How does a Conventional Fire Alarm Audio Speaker Work?

A fire alarm speaker is identical to an overhead office paging speaker, with three exceptions:

----- The speaker assembly has a conspicuous "FIRE" label on it

----- The speaker has fire alarm rated connections

----- The speaker has an internal DC blocking capacitor inside the speaker assembly

Of course, it also has been tested and listed for use in that fire alarm system by a third party, nationally known testing laboratory, like UL, ULC, FM, CE, DE, CCC, etc.

If the "FIRE" label was removed, the fire alarm's speaker could be used in an office's overhead paging system, and work like any other speaker.

Impedance Matching - Step-down Transformer

The 25-volt or 70-volt audio from the audio amplifier would cause the speaker's 8-ohm voice coil to rip apart, so an impedance matching transformer is used between the relatively high voltage of the Notification Appliance Circuit (NAC) and much lower voltage requirements of the speaker itself.

In other words, the transformer steps-down the power taken from the NAC circuit to a manageable level for the speaker.

There are several power taps on the primary side of the transformer, so the amount of power the speaker receives (and turns into audio sound) is adjustable.

DC Blocking Capacitor

In a Notification Appliance Circuit (NAC), for its wire supervision, there is a DC voltage applied to the circuit wires so it can perform a continuity test of the NAC circuit wires themselves.

Because the primary winding on the transformer is just a coiled-up piece of wire, the speaker's transformer would be naturally be shorting out the NAC's DC supervision, so a DC blocking capacitor is in series with the transformer's primary winding.

The audio signal itself, though, is AC; the capacitor in this configuration passes the AC audio signal, so the audio is passed through to the transformer.