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Steam whistle

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For the "Steam Whistle" beer brand, see Steam Whistle Brewing

A steam whistle is a device used to produce sound with the aid of live steam. Unlike a horn, a 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

A high-pitched plain whistle (left) and a low-pitched plain whistle (right).
3-bell multi-tone (chime) whistle sounds a musical chord.
Single-bell multi-tone (chime) whistle with compartments of differing length and pitch tuned to a musical chord.
6-note "step-top" multi-tone (chime) whistle with 6 compartments of differing length and pitch. The mouth of each chamber is partially walled.
A partial mouth whistle ("organ whistle") in which the mouth extends less than 360 degrees around the whistle circumference.
A "gong" chime whistle, two whistles aligned on the same axis.
A variable pitch whistle; note the internal piston used for adjusting pitch.

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 [1] in the 1700s [2] and early 1800s. [3] During the 1830s, whistles were adopted by railroads [4] and steamship companies.[5]

Steam whistles for use on locomotives have since been replaced by air horns.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 usch 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 whistle – a whistle with a wide column in the center so the working part of the whistle is a ring at the periphery, therefore the mouth (sound radiating area) is large in relation to the working area.

Whistle Acoustics

Resonant Frequency

A whistle has a characteristic frequency [6] 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. [7]
  • Blowing Pressure – Frequency increases with blowing pressure, [8] 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 pressure range of 100 to 300 psig, although some were constructed for use on pressures as high as 600 psig. All of these pressures are within the choked flow regime, where mass flow is proportional to upstream absolute pressure.
  • Whistle Scale – The more squat the whistle, the greater is the change in pitch with blowing pressure [9] due to a lower Q value. [10] The pitch of a very squat whistle may rise several semitones as pressure is raised. [11] A set of whistles of different scales may fail to track a musical chord as blowing pressure changes. 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. [12]
  • Mouth Vertical Length (“cut-up”) – Frequency declines as the whistle resonant chamber is raised away from the steam source, that is as the mouth is lengthened and the whistle ceiling is raised. [13]
  • 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. [14]
  • Steam Aperture Width – Frequency rises as steam aperture width declines. [15]
  • 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

Sound pressure level varies with several factors:

  • Blowing Pressure – Sound level increases as blowing pressure is raised. [16] [17]
  • Whistle Scale – Sound level increases as whistle length/width ratio decreases. A halving of length may require a doubling of absolute pressure to realize the sound potential of the whistle. [18] The sound level of a very squat single-note six-inch diameter whistle recorded at the Boot Hill annual whistle blow in 1992 measured 116 C-weighted decibels at 100 feet. A six-inch diameter “organ-pipe” design (about 6x as long as wide) tested elsewhere at about the same pressure sounded 110 dBC at 100 feet. [19]
  • Whistle Diameter – Sound level increases with whistle diameter, as the sound radiating area increases with diameter. [20] 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. [21] A 20-inch diameter toroidal whistle operating at 15 psig produced 124 dBC at 100 feet, eight decibels greater than the six-inch diameter conventional plain whistle mentioned under "Whistle Scale," above. [22] [23] 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 and gas consumption. By comparison to these whistles, a Bell-Chrysler air-raid siren generates 138 dbC at 100 feet. [24]
  • Steam Aperture Width – Sound pressure level increases as the steam aperture width is increased. [25] 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. [26] Data on the compensatory relationship between pressure and aperture size are scant, but tests indicate that a halving of psia requires that the aperture be at least doubled in width to maintain the original sound level. [27]
  • Mouth Vertical Length (“cut-up”) – The mouth length (cut-up) that provides the highest sound level varies with whistle scale. Some makers of multi-tone whistles thus cut a mouth height unique to the scale of each resonant chamber, maximizing sound output of the whistle. [28]. Antique whistle makers commonly used a mouth area of about 1.4x whistle cross-sectional area.


References

  1. ^ http://www.lincolnarchives.us/index.php?act=election1860/sciencetech&sub=invention01221859
  2. ^ Stuart, Robert (1829). Historical and Descriptive Anecdotes of Steam Engines and of their Inventors and Improvers, London: Wightman and Cramp, page 301.
  3. ^ Ommundsen, Peter (2007). Pre-1830 steam whistles. Horn and Whistle 117:14.
  4. ^ Wood, Nicholas (1838). A Practical Treatise on Railroads. London: Longman, Orme, Brown, Green and Longmans, page 340.
  5. ^ Pringle, R.E. and J. Parkes (1839). The causes and means of prevention of steam-boat accidents. Mechanics Magazine 31:262.
  6. ^ Liljencrants, Johan (2006).“End correction at a flue pipe mouth.”
  7. ^ Liljencrants, Johan (2006).“End correction at a flue pipe mouth.”
  8. ^ Ommundsen, Peter (2003). Effects of pressure on whistle frequency. Horn and Whistle 101:18.
  9. ^ Ommundsen, Peter (2003). Effects of pressure on whistle frequency. Horn and Whistle 101:18.
  10. ^ Liljencrants, Johan (2006).“Q value of a pipe resonator”
  11. ^ Ommundsen, Peter (2004). Whistle mouth area and lip height in relation to manifold pressure. Horn and Whistle 103:7-8.
  12. ^ Atchison, Topeka, and Santa Fe Railway 1925 engineering drawing, published 1984, Horn and Whistle 13:12-13.
  13. ^ Ommundsen, Peter (2007). Observations on whistle cut-up and frequency. Horn and Whistle 116:4-7.
  14. ^ Liljencrants, Johan (2006).[http://www.fonema.se/mouthcorr/mouthcorr.htm “End correction at a flue pipe mouth.”
  15. ^ Ommundsen, Peter (2007). Observations on whistle cut-up and frequency. Horn and Whistle 116:4-7.
  16. ^ Burrows, Lewis M. (1957). “Whistle Patent Number 2784693" United States Patent Office, column 5, lines 29-31.
  17. ^ Ommundsen, Peter (2005). Effect of slot width on whistle performance. Horn and Whistle 109:31-32.
  18. ^ "Whistle sound level examples"
  19. ^ Barry, Harry (2002). Sound levels of my whistles. Horn and Whistle 98:19.
  20. ^ Burrows, Lewis M. (1957). “Whistle Patent Number 2784693" United States Patent Office, column 5, lines 30-34.
  21. ^ Barry, Harry (2002). Sound levels of my whistles. Horn and Whistle 98:19.
  22. ^ Weisenberger, Richard (1983). The loudest whistle. Horn and Whistle 6:7-9.
  23. ^ U.S. Patent 4429656, Feb 7, 1984 "Toroidal Shaped Closed Chamber Whistle"
  24. ^ Carruthers, James A. (1984). More on loudest sounds. Horn and Whistle 10:6
  25. ^ Ommundsen, Peter (2005). Effect of slot width on whistle performance. Horn and Whistle 109:31-32.
  26. ^ Pringle, R.E. and J. Parkes (1839). The causes and means of prevention of steam-boat accidents. Mechanics Magazine 31:262.
  27. ^ Ommundsen, Peter (2007). Factors to consider in whistle slot width prescriptions. Horn and Whistle 115: 6-8.
  28. ^ Burrows, Lewis M. (1957). “Whistle Patent Number 2784693" United States Patent Office, column 5, lines 20-28.

Further Reading

Fagen, Edward A. (2001). The Engine's Moan: American Steam Whistles. New Jersey: Astragal Press. x+277 pages.

See also

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