The Technics of Foghorns
This page is under constructionA foghorn is a fog signal that uses sound to warn vessels of navigational hazards like rocky coastlines or of the presence of other vessels, in foggy conditions. The term 'fog horn' is most often used in relation to marine transport. When visual navigation aids such as lighthouses are obscured, foghorns provide an audible warning of rock outcrops, shoals, headlands, or other dangers to vesselping.
Sound signalsThe limitations of purely visual navigation very early led to the idea of supplementary audible warning in lighthouses. The first sound signals were explosive. At first cannon were used, and later explosive charges were attached to retractable booms above the lantern and detonated electrically. Sometimes the charges contained magnesium in order to provide an accompanying bright flare. Such signals could be heard up to four nautical miles away. Bells also were used, the striker being actuated by weight-driven clockwork or by a piston driven by compressed gas (usually carbon dioxide). Some bells were very large, weighing up to one ton.
Compressed airAbout the beginning 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 metres) 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.
ElectricityModern 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 750 hertz. Emitters can be stacked vertically, half a wavelength apart, in order to enhance the sound horizontally and reduce wasteful vertical dispersion.
Effective rangePropagation of sound in the open air is extremely accidentally, owing to the vagaries of atmospheric conditions. Wind direction, humidity, and turbulence all have an effect. Vertical wind and temperature gradients can bend the sound up or down; in the latter case it can be reflected off the sea, resulting in shadow zones of silence. The range of audibility of a sound signal is therefore extremely unpredictable. Also, it is difficult to determine with any precision the direction of a signal, especially from the bridge of a vessel in the fog. Contents 1 Description 2 History 2.1 Early fog signals 2.2 Mechanization 2.3 Diaphone 2.4 Obsolescence 3 Railway fog signals 4 See also 5 References 6 External links Description All foghorns use a vibrating column of air to create an audible tone, but the method of setting up this vibration differs. Some horns, like the Daboll trumpet, used vibrating plates or metal reeds, a similar principle to a modern electric car horn. Others used air forced through holes in a revolving cylinder or disk, in the same manner as a siren. Semi-automatic operation of foghorns was achieved by using a clockwork mechanism (or "coder") to sequentially open the valves admitting air to the horns; each horn was given its own timing characteristics to help mariners identify them. History Early fog signals An early form of fog signal. The fog bell at Fort Point Light Station, Maine. Audible fog signals have been used in one form or another for hundreds of years, initially simply bells or gongs struck manually. At some lighthouses, a small cannon was let off periodically to warn away vessels, but this had the obvious disadvantage of having to be fired manually throughout the whole period the fog persisted (which could be for several days). Lighthouse windows and lighting apparatus were susceptible to damage depending on the proximity of the explosion. One incident of lax handling of explosives nearby resulted in a concussion that propelled the lighthouse keeper at Fort Amherst, who was seated, to the other end of the room. In the United States, whistles were also used where a source of steam power was available, though Trinity House, the British lighthouse authority, did not employ them, preferring an explosive signal. Throughout the 19th century efforts were made to automate the signalling process. Trinity House eventually developed a system (the "Signal, Fog, Mk I") for firing a gun-cotton charge electrically. However, the charge had to be manually replaced after each signal. At Portland Bill, for example, which had a five-minute interval between fog-signals, this meant the horns had to be lowered, the two new charges inserted, and the horns raised again every five minutes during foggy periods. Clockwork systems were also developed for striking bells. Captain James William Newton claimed to have been the inventor of the fog signalling technique using loud and low notes. Mechanization Foghorns near Lizard Point, Cornwall. This installation uses a siren to produce sound. Another Trinity House fog siren installation on Flat Holm, now restored by the Flat Holm Project Bearing mechanism of Sumburgh lighthouse Foghorn (Shetland) The first automated steam-powered foghorn was invented by Robert Foulis, a Scotsman who emigrated to Saint John, New Brunswick, Canada. Foulis is said to have heard his daughter playing the piano in the distance on a foggy night, and noticed the low notes were more audible than the higher notes: he then designed a device to produce a low-frequency sound, as well as a code system for use with it. Foulis repeatedly presented his concept to the Commissioners of Light Houses for the Bay of Fundy for installation on Partridge Island. While the Commissioners initially rejected Foulis's plan, one commissioner eventually encouraged Foulis to submit detailed plans to the Commission. For reasons unknown, the plans were given to another Canadian engineer, T. T. Vernon Smith, who officially submitted them to the Commissioners as his own. The foghorn was constructed at Partridge Island in 1859 as the Vernon-Smith horn. After protest by Foulis and a legislative inquiry, Foulis was credited as the true inventor, but he never patented or profited from his invention. The development of fog signal technology continued apace at the end of the 19th century. During the same period an inventor, Celadon Leeds Daboll, developed a coal-powered foghorn called the Daboll trumpet for the American lighthouse service, though it was not universally adopted. A few Daboll trumpets remained in use until the mid-20th century. In the United Kingdom, experiments to develop more effective foghorns were carried out by John Tyndall and Lord Rayleigh, amongst others. The latter's ongoing research for Trinity House culminated in a design for a siren with a large trumpet designed to achieve maximum sound propagation (see reference for details of the Trials of Fog Signals), installed in Trevose Head Lighthouse, Cornwall in 1913. One of the first automated fog bells was the Stevens Automatic Bell Striker. Some later fog bells were placed under water, particularly in especially dangerous areas, so that their sound (which would be a predictable code, such as the number "23") would be carried further and reverberate through the vessel's hull. For example, this technique was used at White Shoal Light (Michigan). This was an earlier precursor to RACON. Diaphone Main article: Diaphone From the early 20th century an improved device called the diaphone, originally invented as an organ stop by Robert Hope-Jones, and developed as a fog signal by John Northey of Toronto, became the standard foghorn apparatus for new installations.[where?] Diaphones were powered by compressed air and could emit extremely powerful low-frequency notes. In 1982, the Dutch broadcaster VPRO aired a live foghorn concert on national radio, relaying the sound of the foghorns in Emden, Calais, Nieuwpoort, Scheveningen, Den Helder, Lelystad, Urk, Marken and Kornwerderzand, mixed with studio music by sound artist Alvin Curran.  Obsolescence Foghorn on Ailsa Craig, where the fog signal was discontinued in 1966. Since automation of lighthouses became common in the 1960s and 1970s, most older foghorn installations have been removed to avoid the need to run the complex machinery associated with them, and have been replaced with electrically powered diaphragm or compressed air horns. Activation is completely automated: a laser or photo beam is shot out to sea, and if the beam reflects back to the source (i.e. the laser beam is visible due to the fog), the sensor sends a signal to activate the foghorn. In many cases, modern navigational aids have rendered large, long-range foghorns completely unnecessary, according to the International Association of Lighthouse Authorities. FOG SIGNALS The function of a fog signal in the system of aids to navigation is to warn of danger and to provide the mariner with an audible means of approximating his position relative to the fog signal when the station, or any visual signal which it displays, is obscured from view by atmospheric conditions. Fog signals depend upon the transmission of sound through air. As aids to navigation, they have certain inherent defects that should be considered. Sound travels through the air in a variable and frequently unpredictable manner. It has been established that: fog signals are heard at greatly varying distances and that the distance at which a fog signal can be heard may vary with the bearing of the signal and may be different on different occasions; under certain conditions of atmosphere, when a fog signal has a combination of high and low tones, it is not unusual for one of the tones to be inaudible. In the case of sirens, which produce a varying tone, portions of the blast may not be heard; there are occasionally areas close to the signal in which it is wholly inaudible. This is particularly true when the fog signal is screened by intervening land or other obstructions; fog may exist a short distance from a station and not be observable from it, so that the signal may not be in operation; even though a fog signal may not be heard from the deck or bridge of a ship when the engines are in motion, it may be heard when the ship is stopped, or from a quiet position. Sometimes it may be heard from aloft though not on deck; the intensity of the sound emitted by a fog signal may be greater at a distance than in immediate proximity. All these considerations point to the necessity for the utmost caution when navigating near land in fog. Particular attention should be given to placing lookouts in positions in which the noises in the ship are least likely to interfere with hearing a fog signal. Fog signals are valuable as warnings, but the mariner should not place implicit reliance upon them in navigating his vessel. They should be considered solely as warning devices. Among the devices in common use as fog signals are: Radiobeacons which broadcast simple dot-and-dash combinations by means of a transmitter emitting modulated continuous waves; Diaphones which produce sound by means of a slotted reciprocating piston actuated by compressed air. Blasts may consist of two tones of different pitch, in which case the first part of the blast is high and the last of a low pitch. These alternate pitch signals are called “two-tone;” Diaphragm horns which produce sound by means of a diaphragm vibrated by compressed air, steam, or electricity. Duplex or triplex horn units of differing pitch produce a chime signal; Nautophones, electrically operated instruments, each comprising a vibrating diaphragm, fitted with a horn, which emits a high note similar in power and tone to that of the reed; Reed horns which produce sound by means of a steel reed vibrator by compressed air; Sirens which produce sound by means of either a disk or a cup-shaped rotor actuated by compressed air or electricity; Whistles which produce sound by compressed air emitted through a circumferential slot into a cylindrical bell chamber; Bells which are sounded by means of a hammer actuated by hand, wave motion, by a descending weight, compressed gas, or electricity; Guns and explosive signals which are produced by firing of explosive charges, the former being discharged from a gun, and the latter being exploded in midair; Fog Detector Lights—certain light stations, in addition to the main light, are equipped with fog detector lights for automatic detection of fog. These lights sweep back and forth through an area over which the fog watch is necessary, showing a powerful bluish-white flash of about 1 second in duration. The interval between successive flashes will vary with the position of the vessel within the sector. At the limits of the sector the duration of the flash may be considerably longer than 1 second. Fog detector lights operate continuously. Standby fog signals are sounded at some of the light and fog signal stations when the main fog signal is inoperative. Some of these standby fog signals are of a different type and characteristic than the main fog signal. Radiobeacons, RACONs, RAMARKs, and radio direction- finders are mentioned in the List of Lights, but for detailed information, including the synchronization of radio signals and sound signals for distance finding, the navigator should consult Pub. 117, Radio Navigational Aids. Note—use Chart No. 1 for the complete list of symbols and abbreviations commonly used in presenting the essential characteristics of lights, fog signals, and radio aids found on charts.