Sirens are low frequency devices usually continuous rated and are electric motor driven. Air is pulled in through a multi bladed impeller and pushed out through radial vents. The combination of motor speed and the number of impeller blades, with the number and spacing of the radial outlets determine the frequency. The siren is used extensively for disaster warning but also has many other applications. These include, uses as fire alarms, anti-theft/security alarms, process control, time signalling, fault indication and machinery stop-start alarms. Most individual requirements are covered by sirens rated from 100 W to 7.5 KW, which have sound outputs from 95 dB to 145 dB at 1 metre distances and giving an audible signal over a range of 100 to 15000 metres.

Buzzers and Hooters
These are electro-mechanical devices where the diaphragm is deflected by an electro magnet. In general, hooters are
low cost and robust. Their applications include fire and security alarms in Industrial and Marine applications, truck horns, process control, time signalling, telephone signalling, boat horns and public vehicle attack alarms. Buzzers can be used with projectors to give more directional sound and are used as general industrial alarms, crane warning alarms, control panel warning alarms, industrial truck alarms and boat alarms.

Electronic Sounders
The electronic sounder is by far the most versatile device available and has many distinct advantages as far as the design engineer and contractor is concerned. Not least of these is the low current consumption and relatively high sound output, which make the electronic sounder ideal for use in conjunction with battery powered systems. Consequently, this type of sounder is also used extensively for fire and security alarm systems, as well as industrial mining, process control, marine and hazardous area applications.

The beacons can assist by adding additional visual attraction by means of a flashing, static or rotating format. These units are normally available in RED, AMBER, GREEN and BLUE, but other colours are available on request. The LED beacon can be set to flash at up to 120 flashes per minute or remain static.The Xenon flasher is available in all the colours and the
LED flasher usually only in RED or AMBER, other colours available on request. When beacons are used in conjunction with sirens or sounders, the unit can be connected to set the siren and beacon off simultaneously or with the siren sounding for a short period and the beacon flashing until the condition for alert has been rectified.

These are motor driven or coil driven devices mainly used for School alarms, Fire and Security alarms and time signalling. These units have high frequencies and low current consumption. Of particular interest to the contractor is that models have been designed to make installation very simple.