I was playing around with some Sony wireless mics and i noticed a label on the back stating the frequency they operate in. The label said "638.125-661.875". I grabbed my hackrf one and plugged it in. It should be noted that in that frequency range, an RTL-SDR dongle would work as well.
Once I plugged in my HackRF One, I ran hackrf_info to make sure it was recognized and then started gqrx.
I tuned gqrx to the start of the range, 638.125. I was pleased to find that this was the frequency used by "Channel 1" setting on the wireless mic. I flipped the switch on the mic, and bam. A very strong signal appeared on my screen. I centered the cursor over that, adjusted the squelch to tune out the background noise and turned up the volume so i could hear. Lo, and behold thats all that was needed.
Turns out that the wireless mics simply are FM transmitters broadcasting the audio so the receiver unit can pick it up. Well, in this set up my hackrf is the receiver.
Now, once i learned this i kind of facepalmed a little.
Someone asked me what the big deal is with this. I told them that lots of times the communications being held over these wireless mics can be confidential. Broadcasting confidential info usually is what people try to avoid.
A great real word example of utilizing this info would be if a reporter is doing an interview with a high profile individual and wants everything to be kept secret until the airing. Well, if someone knew the building the recording was happening, they could record the audio of the wireless mic and break the story first.
Or worse, use the story info to determine if they should carry out hits to keep people quiet.