Siren (alarm)
Civil defense sirens are mounted in fixed locations and used to warn of natural disasters or attacks.
Most fire sirens are single tone and mechanically driven by electric motors with a rotor attached to the shaft.
Civil defense sirens also used as fire sirens often can produce an alternating "hi-lo" signal (similar to emergency vehicles in many European countries) as the fire signal, or attack (slow wail), typically 3x, as to not confuse the public with the standard civil defense signals of alert (steady tone) and fast wail (fast wavering tone).
In 1819, an improved siren was developed and named by Baron Charles Cagniard de la Tour.
[4] De la Tour's siren consisted of two perforated disks that were mounted coaxially at the outlet of a pneumatic tube.
[9] The earliest such sirens were developed during 1877–1880 by James Douglass and George Slight (1859–1934)[10] of Trinity House; the final version was first installed in 1887 at the Ailsa Craig lighthouse in Scotland's Firth of Clyde.
During the Second World War, the British civil defence used a network of sirens to alert the general population to the imminence of an air raid.
Because of its high current draw (100 amps when power is applied)[citation needed] its application is normally limited to fire apparatus, though it has seen increasing use on type IV ambulances and rescue-squad vehicles.
It is not unusual, especially in the case of modern fire engines, to see an emergency vehicle equipped with both types of sirens.
In many sirens, a centrifugal blower and rotor are integrated into a single piece of material, spun by an electric motor.
The frequency, i.e. the cycles per second of the sound of a mechanical siren is controlled by the speed of its rotor, and the number of openings.
The characteristic timbre or musical quality of a mechanical siren is caused because it is a triangle wave, when graphed as pressure over time.
Distant sirens sound more "mellow" or "warmer" because their harsh high frequencies are absorbed by nearby objects.
To achieve tempered ratios in a mechanical siren, the rotors must either be geared, run by different motors, or have very large numbers of openings.
A mechanical siren that can alternate between its tones uses solenoids to move rotary shutters that cut off the air supply to one rotor, then the other.
A variation of a siren, played on a keyboard, are the opening notes of the REO Speedwagon song "Ridin' the Storm Out".
Some heavy metal bands also use air raid type siren intros at the beginning of their shows.
[citation needed] The opening measure of Money City Maniacs 1998 by Canadian band Sloan uses multiple sirens overlapped.
For example, the California Highway Patrol approves specific models for use on emergency vehicles in the state.
Though this version remains quite similar to the California Title 13 standard for sound output at various angles, this updated practice enables an acoustic laboratory to test a dual speaker siren system for compliant sound output.
This reduces the noise for occupants and makes two-way radio and mobile telephone audio more intelligible during siren use.
[18] Research has shown that sirens mounted behind the engine grille 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.
[20] The worst installations are those where the siren sound is emitted above and slightly behind the vehicle occupants such as cases where a light-bar mounted speaker is used on a sedan or pickup.
In some cases, concealed or poor installations produce noise levels which can permanently damage vehicle occupants' hearing.
[21] Electric-motor-driven mechanical sirens may draw 50 to 200 amperes at 12 volts (DC) when spinning up to operating speed.
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.
Original: "Si l'on fait passer de l'eau dans la sirène, au lieu d'air, elle produit également le son, lors même qu'elle est entièrement immergée dans ce fluide, et les mêmes nombres de chocs produisent les mêmes nombre de notes par l'air.
C'est à cause de cette propriété d'être sonore dans l'eau, que j'ai cru pouvoir lui donner le nom sous lequel elle est désignée.
