NFPA 72-2010, Chapter 18 requires that audible appliances provide a minimum sound pressure level of 15dBA above the ambient noise level
or 5dBA above a maximum sound level lasting for at least 60 seconds, whichever is greater. In addition the Life Safety Code – 2010 edition
and the Americans with Disabilities Act (ADA) requires that visible appliances be installed to assist in the alarm notification of the hearing
impaired. Strobes must be placed in accordance with NFTA 72-2010, Chapter 4 requirements to ensure proper coverage while avoiding excessive
flash rates that may trigger a seizure with photosensitive epileptic prone individuals.
NFPA 72 also requires that all audible evacuation signals conform to the American National Standard Evacuation Signal, ANSI S3.41. This temporal code 3 signal must be synchronized within a notification zone. The temporal three code is only to be used when total evacuation of a building is to occur.
Bells may be used for fire alarm signals where their sound is distinctive and will not be confused with similar audible signals used for other purposes. Bells are normally operated by 12 or 24 volts DC (direct current) and may be of the single-stroke or vibration type connected in
Bells may be provided with 4-inch through 12-inch gongs (in 2-inch increments). The 6- and 10-inch sizes are the most commonly used.
Usually, bells with 4-inch gongs are reserved for use as trouble signals. Generally, the larger the diameter of the gongs the lower the frequency
and the louder the audible signal (expressed in terms of decibels [dB]).
Horns are provided for applications that require louder or more distinctive signals, or both. Horns may be operated by either alternate or direct current and may be connected in series or parallel. Care should be exercised to see that circuits are electrically compatible when powering
both types of appliances. Horns that are manufactured today are generally 12 or 24 VDC.
Horns are usually of the continuous vibrating or electronic type and may be used to provide either coded of non-coded audible alarm signals.
They may be of the surface, flush, semi-flush, single projector, double projector, or trumpet type.
In very noisy areas, resonating, air-powered or motor-driven horns are sometimes used because of their inherently higher decibel output. NFPA
72 stipulates that the sound pressure from a notification appliance may not exceed 110 dBA.
Speakers are frequently used as fire alarm signaling appliances. Since they reproduce electronic signals, they can be made to sound like any
mechanical signaling device and have the capability of reproducing unique sounds that are not practical on mechanical appliances. In addition,
they may be used to give live or recorded voice instructions. Speakers are either direct radiating cone type, or of the compression driver and horn
Speakers are generally operated from audio amplifiers delivering standard output line levels of 70.7 or 25 volt AC rms. The speakers are driven
by an electronic tone generator, microphone, or voice synthesizer and an electronic amplifier. Two types are in wide use:
Sirens usually are limited to outdoor applications but are sometimes used in extremely noisy indoor areas. Sirens are motor-driven or electronic
appliances and may be either alternating or direct current operated. They are not very practical for use as coded audible signals
Strobe lights operate on the energy discharge principle to produce a high intensity flash of short duration. These lights are very efficient.
The short bright flash is not only attention getting but is effective when general visibility is low. Strobe appliances come in a wide range of
light intensities and operating voltages. Repetition rates are not allowed to exceed two flashes per second nor less the one flash every second
throughout the listed voltage range of the appliance.
The audible and visible functions can be combined in one unit to produce both sound and light from a single appliance. For example, the
sounder can be a horn, bell, or speaker. The light is required to be a strobe with specific characteristics as described in Chapter 18 of the
2010 National Fire Alarm and Signaling Code. Advantages of the combined signals are:
• The visible signal localizes the particular audible alarm appliance that is operating.
• The visible signal produces a recognizable alarm when an ambient noise level may affect the audible signal.
• Persons having impaired hearing can see the visible portion of the alarm signals.
The combined signals are available in all voltages up to line voltage. Twenty-four volt dc units are the most prevalent. Polarized versions facilitate
line monitoring. Two or four-wire connected types permit application of either a common or separate power supply.