Siren (alarm) - Wikipedia. This article is about the alarm device. For other uses, see Siren.
A siren is a loud noise making device. Civil defense sirens are mounted in fixed locations and used to warn of natural disasters or attacks. Sirens are used on emergency service vehicles such as ambulances, police cars and fire trucks. There are two general types: pneumatic and electronic.
The pneumatic siren, which is a free aerophone, consists of a rotating disk with holes in it (called a chopper, siren disk or rotor), such that the material between. 1940 Tacoma Narrows Bridge Slender, elegant and graceful, the Tacoma Narrows Bridge. Everything in the universe is weighable and measurable, although the matter from which everything is made exists in differing degrees of density. See all clips from Sound Waves: The Symphony of Physics.
Many fire sirens (used for calling the volunteer fire fighters) serve double duty as tornado or civil defense sirens, alerting an entire community of impending danger. Most fire sirens are either mounted on the roof of a fire station, or on a pole next to the fire station. Fire sirens can also be mounted on or near government buildings, on tall structures such as water towers, as well as in systems, where several sirens are distributed around a town for better sound coverage. Most fire sirens are single tone and mechanically driven by electric motors with a rotor attached to the shaft. Some newer sirens are electronically driven by speakers, though these are not as common.
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Fire sirens are often called . Although there is no standard signaling of fire sirens, some utilize codes to inform firefighters of the location of the fire. Civil defense sirens pulling double duty as a fire siren often can produce an alternating . Fire sirens are often blasted once a day at noon and are also called . Robison's siren consisted of a stopcock that opened and closed a pneumatic tube. The stopcock was apparently driven by the rotation of a wheel.
In 1. 81. 9 an improved siren was invented and named by Baron Charles Cagniard de la Tour. One disk was stationary, while the other disk rotated. The rotating disk periodically interrupted the flow of air through the fixed disk, producing a tone. The inner cylinder rotates while the outer one remains stationary.
As air under pressure flows out of the slots of the inner cylinder and then escapes through the slots of the outer cylinder, the flow is periodically interrupted, creating a tone. As electricity became available, the first fire sirens were manufactured.
In 1. 88. 6 French electrical engineer Gustave Trouv. Two early fire sirens were the Decot siren and Sterling Siren. Both started manufacturing fire sirens around 1. Many communities have since deactivated their fire sirens as pagers became available for fire department use. During the Second World War the British civil defence used a network of sirens to alert the general population to the immanence of an air raid.
A single tone denoted a raid. A series of tones was an . As the holes in the rotating disk alternately prevent and allow air to flow it results in alternating compressed and rarefied air pressure, i. Such sirens can consume large amounts of energy. To reduce the energy consumption without losing sound volume, some designs of pneumatic sirens are boosted by forcing compressed air from a tank that can be refilled by a low powered compressor through the siren disk.
In United States. English language usage, vehicular pneumatic sirens are sometimes referred to as mechanical or coaster sirens, to differentiate them from electronic devices. Mechanical sirens driven by an electric motor are often called . One example is the Q2. B siren sold by Federal Signal Corporation. Because of its high current draw (2. Its distinct tone of urgency, high sound pressure level (1.
B at 1. 0 feet) and square sound waves account for its effectiveness. 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. Its sound power varies, but could get as high as approximately 1.
B, depending on the compressor and the horns. Comparing with the mechanical sirens, it uses much less electricity but needs more maintenance.
In a pneumatic siren, the stator is the part which cuts off and reopens air as rotating blades of a chopper move past the port holes of the stator, generating sound. The pitch of the siren's sound is a function of the speed of the rotor and the number of holes in the stator. A siren with only one row of ports is called a single tone siren. A siren with two rows of ports is known as a dual tone siren. By placing a second stator over the main stator and attaching a solenoid to it, one can repeatedly close and open all of the stator ports thus creating a tone called a pulse.
If this is done while the siren is wailing (rather than sounding a steady tone) then it is called a pulse wail. By doing this separately over each row of ports on a dual tone siren, one can alternately sound each of the two tones back and forth, creating a tone known as Hi/Lo. If this is done while the siren is wailing, it is called a Hi/Lo wail. This equipment can also do pulse or pulse wail. The ports can be opened and closed to send Morse code.
A siren which can do both pulse and Morse code is known as a code siren. Electronic. It is not unusual, especially in the case of modern fire engines, to see an emergency vehicle equipped with both types of sirens. Often, police sirens also use the interval of a tritone to help draw attention. The first electronic siren that mimicked the sound of a mechanical siren was invented in 1. Motorola employees Ronald H. Chapman and Charles W.
These were common before fire sirens became widely available. Fire horns, large compressed air horns, also were and still are used as an alternative to a fire siren. Many fire horn systems were wired to fire pull boxes that were located around a town, and thus would .
For example, pull box number 2. In the days before telephones, this was the only way firefighters would know the location of a fire. The coded blasts were usually repeated several times. This technology was also applied to many steam whistles as well. Some fire sirens are fitted with brakes and dampers, enabling them to sound out codes as well.
These units tended to be unreliable, and are now uncommon. In music. 1. 92. 7), Hyperprism (1. Ionisation (1. 93. George Antheil's Ballet M. Henry Fillmore's . A variation of a siren, played on a keyboard, are the opening notes of the REO Speedwagon song .
Some Heavy Metal bands also use air raid type siren intros at the beginning of their shows. Vehicle- mounted. For example, in California, sirens are designated Class A or Class B. A Class A siren is loud enough that it can be mounted nearly anywhere on a vehicle. Class B sirens are not as loud and must be mounted on a plane parallel to the level roadway and parallel to the direction the vehicle travels when driving in a straight line. Sirens must also be approved by local agencies, in some cases. For example, the California Highway Patrol approves specific models for use on emergency vehicles in the state.
The approval is important because it ensures the devices perform adequately. Moreover, using unapproved devices could be a factor in determining fault if a collision occurs. The Society of Automotive Engineers, (SAE), Emergency Warning Lights and Devices committee oversees the SAE emergency vehicle lighting practices and the siren practice, J1. This practice was updated through cooperation between the SAE and NIST, the National Institute of Standards. Though this version remains quite similar to the California Title 1. Best practices. Vehicles with concealed sirens also tend to have high noise levels inside.
In some cases, concealed or poor installations produce noise levels which can permanently damage vehicle occupants' hearing. Siren speakers, or mechanical sirens, should always be mounted ahead of the passenger compartment. This reduces the noise for occupants and makes two- way radio and mobile telephone audio more intelligible during siren use. It also puts the sound where it will be useful. Studies in some agencies operating emergency vehicles show sound levels over 1. B(A) in the passenger compartment. In one study, a specific vehicle's engine sounds and the siren produced sound levels over 1.
B(A) in the passenger compartment. Research has shown that sirens mounted behind the engine grill or under the wheel arches produces less unwanted noise inside the passenger cabin and to the side and rear of the vehicle while maintaining noise levels to give adequate warnings.
Appropriate wiring and transient protection for engine control computers is a necessary part of an installation. Wiring should be similar in size to the wiring to the vehicle engine starter motor. Mechanical vehicle mounted devices usually have an electric brake, a solenoid that presses a friction pad against the siren rotor. When an emergency vehicle arrives on- scene or is cancelled en route, the operator can rapidly stop the siren. Multi- speaker electronic sirens often are alleged to have dead spots at certain angles to the vehicle's direction of travel. These are caused by phase differences. The sound coming from the speaker array can phase cancel in some situations.
This phase cancellation occurs at single frequencies, based upon the spacing of the speakers. These phase differences also account for increases, based upon the frequency and the speaker spacing.
However, sirens are designed to sweep the frequency of their sound output, typically, no less than one octave. This sweeping minimizes the effects of phase cancellation. The end result is that the average sound output from a dual speaker siren system is 3 d.
B greater than a single speaker system. See also. The dials allow the siren's frequency to be determined. During the 1. 9th century, sirens were among the few sources of sound having a known frequency. Hence they were used in research on hearing and sound.^See: John Robison, Encyclop.
Ernst Robel, Die Sirenen: Ein Beitrag zur Entwickelungsgeschichte der Akustik . Gaertners, 1. 89. Charles Cagniard de la Tour (1. For descriptions of Robison's and de la Tour's sirens, see: Robert T.