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Mayday is an emergency procedure word used internationally as a distress signal in voice-procedure radio communications.. It is used to signal a life-threatening emergency primarily by aviators and mariners, but in some countries local organizations such as firefighters, police forces, and transportation organizations also use the term.
A distress signal, also known as a distress call, is an internationally recognized means for obtaining help.Distress signals are communicated by transmitting radio signals, displaying a visually observable item or illumination, or making a sound audible from a distance.
An air horn consists of a flaring metal or plastic horn or trumpet (called the "bell") attached to a small air chamber containing a metal reed or diaphragm in the throat of the horn. Compressed air flows from an inlet line through a narrow opening past the reed or diaphragm, causing it to vibrate, which creates sound waves .
Diagram of a typical locomotive air horn power chamber, showing operation. Train horns are operated by compressed air, typically 125–140 psi (8.6–9.7 bar), and fed from a locomotive main air reservoir. When the engineer opens the horn valve, air flows through a supply line into the power chamber at the horn's base (diagram, right).
A fire truck uses an air horn to alert cars of its presence. Air horn - These devices force compressed air from the vehicle's air brake system against a diaphragm, creating a loud noise. Air horns used on emergency vehicles usually have a distinctive tone so they can easily be distinguished from other large vehicles, commanding urgency.
In Germany and some other European countries, the pneumatic two-tone (hi-lo) siren consists of two sets of air horns, one high pitched and the other low pitched. An air compressor blows the air into one set of horns, and then it automatically switches to the other set. As this back and forth switching occurs, the sound changes tones.
Whoever receives distress signals, should confirm and alert the emergency services. The distress signal can also be a sound, light, or visible signs: Audible signals: e.g. loud calling, whistles, yodeling (Deep tones are audible over a long distance.) Optical signals: e.g. flashlights, blinkers, sunlight mirrors, flares, flames
Initially, there were only two siren signals, similar to the signals used in Britain at the time: Red Alert, or Air Raid Warning signal, indicating an imminent threat, and All Clear, indicating the danger had passed. In 1944, the signal system was updated to four total signals: Air Warning – 30 seconds of continuous siren sounding. Used when ...