What you are listening to is crosstalk. The sound is cross-talking from the Signaling Line Circuit (SLC) to the speaker wires.
This crosstalk is a problem that occurs whenever any speaker wire is installed or run next to any other signal carrying wires, or even power wires. The crosstalk is two-way: other signals can get into the speaker wire, audio being sent to the speakers on the speaker wire can get into other wires. This crosstalk is particularly a problem with some manufacturer's systems where the SLC (Signaling Line Circuit) data is low frequency, like EST.
Listen to the Transmitted Electronic Noise
Take your signal tracing wand you use for tracing telephone wiring and listen to the electrical noise inside the panel. Don't touch the wires or terminals, just listen to the transmitted noise. You get a better idea of what you're up against if you just hold the wand inside the panel. That is the sound you are hearing on the speakers. Just for kicks and giggles, use the wand and listen to the SLC noise at any detector head or module anywhere in the building. That, again, is what you are hearing on the speakers.
Question - How Is That Noise Getting Into The Speakers?
The good news in all this is that you can use your newfound knowledge about the signal transmission of wiring to easily trace the EST SLC wiring through finished walls and ceilings (as long as the wiring is not shielded by conduit). All of the SLC wiring emits that noise because all wiring of any type emits any noise of the signals it carries.
The bad news is that is how the noise is getting into the speaker wiring. The SLC is transmitting the signal you're hearing into the speaker wiring using transformer coupling. The only way of getting rid of the noise is to separate the speaker wires from everything else in the building (expensive once the building is finished).
Transformer Coupling or Crosstalk
Think of how a transformer is built. There are two wires, each one coiled up, and there is an iron core.
The two wires transmit power to each other through magnetism. One wire builds up the magnetic field and the other wire generates electricity using this magnetic field. It doesn't matter which one is transmitting and which one is receiving, if electrical current is flowing in one of them, the other wire will generate power from the transmitted magnetic field.
Of course, having an iron core around the wires just makes the magnetic field better so the power transmission is stronger.
Remember that any wire, whether it's straight or coiled, creates the magnetic field, and any other wire, whether it's straight or coiled, generates power from this magnetic field.
Yes, the wires in the transformer are coiled, and have an iron core, all of which makes for better power transfer. However, the wires in the building are much longer. This increases the power transfer just because of the longer length of the wires. In many cases, the iron core of the transformer is lengthened and called conduit.
Yes. Conduit has a shielding effect that greatly reduces the signals from getting outside the conduit to interfere with other systems, and this shielding effect also greatly reduces the signals from outside the conduit so it doesn't interfere with protected wiring inside the conduit.
The conduit, however, enhances the power transfer from one wire inside it to another wire inside the same conduit because the conduit, like the magnetic core of any transformer, enhances the magnetic field that has been generated inside it.
Once a wire has an extra unwanted signal that is in the same frequency range as the signal that is wanted (in this case, the unwanted signal is in the audible frequency range) the unwanted signal is extremely difficult to remove. There are some methods to reduce the crosstalk, but they often void the type acceptance of the system, are somewhat costly themselves, and make updating the system later almost impossible without reengineering the noise reduction techniques.
Methods to Solve the Crosstalk Problem
The best method to solve this crosstalk problem is to use proper wiring etiquette in the first place and install the fire alarm system with all the speaker wires in separate conduit, or separated several inches from any other wire in the building. This is what is needed to prevent the crosstalk. Doing the separation at the time the building is put up is not that expensive.
After the building is finished and all the wiring is hidden in walls and ceilings, the second best method to solve this crosstalk problem is to install new speaker wiring, away from any other wiring in the building. Then the speaker wires won't pick up the crosstalk. This solution is very expensive.
The third method (this is what I told the salesperson to use when the condo owners complained about the noise, and the salesman didn't want to address the cost of rewiring a finished building) is to tell the people there that "This is the way it is." This is the trickiest method of dealing with the noise (people's feelings involved and all that), but it's also much cheaper than rewiring the building.
Long Term Solution
Get together with the project managers of the company, with the salespeople, with the installers, and with the other technicians. Show them the problem, and show them the solution. Remember that the best method of solving your problem is to start before the first wire is installed in a building. Design the system and install it to keep that speaker wire away from any other wire.
This problem and other induced ground fault issues are explained further in the book Make It Work - Hunting Ground Faults
. The book shows the three types of ground fault, what equipment should be used with each type of ground fault, and how to locate those hard-to-find ground faults.