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Dead medium: Mechanical Sirens and Foghorns
From: (Ron Bean)

Source(s): Encyclopaedia Brittanica, 15th edition

From Volume 10, page 844:

"Siren. Noisemaking device producing a piercing sound of definite pitch. Used as a warning signal, it was invented in the late 18th century by the Scottish natural philosopher John Robinson. The name was given it by the French engineer Charles Cagniard de LaTour, who devised an acoustical instrument of the type in 1819.

"A disk with evenly spaced holes around its edge is rotated at high speed, interrupting at regular intervals a jet of air directed at the holes. The resulting regular pulsations cause a sound wave in the surrounding air. The siren is thus classified as a free aerophone.

"The sound-wave frequency of its pitch equals the number of air puffs (or holes times the number of revolutions) per second. The strident sound results from the high number of overtones (harmonics) present."

Volume 26 page 378:

"Public Works: Lighthouses: Sound Signals:

"Compressed air. About the turn of the 20th century, compressed-air fog signals, which sounded a series of blasts, were developed. The most widely used were the siren and the diaphone. The siren consisted of a slotted rotor revolving inside a slotted stator that was located at the throat of a horn. The diaphone worked on the same principle but used a slotted piston reciprocating in a cylinder with matching ports.

"The largest diaphones could be heard under good conditions up to eight nautical miles away. Operating pressures were at 2 to 3 bars (200 to 300 kilopascals), and a large diaphone could consume more than 50 cubic feet (approximately 1.5 cubic meters) of air per second. This required a large and powerful compressing plant, 50 horsepower or more, with associated air-storage tanks.

"A later compressed-air signal was the tyfon. Employing a metal diaphragm vibrated by differential air pressure, it was more compact and efficient than its predecessors.

"Electricity. Modern fog signals are almost invariably electric. Like the tyfon, they employ a metal diaphragm, but in the electric signal they vibrate between the poles of an electromagnet that is energized by alternating current from an electronic power unit. Powers range from 25 watts to 4 kilowatts, with ranges from half a nautical mile to five nautical miles. Note frequencies lie between 300 and 400 hertz. Emitters can be stacked vertically, half a wavelength apart, in order to enhance the sound horizontally and reduce wasteful vertical dispersion."

Ron Bean (