Fire Alarm Systems -- Types of Fire Alarm Systems -- System Design -- Control of Indicating Panels -- Detector Heads -- Manual Call Points -- Alarm Sounders -- Power Units -- Diversion Relays -- Self Contained Fire Alarms -- Wiring & Installation -- Inspection & Servicing -- Statutory Regulations

1. Dome bells – operating mechanism contained within the bell.
2. Bells with operating mechanism external to the bell.
3. Electronic solid state sounders with mono or multi tone output normally in the range of 800 – 1000 Hz.
4. Small sirens operating in the range of 1,200 – 1,700 Hz.
5. Sirens ranging widely in size from 0.17kw to 11kW generally operating in the frequency range of 400 – 800 Hz.
6. Horns operating in the range of 300 – 400 Hz and either motor or pneumatic operations.

The following figures gives a broad indication of the sound levels of the various alarm sounders. Also indicated are typical sound levels for various industrial and commercial situations. An alarm noise level of not less than 5 decibels above ambient should be provided in general areas for adequate audibility but in sleeping areas a minimum level in the order of 65 decibels and 75 decibels at a bed head to wake sleeping occupants. It should be noted that most dome hells are intended for use with flush conduit or wiring systems. For surface wiring or conduit installations surface mounting adaptor boxes are normally required. Generally bells, electronic sounders and small sirens are available for use with 6, 12, 24 and 48V d.c. Sirens are normally operated at mains voltage, single or three-phase depending on the motor rating. Outdoor sirens should be fitted with heaters and thermostats to protect against low temperature conditions. Public address systems may be used for alarms, and visual signals may be used in special circumstances.

Points to consider

• Operating voltage
• Installation with surface or flush conduit system
• Internal or weather-proof installations
• Bell gong size
• Colour
• Motor rating of sirens, note some sirens are rated for continuous duty whilst others are rated for limited time periods.
• Television interference suppression

Whisper at three feet = 30dB

Casual conversation at three feet = 65dB

Threshold of pain = 130dB