Conventional Supervision is Over a Century Old
Speakers, horns, strobes, etc. require power to operate. Most methods of sending signals don't send much power. For instance, powering a horn or strobe off fiber optic communications just does not work. Copper wires are needed to transfer power, so copper wires carry power to horns, strobes, and even speakers.
The idea of sending fire alarm power over wires is over a century old, and the idea of supervising the wiring for fire alarm systems is over a century old. When wires were first supervised for fire alarm systems, only relays existed, so only relays could be used.
This older era is where the idea of supervising wires using DC current came into being. Supervising wires using DC current seems to have worked until now, so in all probability, DC current will continue to be used to supervise wire in conventional fire alarm circuits like Initiating Device Circuits (IDC) and Notification Appliance Circuits (NAC).
Understanding why DC current is used to supervise the wires in an Audio NAC circuit takes a little explanation.
A Speaker Notifies
A speaker is a Notification Appliance; during a fire alarm, a speaker makes noise to warn people. As far as how a fire alarm speaker works as opposed to a home stereo speaker, with only one small exception, the fire alarm speaker is identical to a home stereo speaker.
Magnet and Coil "Motor"
Inside all home speakers and all fire alarm speakers is a permanent magnet. This magnet is held in place by what is called a basket, which is part of the overall metal speaker housing. In the magnetic field of the permanent magnet "floats" a coil of wire.
To move the air so we can hear things is a paper or plastic cone that can move the air. It compresses the air or "rarefies" (un-compresses) the air. The floating coil of wire is firmly attached to the cone using a coil form.
When there is an electrical current flowing through the coil, the magnetism around the coil forces it to move. This is motor action. This movement is transferred to the cone, which causes the air to be compressed or rarified.
Any Change in Current is Motor Action
Whenever the amount of current flowing through the coil is changed, the coil is forced by the magnetism to move. A change in DC current from none to some causes the coil to move. A change in DC current from some to none causes the coil to move. AC current (which is always changing) causes the coil to continuously move.
The human ear is a wonderful device. The ear can adjust its sensitivity hear a jet airplane as the jet is passing close overhead; the ear can adjust its sensitivity hear a sewing pin hit the floor.
What that means is that, no matter how slight the current change, any current change in the coil will be heard by people. The only way to keep a speaker from making noise that people with their sensitive ears will hear is to not change the electrical current through the coil. That is why the supervision method used by the Audio Notification Appliance Circuit (Audio NAC) is DC; so there won't be a change in the electrical current in the coil.
Supervising the Wires
The small exception mentioned at the start of this is based in what the fire alarm panel is doing when it is supervising the wires. It is supervising a NAC circuit for open wires, and it is supervising a NAC circuit for wire-to-wire shorts.
When it is supervising, one way of looking at wire supervision is the panel is measuring the resistance of the end of line resistor. The end of line resistor is somewhere else in the building, so the panel is really measuring the wires of the NAC circuit.
When the resistance of the end of line resistor is the resistor's value (1K Ohm, 1.5K Ohm, 2K Ohm, 10K Ohm, 15K Ohm, 47K Ohm, whatever the panel is expecting), the panel shows "Normal" for the NAC circuit. If there's a fire, whether it's a horn/strobe circuit or a speaker circuit, the panel disconnects its supervision circuitry and activates (or powers) the circuit.
During Supervision, when the resistance of the end of line resistor is higher than expected, the panel assumes that a wire is broken or disconnected, and shows "Trouble Open" for the NAC. Even if the NAC circuit is open when there's a fire, whether it's a horn/strobe circuit or a speaker circuit, the panel activates the circuit; the panel applies power.
When the resistance of the end of line resistor is lower than expected, the panel assumes that there's a wire-to-wire short, and shows "Trouble Short" for the NAC.
This is the dangerous part, though. If there's a fire, whether it's a shorted horn/strobe circuit or a shorted speaker circuit, the panel WILL NOT
activate the NAC circuit. The panel has to protect its own power supply from being shorted out, and the panel also has to protect any other NAC circuits from being shorted out. Remember that connecting a shorted circuit to anything else shorts out whatever it's connected to.
DC Blocking Capacitor
Fire alarm speakers, just like fire alarm horns and strobes, are very low-tech devices. They cannot send a separate signal that they have a problem, or even say that they are dis-attached from the NAC circuit. They have to be listened to by a person walking around in order to find out if they even work.
So, speakers, horns, and strobes are not supervised by the panel.
Worse yet, though, is electrical shorts. If the speakers didn't have a DC blocking capacitor in line with the coil, or the horns and strobes didn't have reverse current blocking diodes in line with their circuitry, the speakers, horns, and strobes would conduct electricity while the wires were being supervised by the panel.
Another way of looking at "conducting electricity" is that by conducting more electricity, they reduce the resistance of the NAC circuit as the panel is supervising the wires. The panel, seeing lower than expected resistance, would display a "Trouble Short" and never activate the circuit during a fire.
No Other Method
With conventional fire alarm systems that have conventional Notification Appliance Circuits (NACs), there is no other accepted method of supervising the wires. Because Fire Detection and Alarm Systems are Life Safety Systems, any changes to a method have to be a significant improvement.
There are different ways of supervising, but the different methods are usually addressable circuits. The addressable methods are proprietary to manufacturers, and usually involve being able to turn on and off individual speakers, horns, and strobes without directly affecting the power circuit supplying the Audio to the speakers, or the DC power for the horns and strobes.