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A whir is a sound made by an object rotating round its axis, usually with a fairly high speed. The word comes from the North Germanic word hwītjan, which means "to whine." The most common forms of whirring are those generated by electric motors or that produced when air circulates through an area cooled to its dew point. Whistling is also heard when liquid boils. In audio technology, a whirring sound is often an undesirable artifact, associated with a low-frequency rumble or hiss. Such sounds are often caused by minor variations in eccentric mechanism positions due to manufacturing tolerances. In film and video production, a whirring sound may be used as material for montage sequences.Whistling has been reported from as early as 1763, when it was referred to as "whistling in the head". Whistling began to appear widely in the 1860s after the development of sheet metal screws made it possible to mass-produce cheap whistles. These early whistles were bulky and required frequent clearing of the mechanism. Whistles are usually categorised by the number of pressure holes through which air enters the whistle, and by the type of whistle mechanism. The most common type of whistle uses a set of blades pivoted on a shaft to create chords which are struck against each other. This produces a penetrating tone which can be varied in pitch by opening or closing valves. Early military whistles were made with brass bodies and bronze blades, but modern whistles are made from metal that is less prone to corrosion. The majority of high-quality whistles have lead weights at both ends to maintain their pitch without any loss in volume if dropped. Whistles have been used for signaling since ancient times, both in emergencies and in battle. However, the whistles described here do not produce the simple tones described by the military standard signal codes. Whistles which give a musical sound are produced using a reed. Such whistles have many different shapes and sizes. The classic shape was made from copper but is now often made from stainless steel or plastic. Whistles which do not give a musical sound use a blade that is flapped against the inside of a pipe to create a noise similar to that produced by an owl, or may be simply shaped into a nozzle which directs the air in pulses rather than in distinct notes. The siren was first used as a warning device, but is now used as a musical instrument. The siren has undergone many changes over the years, developing from a simple buzzing to a modern electronic noise making machine. In recent years it has been found that many people who use sirens as musical instruments have no idea how they work or what features they have. In engineering applications, an emergency vehicle siren is an audible warning device fitted to emergency vehicles. Typically these are permanently mounted below the vehicle's bodywork and electronically connected to the vehicle's lighting and/or hazard warning lights. They can also be found on ambulances and other emergency vehicles which do not use lightbars. cfa1e77820
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