Steam Whistles

A steam whistle is a device for making a [loud] noise, usually for warning or communications purposes, (or just for fun), using steam for the input. There are plain whistles, which produce a single note, and chime whistles where more than one whistle contained in the one assembly, creating a chord (3-note and 5-note are popular). Air whistles are very similar except compresssed air is used as the blowing medium. The same whistle will sound different when powered by air or steam due to their different characteristics of the gases. While there are many designs of whistles, all operate in a similar way with a gas (steam or air) flowing across a cavity which causes resonance of the air in the cavity volume. There are many factors which affect the frequency (tone) and loudness of a steam whistle.

In general, the Frequency of a whistle varies due to the following factors:-

Whistle Length - Frequency decreases as the length of the whistle is increased.
Blowing Pressure - Frequency increases with blowing pressure.
Mouth Vertical Length - Frequency decreases as the whistle bell is further away from the steam source.
Steam Aperture Width - Frequency increases as steam aperture width decreases.

and, the Sound level varies due to:-

Blowing Pressure - Sound level increases as blowing pressure is raised.
Whistle Diameter - Sound level increases with whistle diameter.
Steam Aperture Width - Sound level increases as the steam aperture width is increased.
Whistle Scale - Sound level increases as whistle length/width ratio decreases.

There are two main types of construction for whistles commonly used, the 'bell' type and the 'tube' type. See the illustrations below for typical constructions of each type.

Bell style whistle	The bell type whistle is where steam is fed end-wise around the entire periphery of the open end of the bell. The bell is a tube open at one end and closed at the other, and supported by a central rod or post. The bell is usually steadied by a 'spider' with fine fingers to ensure alignment of the circular steam oriface with the edge of the bell casing.
Tube (or organ) style whistle	The tube type whistle (organ whistle) is where steam is fed from a slotted oriface across an aperture cut into the side of the tube, and is usually a long whistle in relation to diameter, hence the name.

Tube and bell type whistles produce only a single-note. However, this note is quite complex with many overtones and steam noise on top, and is not just a simple plain note. The basic note can also be varied by the blowing pressure, and many locomotive drivers can make quite varying sounds by the way they operated the operating valve.

A Chime whistle is where two or more resonant bells or chambers that sound simultaneously, and a number of notes are produced, either from multiple cavities in the whistle, or from a number of individual whistles grouped and mounted together. Typically compact chime whistles with more than one whistle contained within, creating a chord, and 3-note, 5-note and 6-note whistles are popular.

A selection of chime whistles of different types.
Chime whistle with 3 tube whistles	Chime whistle with 3 bell whistles with a common feed	Bell chime whistle with 3 cavities in the bell	Bell chime whistle with 5 cavities in the bell
Bell chime whistle with 6 cavities in the bell

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