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A fire alarm notification appliance is an active fire protection component. A notification appliance may use audible, visible, or other stimuli to alert the occupants of a fire or other emergency condition requiring action. Audible appliances have been in use longer than any other method of notification. Most of today's appliances produce sound pressure levels between 45 and 120 decibels at ten feet. ... The primary function of the notification appliance is to alert persons at risk. Several methods are used and documented in industry specifications published by UL. Alerting methods include ... Sound (audible signals): (1) ~3 KHz / ~3100 Hz tone (high frequency). Used in many current notification devices. (2) 520 Hz (low frequency). Used in newer notification devices. (3) 45dB to 120dB A weighted for human hearing (higher decibels, in the 100 to 120dB range, were common with older fire alarm horns) [Fire alarm notification appliance. Wikipedia]