ElectroMagnetic Interference or EMI
There seems to be two effects that you are looking at: electrical noise getting into speaker lines, and electrical noise getting into other building circuits in the fire alarm system. When they occur in a building, both of these signals can be considered "Crosstalk".
Crosstalk is ElectroMagnetic Interference (EMI) that, to one extent or another, affects all wiring. Usually, the amount of EMI is low enough that it's not detectable with most instruments. Also, typically between building circuits, the EMI is achieved through magnetism.
At one time or another, because of extraneous signals in the building's fire alarm building circuits, your boss has probably been burned by extraneous EMI signals interfering with the fire alarm system. These signals could show up as intermittent or continuous "Ghost Troubles", or maybe they could show up as a quiet noise in the speakers.
Whether it's the ghost troubles, or noise in the speakers, the noise or interference appears to seemingly come into the fire alarm system through thin air. The only way of correcting this signal crossing-over into the system through thin air is to separate the wiring and keep it from any other system. Often times, some fancy rewiring, along with guesses is required.
The rewiring costs money.
Crosstalk is a Form of EMI
Crosstalk in wiring is an issue. People working in all areas of the electrical and electronic fields have had to address the issues of crosstalk many times.
Most crosstalk in a building is caused by unsolicited power being transferred from one wire to another. Usually, like in a transformer, the power transfer from one wire to another is performed through invisible, varying intensity, magnetic fields.
A Transformer Transfers Power
A transformer is two wires transferring AC power between them, using magnetism. This power transfer between two wires can occur both ways, and sometimes can cause interference in both directions at once.
Anytime there's electrical current passing through one of the wires, there's a magnetic field around the wire. If the current increases, the magnetic field increases its strength; if the current decreases, the magnetic field decreases its strength. When the current is varying, the magnetic field it's creating is varying in strength.
On the other hand, anytime there's an increasing strength or decreasing strength magnetic field crossing a wire, there's an additional voltage created inside the wire that increases or decreases the current in the wire.
Magnetism Transfers Power
Current and voltage aren't directly transferred from one wire to the other wire; the insulation is almost always good enough to stop the effects the current (caused by voltage) from leaking from one wire to the other. However, the varying strength a magnetic field still transfers power from one wire to the other wire.
Either wire can be creating the varying strength magnetic field, and all wires in the varying strength magnetic field are affected. The closer together the wires are, the greater the effect, the farther apart the wires are, the lessor the effect.
The effect of the created magnetic field increases with less distance between the wires. Wires that are closer together are more affected by the varying strength magnetic fields than wires that are some distance away. Because the magnetic field is greater when the distance between the wires is decreased, the crosstalk increases.
The effect of the created magnetic field increases when an iron core is added: either inside a coil, or surrounding the wires. A conduit can be considered to be a magnetic core that increases the strength of a magnetic field. When the wires are together inside the same conduit, the crosstalk increases.
Speaker wires carry a lot of current. Any wires in the same bundle or in the same conduit such as signaling line circuit wires will pick up audio from the speaker wires.
When the distance that the wires run together is increased, the crosstalk increases. When the wires just get close where they cross each other, because they're close only for a very short distance, the crosstalk is less than if they run close together for a long distance.
Speakers and Hearing
Speakers are a special case. Most of the signals in a fire alarm system start in electronic equipment (like detectors, modules, switches, and panels), and end in electronic equipment (like horns, strobes, relays, and other panels).
Signals in speaker circuits, on the other hand, are an exceptional problem. The human ear has to be included in the effects of a speaker. Whereas the signals may start in electronic equipment (like speaker amplifiers), because of the sensitivity of the ear, even quiet sounds can be heard.
Keep in mind that signals being carried by other wires next to speaker wires will be transferred to speaker wires, and even though there isn't much power being transferred, those low powered signals can sometimes be heard from all of the speakers.
Whereas the signals being sent from equipment-to-equipment are immune to most low levels of EMI, because of the sensitivity of the human ear, even a very low-level EMI noise can easily be heard by the human ear.
In essence, because of the AC crosstalk from signals in other wires passing into speaker circuits, EMI can be heard over all the speakers.
Speaker Circuits and Crosstalk
Think of diminishing the effects of crosstalk as taking the magnetic core of out of a transformer (which is similar to placing the affected circuits in different conduits), moving the coils of primary and secondary transformer coils apart (which is placing the affected circuits in different bundles, away from other wiring), or both.
These actions, while more costly, reduce the overall cost of trying to fix problems after installation.
This crosstalk problem with speaker circuits and other circuits has been the bane of many electronic installations: fire alarm, overhead paging, sound reinforcement, etc. Just keep the speaker wires away from everything else.
Radio Station EMI
Having worked for 15 years as a broadcast engineer, I've seen lots of EMI interference.
Fire alarm systems, though, have a fairly high tolerance to EMI. EMI interference from close by AM radio stations has been an issue with only a few fire alarm systems. However, even after 20 years working in the fire alarm industry, I've seen only a single instance where a really, really close by TV or FM radio station interfered with a fire alarm system.
I've seen where AM radio has interfered with a number of fire alarm systems, but the one manufacturer that had a major problem with this has since fixed most of their EMI issues.
I can't say for sure, but unless it's an AM radio station with its tower located on the campus, there probably isn't a problem with EMI.
Redundant Class A Wiring
To get more information on redundant wiring, see:
How Does Conventional Class A Fire Alarm Wiring Work?
Remember that Class A wire paths are there to make sure that if a wire breaks or comes loose from a connection, all devices will be connected to the panel. If all circuits, the feed and the return for the same Class A circuit, are enclosed in one conduit, and the conduit is broken, some or all of the devices beyond the break will be useless.
To keep all of the devices working is a major reason to go to the extra step of using the increased life safety of Class A circuits.
Douglas Krantz
