Whilst conducting alarm service checks, a constant query for our field team is “how loud should the building’s fire alarm be?” Building occupants will quite often be concerned that their alarms are too loud, or that they do not hear them at all.
So in regards to fire alarms, how loud is loud?
Some Context on Sound
Before we discuss the required noise a smoke alarm has to produce, we will give some context in regard to average noise levels and how loud a Decibel (dB) actually is.
We have some examples below;
Ambient noise of a library, bird calls, lowest limit of urban ambient sound
Conversation in restaurant, ambient office noise, background music
Average factory noise, car wash, propeller plane at 1000 ft, food blender
Outboard marine motor, lawn mower, motorcycle, farm tractor, jackhammer, garbage truck
So how loud does an alarm need to be?
AS 1670.4 2004 Fire detection warning control and intercom systems – System design installation and commissioning, states the following;
“The A-weighted sound pressure level during the ‘on’ phases of the audible warning signals shall exceed by a minimum of 10 decibels (dB) and the ambient sound pressure level averaged over a period of 60 seconds, shall not be less than 65 dB and not more than 105 dB. If the audible warning signals are intended to arouse sleeping occupants, the minimum A-weighted sound pressure level of the signal shall be 75 dB at the bedhead, with all doors closed. Where occupants, such as patients in hospital wards, must not be subject to possible stress imposed by loud noises, the sound pressure level and content shall be arranged to provide warning for the staff and minimize trauma.”
We break this down to mean;
- Alarms must be at least 65 dB
- Alarms must be at least 10 dB above ambient noise level
- Alarms must be no louder than 105 dB
- Alarms must be at least 75 dB to wake sleeping occupants (measured with the bedroom door shut)
- Alarms may be adjusted for special use such as hospitals where there is on site staff to hear the signal and begin evacuation
Why are the smoke alarms required to be so loud?
Fire alarm systems are designed to convey the signal to every occupant of the whole building even while they are sleeping. A very important factor for alarm sound is that it will be reduced significantly when it travels over distance and through walls. Approximately 6 dB is lost every time the alarm moves 10 feet (3.048 metres) away from the speaker. If a door is open, the sound level can be lost by 4-12 decibel, while shut, the loss will be increased to 10-24 decibel.
Given these factors, a fire alarm should be adjusted at the speaker so that it is quite loud in regard to the ambient noise levels for a building.
There are also smoke alarms designed for those who are deaf or hard of hearing. These alarms come with a strobe light and/or a vibrating pad that can be placed under the pillow. The light or pad can be interconnected with smoke alarms in different locations within the home; when one of the alarms senses smoke, all alarms will go off, which triggers the strobe to flash and vibrating pad to vibrate.
For any queries or to have your fire alarm systems tested and serviced please contact Fire Safe ANZ.