Different types of noise maker


Sirens:  A siren is a loud noise maker. The original version would yield sounds under water, suggesting a link with the sirens of Greek mythology. Most modern ones are civil defense or "air raid" sirens, tornado sirens, or the sirens on emergency service vehicles such as ambulances, police cars and fire trucks. There are two general types, pneumatic and electronic. The device was invented by the Scottish natural philosopher (physicist) John Robison. It was improved and given its name by Charles Cagniard de la Tour. The most common types of sirens include the Federal Signal Model 1, Model 2, Model 5, 3T22, Thunderbolt 1003, STH10, STL-10, and the Sterling (now Sentry) siren Model M.

Pneumatic sirens:  The pneumatic siren, which is a free aerophone, consists of a rotating disk with holes in it (called a siren disk or rotor), such that the material between the holes interrupts a flow of air from fixed holes on the outside of the unit (called a stator). As the holes in the rotating disk alternately prevent and allow air to flow it results in alternating compressed and rarefied air pressure, i.e. sound. Such sirens can consume large amounts of energy.

Mechanical sirens:  A mechanical siren uses a rotor and stator to "chop" an air stream, which is forced through the siren by radial vanes in the spinning rotor. An example of this type of siren is the The Federal Signal "2T22", which was originally developed during the Cold War and produced from the early 1950s to the late 1980s. This particular design employs dual rotors and stators to sound each pitch. Because the sound power output of this type of siren is the same in every direction at all times, it is described as omnidirectional. The Federal 2T22 was also marketed in a 3-signal configuration known as the Federal 3T22, which had capabilities for a "hi-lo" signal. While some mechanical sirens produce sound in all directions simultaneously, other designs produce sound in only one direction, while employing a rotator mechanism to turn the siren head throughout 360 degrees. One such siren is the American Signal "Allertor". This siren also produces two pitches simultaneously in a musical interval, but in this case the rotor and stator incorporate separate sections for producing each pitch. An example of such a siren being produced today is the Federal Signal Model 2001 series. Introduced in the late 80s, it is capable of battery backup. There are two separate motors in the 2001. One powers the siren rotor, while another powers the rotator mechanism.

Electronic sirens:  Electronic sirens incorporate circuits such as oscillators, modulators, and amplifiers to synthesize a selected siren tone (wail, yelp, pierce/priority/phaser, hi-lo, scan, airhorn and a few more) which is played through external speakers. It is not unusual, especially in the case of modern fire engines, to see an emergency vehicle equipped with both types of sirens. Electronic sirens  consist of an electronic tone generator, a high-power amplifier, and a horn speaker typically incorporating one or a multiple of electrodynamic transducers. Typically the loudspeaker unit incorporates horn loading, which causes them to be similar in appearance to some supercharged electromechanical sirens. For siren applications, high-fidelity sound is a secondary concern to high output, and siren drivers typically produce large amounts of distortion which would not be tolerable in an audio system where fidelity is important. As with electromechanical sirens, there are both omni-directional and rotating categories. Also, these sirens can be set to rotate any amount from 0 to 360 degrees, allowing sirens to broadcast only in (a) certain direction(s).

"Supercharged" electromechanical sirens:  The Federal Signal Thunderbolt series is the most recognizable of all warning sirens due to its unique shape and design. It creates a very distinct tone, made specifically to get the attention of people. "Thunderbolt" sirens use a separate blower to force air through the rotor and produce greater air movement with each pulse, thus they are described as supercharged. Specially-designed horns having an exponential profile amplify the sound, causing the air at the end of the horn to be displaced the same distance as air in the throat of the horn with the passage of each wavefront. This lends a unique "distorted" character to the sound of these sirens as the throat of the horn is overloaded. Within the Thunderbolt product line, three different configurations were offered. The Thunderbolt 1000 is a single tone siren, and the Thunderbolt 1000T is a dual tone siren. The Thunderbolt 1003 is essentially the same as the Thunderbolt 1000T, except that it employs solenoid-actuated slide valves to create a "hi-lo" signal.A variation on the 'Supercharged' electromechanical siren is the pneumatic.

An electromechanical siren (a German E57)                                  

Fig. Electromechanical siren                                 Fig. Electronic siren


Electronic chimes:  An electronic chime for generating an aural tone comprising an oscillator, an input signal generator for the oscillator, the input signal generated thereby decaying from an initial value to a lower value. The oscillator includes a signal processor (i.e., an amplifier) with a feedback frequency generator connected to the input of the amplifier and being responsive to the output of the amplifier for impressing a desired frequency on the input signal to the amplifier. A speaker and a driver for the speaker are provided, the latter being responsive to the output of the oscillator so as to sound a chime-like aural signal whose amplitude (volume) decays at a predetermined rate while the frequency of the signal remains substantially constant.

                     An exponentially decaying chime sound is generated with a single tri-statable output pin from a microprocessor which is implemented using a single digital output pin line to create states of low, high and disconnect which are input to a control circuit for output to a speaker. Preferably, the control circuit comprises a low pass filter in series with a buffer. Preferably, the buffer is a Darlington transistor. Preferably, the speaker is an electro-mechanical device, alternatively it may be a piezo-resistive device. A method is disclosed for generating a desired audible chime signal with a chime generating circuit regulating a tri-statable output via a single pin of a microprocessor to approximately generate a desired signal frequency, and preferably includes regulating said tri-statable output to produce amplitude decay of the desired signal frequency over time.

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