Audible notification: horns, speakers and audibility

Ravi hands Sasha a sound level meter and a floor plan of Willowbank School's new addition. 'Before we finish the layout, I want the ambient noise in every space we're covering,' he says. 'The gym during a basketball game, the music room during band practice, the shop with the dust collector running, the library at its quietest. I can't decide how many horns we need, or how loud, until I know what they have to be heard over.' Audible notification is the part of the fire alarm system that most people experience. If it is too quiet, people in a noisy room don't hear it. If it is far too loud, it is painful, it drowns out instructions, and people cover their ears instead of moving. This lesson covers the appliances, the signals they make, and the rules for how loud they must be.

Audible appliances

Horns are the most common audible appliances: an electronic or electromechanical sounder that produces a harsh, broadband tone that cuts through noise. Many are multi-tone and multi-candela combination horn-strobes, with the tone and volume set by switches or jumpers at installation. Bells were the classic appliance in older systems and still appear in some buildings. Chimes are quieter, used in hospitals and other places where a loud signal would disturb patients and staff are trained to respond. Speakers are used in voice systems, where a pre-recorded or live message follows the alarm tone; they play the tone and the message, and each speaker has taps for different power settings. Voice systems and intelligibility are covered in FAT-310.

Audible appliances are listed to CAN/ULC-S525, and in-suite sounders and smoke alarm sounders have their own requirements. Installed appliances must be compatible with the panel and its signalling circuits, and their tone and pattern settings must match the building's system.

The evacuation signal

The evacuation signal in North America is the temporal-three pattern: three half-second tones, each followed by a half-second pause, then a one-and-a-half-second pause, repeated. It is standardized internationally so that the same sound means the same thing in every building. Every audible appliance on the system must produce it in synchrony, so that horns in the same space don't overlap into a confusing jumble; the panel or a sync module keeps them together. Older systems that ring a continuous bell or use a slow whoop may stay until they are replaced, but new and altered systems use the temporal pattern.

In two-stage systems, the first-stage alert signal is different from the evacuation signal, commonly a slower repeating tone, so staff recognize that an investigation is under way, and the building's fire safety plan explains what each sounds like. The National Building Code and CAN/ULC-S524 set the alert and alarm signal patterns.

Measuring sound in decibels

Sound level is measured in decibels, a logarithmic scale, so that each increase of 10 decibels is a tenfold increase in sound power and is heard as roughly twice as loud. The A-weighted scale, written dBA, adjusts the measurement to match the ear's sensitivity, which is lower for very low and very high frequencies. Quiet library: 35 to 45 dBA. Normal conversation at one metre: about 60 dBA. Busy school corridor: 65 to 75 dBA. Gym during a game: 80 to 90 dBA. Workshop with machinery: 85 dBA or more. Because decibels are logarithmic, two horns of equal level together are only 3 dB louder than one, not twice as loud, and doubling the distance from a horn in open space reduces its level by about 6 dB.

# Decibel arithmetic for horns (open space, illustrative)
import math
def combine(*levels):
    return 10 * math.log10(sum(10 ** (L / 10) for L in levels))
def at_distance(L_ref, d_ref, d):
    return L_ref - 20 * math.log10(d / d_ref)
print(f"two horns of 85 dBA together: {combine(85, 85):.1f} dBA")
print(f"one horn of 85 dBA with one of 75 dBA: {combine(85, 75):.1f} dBA")
for d in [3, 6, 12, 24]:
    print(f"horn rated 85 dBA at 3 m, at {d:3d} m: {at_distance(85, 3, d):5.1f} dBA")

two horns of 85 dBA together: 88.0 dBA
one horn of 85 dBA with one of 75 dBA: 85.4 dBA
horn rated 85 dBA at 3 m, at   3 m:  85.0 dBA
horn rated 85 dBA at 3 m, at   6 m:  79.0 dBA
horn rated 85 dBA at 3 m, at  12 m:  73.0 dBA
horn rated 85 dBA at 3 m, at  24 m:  66.9 dBA

The second line shows something technicians find surprising: adding a horn 10 dB quieter than the first barely changes the total. Coverage comes from placing horns where each space needs them, not from adding more horns in one place.

How loud is loud enough

The National Building Code sets the audibility requirement in Canada. The alarm signal must be clearly audible throughout the floor area, at a sound pressure level at least 10 dBA above the ambient noise level, and not less than 65 dBA. In sleeping rooms of residential occupancies, it must reach at least 75 dBA with the intervening doors closed. The code also limits the maximum level in normally occupied spaces so that the signal is not harmful. CAN/ULC-S524 sets how appliances are installed, and CAN/ULC-S537 sets how the levels are measured during verification: with a sound level meter, with all appliances sounding and doors in their normal positions.

Ambient noise is the normal background level when the space is in use, and it has to be measured or estimated with care. A gym measured empty at 45 dBA might reach 85 during a basketball game. A shop with the dust collector off is very different from one with it running. Designers use measured values where they can, published typical values otherwise, and choose the busiest normal condition, not the loudest freak event. Where the ambient is so high that the required level would exceed the maximum, such as above about 95 to 105 dBA, the standards allow visual signals to be used instead of or as well as audible ones, and some machines can be shut down on alarm.

# Required alarm level for each space (NBC: 10 dBA above ambient, at least 65 dBA)
spaces = [  # name, average ambient (dBA), maximum lasting 60 s or more (dBA)
    ("Library", 42, 50),
    ("Classroom", 55, 62),
    ("Corridor at class change", 72, 78),
    ("Gym during a game", 84, 92),
    ("Shop, dust collector on", 88, 91),
]
limit = 110
for name, avg, peak in spaces:
    need = max(65, avg + 10)
    note = "(high: plan visual signals as well)" if need >= 95 else ""
    print(f"{name:26s} ambient {avg} dBA -> alarm at least {need:.0f} dBA {note}")

Library                    ambient 42 dBA -> alarm at least 65 dBA 
Classroom                  ambient 55 dBA -> alarm at least 65 dBA 
Corridor at class change   ambient 72 dBA -> alarm at least 82 dBA 
Gym during a game          ambient 84 dBA -> alarm at least 94 dBA 
Shop, dust collector on    ambient 88 dBA -> alarm at least 98 dBA (high: plan visual signals as well)

Low-frequency signals for sleeping areas

Research on waking people has found that a low-frequency, square-wave tone around 520 Hz wakes sleeping people far more reliably than the high-pitched tones of traditional horns and smoke alarms, particularly for older adults, people with mild hearing loss, children and people who have been drinking. The 3,000 Hz tones of many older smoke alarms are near the top of the range where hearing declines first with age. As a result, codes and standards increasingly require low-frequency sounders in sleeping rooms: in hotel rooms, dormitories and, depending on the code, apartment suites. Low-frequency sounder bases can be fitted under some addressable smoke detectors, so that the detector and the sounder occupy one ceiling location.

In Canada, the requirements for low-frequency signals depend on the code edition the province has adopted and on CAN/ULC-S524 and the smoke alarm standard; designers check the edition in force. At Riverlight Tower, new suite sounders installed during the corridor renovation are low-frequency types, as the design specified.

Placing audible appliances

CAN/ULC-S524 requires wall-mounted audible appliances to be at least 2300 mm above the floor where the ceiling allows, and at least 150 mm below the ceiling, so that the sound spreads over people and furniture rather than being blocked. Appliances are spread through a space rather than grouped, so that the level is even and nobody stands next to a painfully loud horn while the far end is too quiet. Closed doors cut sound sharply, so rooms that are normally closed, such as offices and classrooms, often need their own appliances rather than relying on the corridor.

Horn settings matter. Many horns have high and low volume settings, often several decibels apart, and each choice changes the current the appliance draws, which affects the power supply and battery calculations covered in FAT-130. Speakers have taps for different powers, such as 1/4, 1/2, 1 and 2 watts, where each doubling of power adds about 3 dB. The settings chosen in design must be the settings installed, recorded on the drawings and checked at verification.

Measuring audibility in the field

Audibility is designed on paper but proven with a meter. During verification, the technician sounds all the appliances in the area, with doors in their normal positions and normal equipment running, and measures the level at representative points: the far corners of rooms, the middle of large spaces, the pillow position in sleeping rooms with the doors closed. The meter is set to A-weighting and to a response that captures the alarm's pulses fairly, as the standard specifies, and the readings are recorded on the floor plans. Where a reading falls short, the fix is usually to add or move an appliance, raise a tap setting, or reduce a noise source; turning every horn up to its highest setting can push spaces near the horns over the limit. Rooms that change use, such as a classroom converted to a music room, need new measurements, because their ambient noise has changed.

Sasha's meter readings at Riverlight Tower's renovated floors become part of the building's record. A year later, when a tenant complains that the alarm is too quiet in a corner suite, the record shows that the level met the requirement at verification, and the inspection finds that the tenant has installed a solid-core door with heavy weatherstripping between the living room and the bedroom. The fix is a sounder base under the bedroom's smoke alarm, tied to the building's alarm, and the record is updated.

Speakers in voice systems have their own audibility rules, because a message that is loud but garbled fails as surely as one that is too quiet. FAT-310 covers intelligibility. For tone signals, the audibility rules in this lesson apply to the tone that the speakers play before and between messages.