Sound is a complex parameter to measure and evaluate. Sound levels are measured using a meter that measures sound energy in decibels (dB) which is a non linear scale. For example, doubling the amount of sound energy in a room by replacing one bell with two would only increase the sound pressure measurement reading by 3 dB. Adding 100 times the sound energy would provide only a 20 dB sound level increase. To provide a 20 dB increase in sound level would require a substantial change in the number of signal devices, device power level, wire size circuit capacity, amplifiers and possibly battery power. Increasing the dB sound pressure level can significantly affect the cost of modifying the fire alarm or voice communications system.
Research on the audibility of sound suggests that sound pressure levels (SPL) of 10 to 15 dB above ambient sounds can be heard clearly. Sounds below this difference can still be heard but other characteristics of sound become more significant. The guide in the Appendix of ULC’s previous standard, Standard for the Installation of Fire Alarm Systems (CAN/ULC S524-91), suggested, as a rule of thumb, a 15 dB difference and a minimum level of 65 dB. (As a point of reference, 65 dB is about the level of conversation between two people.) Until the Ontario Building Code or the Ontario Fire Code are amended, the 65 dB guide is acceptable in almost all fire alarm installations and occupancies except where very high ambient sound levels exist such as noisy commercial or industrial locations.
Making Sound Level Measurements
Sound level measurements that are used in an audibility assessment should be made using a suitable instrument:
Most meters require the microphone of the meter to be held at an angle (usually about 70º) from the direction of the sound source and not pointed directly at the sound source. The owners manual should be consulted for this information.
To reduce sound reflections that may affect measurements, the meter should be held 0.5 metres away from the body, approximately 1.5 metres above the floor and at least 0.5 metres from hard reflecting surfaces. At least three measurements should be made in the area to establish an average reading.
The sound level measurements should be taken in the room that is furthest away from the suite door with all intervening doors closed. As the alarm is intended to be heard by occupants who are awake, the measurement need not be taken by the pillows at the head of the bed. The location where alarm sound levels are measured within the room should be the same as where ambient sound levels are measured.
Ambient Sound Level
Background or ambient sound levels are always present. Normal ambient sound levels vary between 30 to 50 dB with the majority in the 40 dB range. The accuracy of the alarm sound level can be affected by its variance from the ambient sound level. If the difference between the ambient sound level and the alarm sound level is less than 10 dB, the alarm signal reading measurement may be incorrect. Calculations or correction charts will be necessary to establish the correct level.
The normal background sound level or ambient level is usually difficult to establish but should not include temporary sound sources such as flushing toilets and heavy vehicles passing. Ambient sound level measurements should be taken at the same location as the alarm sound measurements and under the same conditions. All windows to the exterior and intervening doors should be closed and the Heating, Ventilation and Air Conditioning (HVAC) system operating.
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