Some types of smoke alarms, and carbon monoxide alarms, are designed to communicate with each other, typically wirelessly.
This is so that if an alarm goes off in one room of your house, other alarms can "hear" that that has happened, and repeat the alarm. This is because the original alarm might
be out of hearing range of a human, but within radio range between the two alarms.
It seems a bit redundant to have both a radio and audio alarm going on at the same time... why not use the same sound signal that tells humans that there's smoke/whatever, to signal additional alarms.
The alarm would beep, and encode into that beep some digital data using audio watermarking technology. Other alarms would be equipped with mics, and would decode the watermark, and produce their own beeping.
The data encoded would include:
Whether the alarm was a smoke or CO alarm, the number of times the message as been passed on (number of hops).
The serial number of the alarm that initially signaled.
The location / type of room that the initial alarm is in ("bedroom", "kitchen", "garage", etc), as programmed by the installer. This would be so that talking units could speak the location.
The time that the alarm was initiated.
And probably other things I haven't yet thought of.