Panel's Power Supply
Almost all electronic systems, like home stereos, computers, and fire alarm systems, have internal power supplies. The internal power supplies convert the utility voltage of 120 Volts AC or 230 Volts AC to a lower DC voltage like 5 Volts DC, 12 Volts DC, or 24 Volts DC.
Most fire alarm systems use an internal 24 Volts DC to run on so that if the utility power fails in a blackout, the 24-volt batteries can take over and keep the fire alarm system operating at 24 volts.
Conventional Wire Supervision
Speakers, like almost any other device on a conventional fire alarm system, are not supervised; only the wires going to and from the speakers are being supervised. If something goes wrong with a speaker, a horn, a strobe, a pull station, smoke detector, a waterflow switch, or any other conventional device, a trouble will not show up on the panel. No one will know that there's even a problem.
The devices have to be actually tested by humans; during a fire when the device doesn't work isn't the time to figure out the device should have been replaced.
While the wires are being supervised by the panel, the reason why the devices are not supervised is to prevent current from going through any of the devices. Current going through the devices is considered by the panel to be either an alarm, or to be a shorted circuit.
Input devices like pull stations (they're switches), flow switches, and heat detectors (in reality, they're usually just switches) send in their alarms by turning on; they send an alarm by letting current flow through them. They short out the input circuit (IDC or Initiating Device Circuit).
Smoke detectors are a little different. They require a very small current to flow through them at all times in order for the detection circuitry inside them in order to continue to look for smoke. When in alarm though, they draw a lot more current to partially short out the IDC. The panel then considers that to be an alarm.
Output devices like horns, strobes, and speakers never have any current going through them during supervision times.
To prevent supervision current from going through the devices, either an internal blocking diode prevents the supervision's reverse current from going through the horn or strobe, or an internal blocking capacitor prevents the supervision's DC current from going through the speaker.
To make sure all the devices will continue to detect fires and sound the alarm, the panel doesn't supervise them, we have to test them.
The Speaker Circuit is a NAC Circuit
In a conventional fire alarm system, a Notification Appliance Circuit (NAC) is the circuit (or pair of wires) that carries power to turn on the appliances like horns, strobes, and speakers. The appliances use this power to notify people that there is an alarm. The horns, strobes, and speakers alert people by making noise and flashing lights.
Two Different Power Supplies for Two Different Times
The two screws on the panel for each NAC output are switched inside the panel; the screw terminals are switched between power supplies. In normal times, the two screw terminals are connected to the very weak Supervision Power Supply; in alarm times, the two screw terminals are connected to either the high power 24 Volt Power Supply or to the output of an Audio Amplifier.
Supervision Power Supply
The weak supervision power supply is always a little lower voltage than the high power internal 24-volt power supply inside the panel. It is so weak that the current through the end of line resistor pulls the voltage of the power supply down a little. Because it's so weak, it won't provide enough power for any of the appliances like horns or strobes.
Backward Polarized Supervision Power Supply
On a NAC circuit, the voltage of the Supervision Power Supply is backwards. It is backwards so that the blocking diode inside the horns or strobes won't let any current flow through the horns or strobes.
Because it's a DC voltage, the blocking capacitor inside the fire alarm rated speaker assembly prevents any current from flowing through the speaker assembly.
The Panel is Seeing Voltage at the Terminals
When nothing is connected to the panel's NAC screw terminals, nothing is pulling down the voltage of the power supply at all. Your voltmeter will show a higher-than-normal voltage; the panel sees this higher-than-normal voltage as an open circuit. Depending on which manufacturer made the NAC panel, the higher-than-normal voltage could be anywhere between 16 volts and 23 volts.
When there's an end of line resistor on the screw terminals, the end of line resistor pulls the voltage of the supervision power supply down a little bit. Your voltmeter will show a normal voltage; the panel sees this normal voltage as a normal circuit. Depending on which manufacturer made the NAC panel, the normal voltage could be anywhere between 1.8 volts and 20 volts.
When there's a short on the circuit, somewhere in the building, the extra current draw of the shorted circuit pulls the voltage of the supervision power supply much lower. Your voltmeter will show a lower-than-normal voltage; the panel sees this lower-than-normal voltage as a shorted circuit. Depending on which manufacturer made the NAC panel, shorted circuit voltage is any voltage that is less than 1.5 volts to 8 volts.
You will have to get used to the voltages for each manufacturer, they're all different.
Use the voltage you see on your voltmeter to learn what the panel is seeing, doing that really saves time when troubleshooting.
Switching to the Alarm's 24 Volts Power or Amplified Audio
In an alarm, all the panel has to do is activate the NAC's output relay. The relay switch inside the panel, just like a light switch on the wall, is a physical switch. It can't tell whether it's switching the NAC circuit screw terminals to the high powered, forward voltage 24-volt DC power supply, or to an audio amplifier's 25-volt or 70-volt audio output.
Short Means the Panel Won't Power the NAC Circuit
The panel does not want to short out its power supply, or short out the audio amplifier. To protect itself, the panel checks with the supervision circuitry, and it will never turn on a shorted NAC circuit. If you ever see the words "Shorted NAC" on the panel, assume the panel will never, ever sound the alarm, and job right now is to fix the short.
24 Volts DC from a Power Supply is Real Voltage
Commonly, this power is straight off the 24-volt power supply on a small fire alarm panel. For practical purposes, a short on the NAC circuit is a short on the small fire alarm panel's power supply. That is why, if the supervision circuitry detects a short, the panel will never turn on the NAC circuit.
Audio is AC
The speaker audio on a fire alarm system is exactly the same as the speaker audio on a home stereo system or the speaker audio on a surround sound entertainment system. It's measured as an AC voltage, but the voltage bounces around from zero volts AC to the amplifier's full voltage AC. The voltage you see on the meter shows how much noise is coming out of the speaker at any short period of time.
Amplifier Power is Potential Power
Your home amplifier can be rated at 50 watts; if there isn't a speaker connected to the amplifier, the amplifier is still capable of 50 watts of power. The same with a 50-watt fire alarm amplifier, whether or not there's any speakers on the system, the amplifier is still a 50-watt amplifier.
25-Volt or 70-Volt System is Potential Voltage
If there is voice, the space between the words is close to zero volts. If it's a constant loud tone used for evacuation, it's much closer to the 25 volts or 70 volts that the system is capable of producing. The system may be capable of producing 25 volts or 70 volts, but the actual audio voltage is often much less.
Supervision Switched to Audio or Power Supply
See: How Does a Relay Work? https://www.douglaskrantz.com/ElecHowDoesARelayWork.html
The panel has two possibilities of output to the Notification Appliance Circuit (NAC). When the alarm sounds, the internal relay switches the NAC circuit screw terminals from the supervision power supply to the audio amplifier output or the 24-volt power supply. When the alarm quits sounding, the NAC is switched back.
The blocking diode inside of the horns and strobes, and the blocking capacitor inside the speaker assemblies prevent the horns, strobes, and speakers from looking to the supervision circuitry like a shorted NAC circuit.