging in size from one-inch diameter to six-inch diameter showed a sound level increase with diameter of 15 dBC, or about six decibels for each doubling of diameter. A 20-inch diameter Ultrawhistle operating at 15 pounds per square inch gauge pressure (103.4 kilopascals) produced 124 dBC at 100 feet. It is unknown how the sound level of a toroidal whistle would compare to that of a high frequency conventional plain whistle of the same diameter. By comparison, a Bell-Chrysler air-raid siren generates 138 dBC at 100 feet. The sound level of a Levavasseur toroidal whistle is enhanced by about 10 decibels by a secondary cavity parallel to the resonant cavity, the former creating a vortex that augments the oscillations of the jet driving the whistle.
- Steam Aperture Width – If gas flow is restricted by the area of the steam aperture, widening the aperture will increase the sound level for a fixed blowing pressure. Enlarging the steam aperture can compensate for the loss of sound output if pressure is reduced. It has been known since at least the 1830s that whistles can be modified for low pressure operation and still achieve a high sound level. Data on the compensatory relationship between pressure and aperture size are scant, but tests on compressed air indicate that a halving of absolute pressure requires that the aperture size be at least doubled in width to maintain the original sound level.
- Steam Aperture Profile – Gas flow rate (and thus sound level) is set not only by aperture area, but also by aperture geometry. Friction and turbulence influence the flow rate, and are accounted for by a discharge coefficient. A discharge coefficient of 0.663 was measured for an eight-inch diameter whistle blown on compressed air.
- Mouth Vertical Length (“cut-up”) – The mouth length (cut-up) that provides the highest sound level at a fixed blowing pressure varies with whistle scale, and some makers of multi-tone whistles therefore cut a mouth height unique to the scale of each resonant chamber, maximizing sound output of the whistle. Other antique whistle makers commonly used a compromise mouth area of about 1.4x whistle cross-sectional area. If a plain whistle is driven to its maximum sound level with the mouth area set equal to the whistle cross-sectional area, the sound level may be increased significantly by further increasing the mouth area without raising the blowing pressure.
- Frequency and Distance – Sound pressure level decreases by half (six decibels) with each doubling of distance due to divergence from the source. This relationship is termed inverse proportional, often incorrectly described as the inverse square law; the latter applies to sound intensity, not sound pressure. Sound pressure level also decreases due to atmospheric absorption, which is strongly dependent upon frequency, lower frequencies traveling farthest. For example, a 1000 Hz whistle has an atmospheric attenuation coefficient one half that of a 2000 Hz whistle (calculated for 50 percent relative humidity at 20 degrees Celsius). This means that in addition to divergent sound dampening, there would be a loss of 0.5 decibel per 100 meters from the 1000 Hz whistle and 1.0 decibel per 100 meters for the 2000 Hz whistle. Additional factors affecting sound propagation include barriers and "ground effects."
The Loudest Whistle
Loudness is a subjective perception that is influenced by sound pressure level, sound duration, and sound frequency. High sound pressure level potential has been claimed for the whistles of Vladimir Gavreau, who tested whistles as large as 1.5 meter (59-inch) diameter (37 Hz). Also it has been claimed that the sound level of an Ultrawhistle would be significantly greater than that of a conventional whistle, however, tests of small Ultrawhistles have not supported this claim. A report of the sound level of a large Ultrawhistle did not include a side-by-side comparison of a conventional whistle of equal diameter with output maximized by adjustment of mouth area and scale. The supposed limitations of conventional whistles are based upon certain assumptions that underestimate their potential:(1) The maximum sound level of a conventional whistle cannot significantly exceed 110 dBC at 100 feet. (2) The sound level cannot be increased by raising the mouth area beyond the cross-sectional area. (3) Sound level cannot be significantly increased by increasing the width/length ratio beyond 0.33. (4) halving the aperture size requires a quadrupling of operating pressure. These assumptions are not consistent with references cited in the Whistle Acoustics section of this article.
Fagen, Edward A. (2001). The Engine's Moan: American Steam Whistles.
New Jersey: Astragal Press. x+277 pages.