Compression Horn

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March 1st, 2010 admin Leave a comment Go to comments

Compression Horn

The history of police siren

The word siren first originated in Greek mythology and was also later used to refer to mermaids. Language and literature have used the word siren as indicative of dangerous temptations. The siren as we understand today is not something dangerous, but is generally a warning signal either to stop or proceed, based upon who has used it. So, though the siren is in itself not dangerous, ignoring a siren, especially a police siren, can have dangerous consequences.

The first models of sirens developed during the early 19th century consisted of a pneumatic tube and two disks. The disks were perforated and one of the disks rotated. As the air flowed from the tube to the fixed disk and the other disk rotated, a sound was produced by the interference in the airflow. Later sirens used two slotted cylinders, placed concentrically, with the inner cylinder rotating and the outer remaining stationary.

The tone was created by the interruption of airflow as it passed through the slots. All types of modern sirens, including a police siren, are modifications of this basic pattern.

After the advent of electricity, the compressed air ceased to be a necessity for a siren. The cylinders continued and inside the inner cylinder a centrifugal fan was fitted which produced the necessary airflow. Then to make the sound output more efficient a horn was fitted to the unit. The horn has the ability to transform the sound waves of the siren to the type of sound waves to be released to the outside. Different types of horns and sounds are available for a present day police siren.

The two basic types of sirens are pneumatic sirens and electronic sirens. All conventional sirens were pneumatic sirens and its energy requirements were much more compared to present day electronic sirens. Pneumatic sirens are also known as mechanical sirens. In electronic sirens different types of sounds are synthesized with the combined action of sound modulators, oscillators, and amplifiers. A police siren today is mostly an electronic siren though some police vehicles may be fitted with both a pneumatic siren and an electronic siren.

In some European countries an in-between type of siren known as electro-mechanic siren is also in use. This type of siren will typically have two air horn sets of higher and lower pitches. The air compressor switches between the two sets so that the siren sound alters between high and low. The two tone hi-lo sound it produces is a common sound pattern for a siren. Besides these, a police siren sometimes uses a tri-tone as well. The power consumption of an electro-mechanic siren is less compared a mechanical siren, but its maintenance is not easy.

Though the hi-lo sound is frequently used in a police siren, the most frequently used police siren sounds are wail and yelp. The long dragging sound that typically sounds like a wail is called a wail. And the faster sound that is more akin to a yelp or a bark is called the yelp. Besides these, a modern police siren has many more sound choices, including those, which the operator can manually produce.

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