14.2 Horns, Strobes, Speakers, and Candela

Chapter 9 taught notification appliance circuits: Class A versus Class B, voltage drop, and the idea that strobes in one field of view must flash together. This section is the hardware on the end of that pair. Module 4 fire technology still expects you to know a horn from a speaker, a candela setting from a sound-pressure setting, public mode from private mode, and Temporal-3 fire coding from the commonly taught Temporal-4 carbon monoxide pattern.

Independent OpenExamPrep material is study help for the DOS Security or Fire Alarm Installer written exam. It is not a DOS course packet and it does not reprint NFPA 72. The Uniform Code still gets fire-alarm installation into New York law through the Building Code and Fire Code, which reference NFPA 72. ADA and ICC A117.1 show up because visible notification is how the system reaches people who are deaf or hard of hearing — the next section covers placement; this section covers the appliance itself.

Horns, strobes, and combinations

A horn is an audible appliance. On most protected-premises systems it is a 24 VDC, polarized device on a notification appliance circuit (NAC). A strobe is a visible appliance — a xenon or LED flasher listed for fire-alarm use. Public-mode visible appliances are commonly discussed under UL 1971. Audible fire-alarm appliances are commonly discussed under UL 464. A horn-strobe puts both functions in one housing so you do not mount two boxes on the same wall.

The candela setting and the horn dB or tone setting are separate. Turning the horn to low does not change candela. A 15 cd strobe does not become a 75 cd strobe because the horn is loud. Markings matter: the appliance must be listed for fire-alarm notification, for the voltage, and for wall or ceiling mounting as actually installed. A security siren, a doorbell, or an unmarked “strobe light” from a parts bin is not a fire-alarm visible appliance.

Speakers and speaker-strobes

Emergency voice/alarm communications (in-building fire emergency voice) use speakers, not horns, so a live or recorded message can tell occupants what to do. Speakers typically ride a 25 VRMS or 70.7 VRMS audio circuit with a transformer tap selected in watts. That is not the same electrical system as a 24 VDC NAC. A speaker-strobe combines the speaker with a 24 VDC strobe; you land two systems on that can: the audio pair and the strobe pair (or a listed combination protocol the panel supports). Chapter 9 covered speaker-circuit supervision. Here, remember appliance identity: if the occupancy needs voice, a horn-strobe is the wrong catalog page even if it is loud.

Taps are not candela. A ¼-watt tap that is too quiet in a gym is an audible-coverage problem; it does not change the strobe setting on the same plate. Conversely, selecting 177 cd on the strobe does not raise the speaker watt tap.

Public mode versus private mode

Public mode notifies occupants of the building or area — the people who must evacuate or relocate. Commonly taught NFPA 72 audible practice for public mode is about 15 dB above the average ambient sound, or 5 dB above a maximum sound that lasts 60 seconds or more, measured at a standing ear height in the occupiable area, with a cap so the sound does not injure (commonly taught 110 dBA at the minimum hearing distance). Sleeping-area sound-pressure targets are commonly taught around 75 dBA at the pillow; the next section adds the 520 Hz low-frequency requirement that makes that pillow number actually wake people.

Private mode notifies people who are already responsible for the emergency plan — a nurses’ station, a constantly attended desk, a plant guard office. Private-mode audible levels are commonly taught as about 10 dB above ambient. Private-mode visible appliances may follow a different listing path than public-mode wall strobes. Healthcare and some industrial occupancies use private mode so you do not dump Temporal-3 into every patient room when staff should investigate first. The sequence of operations, not the technician’s preference, chooses the mode. Do not “turn it down because the owner hates noise” and call that private mode.

These dB figures are classroom NFPA 72 practice, not a New York Department of State sound table. Measure ambient on the job when the design requires it; do not guess 15 dB in a mechanical penthouse and 15 dB in a library as if they were the same room.

Candela ratings

Candela (cd) measures the intensity of the visible flash. It is not watts, not dBA, and not “how bright it looks on a phone camera.” Multi-candela appliances commonly list selectable settings of 15, 30, 75, 110, 135, and 177 candela. Those numbers are product settings you will see on System Sensor, Wheelock, Gentex, and similar listed appliances. They are not a New York statute and they are not a unique DOS exam table. The coverage a setting buys in a given room is a lookup in the adopted NFPA 72 — the next section teaches that as a method, with one labeled example, without reprinting the table.

Field-selectable wheels, candela keys, and DIP switches must match the approved drawings. Leaving a 177 cd appliance on 15 cd because it shipped that way is a failed acceptance test. Mixing a wall-listed appliance onto a ceiling, or a ceiling-listed appliance onto a wall, is the same class of error: the listing and the table assume a mounting. Some appliances show different effective intensities on wall versus ceiling; read the marking on that model.

Polarized NACs

Most 24 VDC horns and strobes are polarized. They contain a diode or equivalent so they draw alarm current only when the NAC is in the alarm polarity. Supervision commonly uses the opposite polarity plus an end-of-line device so appliances stay silent while the panel still sees the circuit. Land the plus conductor on the plus terminal. Reverse a single appliance and that appliance may sit dark in alarm while the rest of the circuit works — a commissioning bug that looks like a “dead strobe.”

Polarity is an appliance-landing skill. It does not replace Class A versus Class B topology from Chapter 9. Speakers on a 70.7 V audio circuit are not polarized NAC horns; do not series-diode a speaker transformer and call it NAC supervision.

Temporal-3 and Temporal-4

The public-mode fire evacuation sound is commonly taught as Temporal-3: three pulses, a pause, repeat. That pattern is associated with ISO 8201 and NFPA 72. It is a coded temporal pattern, not “any loud noise.” Continuous horns, security whoops, and doorbell chimes are not automatically the fire-evacuation signal even if they are wired to the FACU.

Temporal-4 is commonly taught as the carbon monoxide audible pattern — four pulses and a pause — so occupants and staff can tell CO from fire. That distinction is a life-safety teaching point. Panels do not magically convert a CO detector into Temporal-4 without the right appliance, the right output, and the right programming. If a CO detector is mapped onto the fire NAC and the building sounds Temporal-3, you have taught occupants the wrong emergency.

Synchronization in a field of view

Visible appliances that can be seen at the same time must flash together. Unsynchronized strobes in one field of view can appear to flash faster than a single listed appliance. That is both a nuisance and a photosensitive-epilepsy concern. Synchronization is a listed protocol: matching appliance families, a sync module, or a panel NAC that outputs that protocol. Two devices that both say “1 Hz” on the datasheet are not automatically synchronized with each other.

Chapter 9 introduced this as a circuit concept. On the wall, it is a matching-appliance concept. Do not mix incompatible strobe protocols in the same view and call it finished. Do not assume Temporal-3 on the horn automatically syncs the strobe; audible coding and visible sync are different functions that often share a housing.

Exam mix-ups to refuse

1. Treating candela as if New York printed a unique table of 18 / 36 / 54.
2. Using a horn where the design requires a speaker.
3. Landing a polarized strobe backwards and blaming the panel firmware.
4. Assuming Temporal-3 covers carbon monoxide.
5. Mixing strobe brands in one view without a listed sync path.

Scenario. A corridor has one manufacturer’s sync protocol on NAC 1 and a second manufacturer’s “1 Hz” addition in the same visual field. Both look busy on a phone camera. They are still not a listed synchronized field of view. Pick one protocol, add a listed sync source, or break the view — do not invent a New York exception.