| • Science | • People | • Locations | • Timeline |
Sound is generated by having a motor drive a shaft at either end of which are mounted fans, one fan having a few more blades than the other. Around each fan is a housing with a number of cut slots to match the number of fan blades. The blades are designed to draw airAir is a name for the mixture of gases present in the Earth's atmosphere. Compressed air is often used in scuba diving as a shallow water breathing gas and to inflate buoyancy devices. Compressed air is also used as a source of energy for pneumatic tools. in at the end and force it out through the slots in the housing. Due to the design, the air output is cut on and off alternately thus producing the sound. Modern sirens can reach 140 dBThe decibel is a "dimensionless unit" (like percent) that is a measure of ratios on a logarithmic scale. Usually, it is ten times the base-10 logarithm of the ratio. It's not an SI unit, although the International Committee for Weights and Measures (BIPM) at 30 metreFor other uses of "metre" and "meter", see Metre (disambiguation). The metre is the basic unit of length in the International System of Units (SI: Systeme International d'Unites). It is defined as the length of the path travelled by light in absolute vacus (100 feet).
Newer "sirens" are far more sophisticated, with the ability to broadcast voiceThe word voice can mean: The human voice. A section of a choir or other musical ensemble that sings or plays the same part. The register of a line of counterpoint, including soprano, alto, tenor, bass. These terms come the section of the choir to which a messageMessage in its most general meaning is the object of communication. Depending on the context, the term may apply to both the information contents and its actual presentation. In the communications discipline, a message is information which is sent from as over large areas, depending on windFor the 1928 film, see The Wind. Wind in the most general sense, is the movement of air. It occurs at all scales, from local breezes generated by heating of land surfaces and lasting tens of minutes to global winds resulting from solar heating of the plans and noise.
A number of different sound forms could be created. During World War II for a "red warning" of approaching danger the siren would be run normally producing a tone that rose and fell regularly between one high and one low tone, corresponding to the number of blades on each fan and the speed at which they turned. A "white warning" (All Clear) was a single continuous tone. Sometimes there was a "take cover" warning for immediate danger, the power to the motor was cut for a moment at intervals to change the tone produced. Post WW II two further warnings were introduced for nuclear attack - "grey warning" indicated approaching fall out with a 2½ minute warning of long steady tones divided by equal periods of silence, the silence being created with a manual shutter. A "black warning", also for manual sirens, was either a Morse code 'D' (–··) or three quick tones, indicating imminent danger of fall out.