How to Install an Alarm Siren and Check Its Output

Why this matters

A siren has two jobs: wake the people inside and tell everyone outside that something is wrong. A siren that cannot be heard in a bedroom with the door closed fails the first, and one an intruder can reach and rip off the wall in the first ten seconds fails the second. Both failures are invisible on the day of install, because every siren sounds loud when the tech stands under it. This card places the sirens for the job they do, sizes the wire so they still sound properly on battery, and closes with a measured output reading on the job record instead of "sounded fine".

Before you start

Siren type has to match the panel output. Panels drive sirens in one of two ways. A bell output delivers DC voltage to a self-contained siren, which makes its own tone and is polarized: plus to plus. A speaker driver output sends a tone signal to a plain speaker, usually rated in ohms. A self-contained siren on a speaker output, or a speaker on a bell output, either does not sound or damages the output. Read the panel manual for which it has and what it can drive.

The panel's bell output has a current rating. Every siren on it draws current, and the total must stay under the rating in the panel manual. On the job below, the manual rated the bell output at 1.0 A; that figure is this panel's, not a general one.

Put the account on test before the first sound. Sounding the sirens means an alarm, and opening the exterior siren's tamper sends a tamper signal. Put the account on test with the central station for the whole job, per How to Put an Alarm Account on Test and Take It Off.

The job this card follows

A two-story house, panel in the basement. Bedrooms upstairs off one hallway. One interior siren drawing 0.3 A and one exterior siren drawing 0.5 A per their spec sheets, both self-contained, both on the panel's bell output. The exterior siren has a cover tamper switch, wired to its own 24-hour tamper zone, zone 9.

Step 1: place the interior siren for the bedrooms

Mount it in the upstairs hallway the bedroom doors open onto, high on the wall or on the ceiling, in the open. Not in a closet, not inside the panel enclosure in the basement, and not in a stairwell two floors below. Sound falls off with distance and loses a great deal through each closed door, so a siren near the panel can be deafening in the basement and a murmur behind a bedroom door.

Why not the panel location: the panel is in the basement because that is where the wire home runs end, not because anyone sleeps there.

Step 2: place the exterior siren out of easy reach

Under the eave at the top of the wall, on the side of the house facing the street or the neighbors most likely to hear it, at a height that needs a ladder to reach. Mount it with the horn or opening facing down or as the maker shows, so rain cannot collect in it.

Ladder work at the eave. Set the ladder at about a 4-to-1 angle, footed or tied off, three points of contact, inspected before use and used only as designed, which is what 29 CFR 1910.23 asks of portable ladders in general-industry work (construction work falls under 1926 Subpart X instead). Keep the ladder and the drill cord clear of the house's overhead service drop through the whole climb. Before drilling siding, identify it: older cement-based siding can contain asbestos, and drilling it releases fibers into the air you are breathing at arm's length. If it may contain asbestos, mount on trim or soffit instead, or stop and have it identified.

Step 3: check the current budget

Interior 0.3 A plus exterior 0.5 A is 0.8 A, against this panel's 1.0 A bell output rating, which leaves 0.2 A of headroom. A third siren at 0.3 A would put the total at 1.1 A, over the rating, and would need a separate listed power supply or a siren driver module.

The sirens are also the biggest part of the panel's alarm current, and that alarm current is a line in the standby battery calculation. If you are adding a siren to an existing system, the battery worksheet changes; see related: How to Size a Standby Battery for an Alarm Panel.

Step 4: size the wire on the voltage at the far end

Siren runs carry far more current than sensor loops, and a thin wire drops enough voltage on a long run that the siren sounds weak, especially on battery when the panel's own voltage is already lower. The run that matters is the longest one at the highest current: here, the exterior siren, 65 ft of cable from panel to eave.

Voltage drop is current times the resistance of the round trip, out on one conductor and back on the other:

• Round trip: 65 ft x 2 = 130 ft.
• 18 AWG copper is about 6.39 ohms per 1,000 ft, so 130 ft is 0.130 x 6.39 = 0.83 ohms.
• At 0.5 A: 0.5 x 0.83 = 0.42 V lost in the cable.

The same run in 22 AWG, about 16.1 ohms per 1,000 ft, is 0.130 x 16.1 = 2.09 ohms, and 0.5 A through it drops 1.05 V, two and a half times as much. On a panel running on a battery that has sagged toward the end of an outage, that difference is the margin between a siren that still makes its rated output and one that does not. The interior run is 40 ft, so 80 ft round trip, 0.08 x 6.39 = 0.51 ohms, and at 0.3 A that drops 0.15 V.

Both runs went in 18 AWG two-conductor. The tamper loop carries almost no current and went in 22 AWG two-conductor. Which cable goes on which run is owned by Alarm Cable Types and Which Run Gets Which.

Step 5: the wiring

BELL OUTPUT (both pairs land on the same terminals,
so the two sirens are in parallel)

PANEL BELL + --18/2, 40 ft-- INTERIOR SIREN +
PANEL BELL - --18/2, 40 ft-- INTERIOR SIREN -

PANEL BELL + --18/2, 65 ft-- EXTERIOR SIREN +
PANEL BELL - --18/2, 65 ft-- EXTERIOR SIREN -

TAMPER LOOP, ZONE 9, 24-HOUR
                       AT THE EXTERIOR SIREN
ZONE 9 + --22/2, 65 ft-- TAMPER SWITCH
                         (closed, cover on)
                               |
ZONE 9 - --22/2, 65 ft-- EOL RESISTOR

Each siren gets its own pair home to the panel, rather than one siren daisy-chained through the other, so a cut cable at the eave does not silence the hallway siren. The tamper switch and its end-of-line resistor sit at the siren, the last and only device on that loop, with the resistor value per the panel's documentation. With the cover on, the switch is closed and the zone sees the resistor: normal. Pull the cover or cut the cable and the loop opens: tamper. Short the pair at the panel end and the zone sees a short instead of the resistor: also a fault. That supervision logic is owned by End-of-Line Resistor Supervision: Open, Short and Normal.

Program zone 9 as a 24-hour tamper or burglary zone so it reports whether or not the system is armed; a siren is most worth attacking when the system is armed, and least likely to be noticed when it is not.

Step 6: sound it and measure voltage at each siren

Hearing protection on first: earplugs under earmuffs, because you will stand within a few feet of the interior siren and on a ladder beside the exterior one. Warn everyone in the house, and the neighbors if they are outside.

Trigger an alarm (account on test) and, while the sirens sound, read DC volts at the panel's bell terminals and then at each siren's terminals.

On this job: 13.2 V at the bell terminals with AC on. 12.8 V at the exterior siren, a 0.4 V drop, matching the 0.42 V calculated in step 4. 13.05 V at the interior siren, 0.15 V down, also matching. A measured drop much larger than the calculation means a poor splice or a loose terminal somewhere in that run, and it gets found now.

Then repeat the exterior reading with the transformer unplugged, so the panel runs on battery, to see the worst case. Here the bell terminals fell to 12.4 V and the exterior siren read 12.0 V, still the same 0.4 V drop and inside the siren's operating range on its spec sheet. Plug the transformer back in.

Step 7: read the output level at 10 feet

Set a sound level meter to A-weighting, stand 10 ft from each siren on its axis, and read the level while it sounds. An eave siren is usually more than 10 ft from anywhere you can stand safely; read it from the ground on a line to the siren and measure that distance instead. Write the reading, the distance, and the meter setting on the job.

On this job: the interior siren read 98 dBA at 10 ft in the hallway, and the exterior 98 dBA from the ground, 20 ft from the siren on a line to it. Those are this job's readings, not targets. The maker rates each siren at a stated distance; outdoors, each doubling of distance costs about 6 dB, so that 98 dBA at 20 ft compares to roughly 104 dBA at 10 ft. A reading well under the rating at the same (or corrected) distance, on a healthy voltage, points to the siren, while a low reading with a low voltage points back to the wire.

Then do the check the reading cannot do: close a bedroom door, have someone sound the alarm, and listen from the bed. The customer should hear it clearly. If they cannot, a second interior siren inside the bedroom wing is a better answer than a louder siren in the hall.

Step 8: test the tamper, set the cutoff, and close up

Pull the exterior siren's cover with the system disarmed. The keypad should show a tamper on zone 9, and the station should receive it. Refit the cover and confirm the restoral.

Set the bell cutoff time in the panel to what the dealer's standard and the local ordinance allow; many jurisdictions limit how long an exterior alarm may sound. Then confirm with the station that every signal from the job arrived, and take the account off test.

References

• Panel manufacturer installation instructions: bell output type and current rating, bell cutoff, tamper zone types.
• Siren manufacturer specification sheets: current draw, operating voltage range, rated output and its measurement distance.
• OSHA 29 CFR 1910.23, ladders, for general-industry work at the eave.
• See related: How to Size a Standby Battery for an Alarm Panel
• See related: End-of-Line Resistor Supervision: Open, Short and Normal