Whistle
A
whistle is a one-note
woodwind instrument which produces
sound from a stream of forced air. (For the sound produced with the mouth, see
whistling and
whistled language.)
Many types exist,from small
police and
sports whistles (also called
pea whistles), to much larger
train whistles, which are
steam whistles specifically designed for use on
locomotives and
ships. Although whistles have a musical characteristic (for example train whistles sound a minor-seventh musical chord) whistles are not usually considered 'musical' in the sense of being able to play a chosen melody, but mainly the small whistles can also be used as a - very shrill and loud -
noise and
rhythm instrument. However, musical whistles exist, including any of several 2-
octave musical instruments known as
tin whistles (sometimes known as pennywhistles or low whistles), as well as the
calliope (an array of separately actuable steam whistles),
organ pipes and the
recorder. Pea whistles are used in
jazz and
Latin music as a
percussion instrument.
The whistle works by causing the smooth flow of air to be split by a narrow blade, sometimes called a
fipple, creating a
turbulent vortex which causes the air to vibrate. By attaching a
resonant chamber to the basic whistle, it may be tuned to a particular note and made louder. The length of the chamber typically defines the resonant
frequency. A whistle may also contain a small light ball, usually called the
pea, which rattles around inside, creating a
chaotic vibrato effect that intensifies the sound.
Japanese bird whistles use several small balls and are half filled with water in order to reproduce the sound of a
bird song.
A steam whistle works the same way, but using steam as a source of pressure: such whistles can produce extremely high sound intensities.
Sometimes, unintentional whistles can be set up. A common one is the opened
sunroof of a
car: air passing over the top of the vehicle can, at certain speeds, strike the back edge of the sunroof, creating a very low frequency whistle which is resonated by the closed interior of the car. Since the sound frequency is
infrasonic, around 4
Hz, the effect is very uncomfortable for occupants, who feel the vibration rather than hear it. Such low frequencies can induce
nausea,
headache,
disorientation and
dizziness. The effect can be prevented by opening a side window a few inches. Subsonic whistles have also been developed for use as weapons, or to deliberately create a sense of uneasiness in an enemy.
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Examples of police whistles |
Industrial whistles are used for
signalling and
timekeeping both on
railroad and
ships, and in
factories. Most of these whistles were
steam powered and not standardized. Individual
locomotives could be identified by their whistles. At noontime in industrial areas up into the
1950s whistles of every pitch could be heard, as each factory had a boiler and a whistle, if not full steam power.
Railroads in particular used elaborate whistle codes for communication both within the
train and with other trains. These methods are maintained today with motor-powered
air horns. Trucks also use air horns, especially since they often have air brakes and so there is already a source of compressed air on board.
Train whistles generally produce three or four different frequencies at the same time, to produce a non-
major chord, that is distinct, loud, and low in pitch.
Whistles are often used as warning devices or as safety devices serving to attract attention to the user. Some
cyclists use a whistle as a substitute for a
bell or
horn. It should be noted, however, that many jurisdictions require that the warning device be permanently attached to the
bicycle.
