A sound system and intercom system for emergency purposes in different to a building occupant warning system. These systems are typically installed where the building satisfies one of the following;
- in a building with an effective height of more than 25m; or
- in a Class 3 buildings having a rise in storeys or more than 2 and used as a residential part of a school, or accommodation for the aged, children or people with disabilities; or
- in a Class 3 building used as a residential aged care building; or
- in a Class 9a building having a floor area of more than 1000m2 or a rise in storeys of more than 2; or
- in a Class 9b building used as a school and having a rise in storeys of more than 3, or used as a theatre, public hall or the like having a floor area of more than 1000m2 or a rise in storeys of more than 2.
Historically these systems were also described as an emergency warning & intercommunication system (EWIS). This term and the relevant Australian Standard AS2220 have however been deprecated in favour of a âsound system and intercom system for emergency purposesâ.
A sound system and intercom system for emergency purposes must be designed in Accordance with the requirements of Australian Standard AS1670.4 and AS2220
The control and indicating equipment that forms the âbrainsâ of an emergency warning system generally comprises the following components;
- Cabinet
- Primary Power Supply Unit & Battery Charger
- Secondary Power (Batteries)
- Control Electronics
- Illuminated Indicators
- Input Interface & Control
- Input Termination & Monitoring
- Output Termination & Monitoring
- Evacuation Zone Amplifiers
Each manufacturer of sound and intercom system for emergency purposes differentiates themselves from other manufactures by the features and layout of the user interface. The following illustration shows a typical interface with two distinct sets of controls;
- Sound System controls on the left; and
- Intercom System controls in the right.
The horizontal rows in this configuration illustrate an evacuation zone.
According to Australian Standard AS1851.4-2004, an evacuation zone is a subdivision of the premises that can be evacuated separately from any other subdivision.
This separation is achieved by providing one unique amplifier and speaker circuit for each evacuation zone. An amplifier for an evacuation zone is rated to provide a defined amount of power, measured in watts (w) to the speaker circuit.
Loudspeakers can be purchased with a two or three power settings, also measured in watts. The greater the number of speakers, the greater the power requirements (demand) on the amplifier.
Example:
An amplifier is rated at 10w there are 20 speakers fitted, each speaker is rated at 0.33w each. The combined speaker load is 20 speakers x 0.33w equals 6.6w load. This is within the available capacity of the 10w amplifier.
Typically loudspeakers are permanently fixed into position and distributed throughout the evacuation zone within a building typically at ceiling height in quantity and distribution to satisfy two criteria;
- a warning signal sound pressure level exceeding 10dB above ambient sound pressure level and not less than 65dB and not more than 105dB; and
- at all places within the evacuation zone where the ambient noise figures are less than 85dB the speech intelligibility shall be equal to or greater than 0.5 STI (speech transmission index)
At this point a little science is required to explain the terms used such as sound, sound pressure, sound pressure level, noise and intelligibility.
- Sound is the pressure variation caused by a sound wave.
- The magnitude of the variations of the air pressure from the static or normal air pressure is a measure of the sound pressure level, measured in decibels (dB).
- The number of cyclic pressure variations per second is the frequency of sound, measured in hertz (Hz).
- Noise is considered to be sound caused by an array of random activities of different sound pressure levels and frequencies.