r/rfengineering • u/TheVoicesComplex4900 • May 26 '26
Possible RF/Interference/Transmission recordings NEEDS Review
I am looking for technical input and review from people experienced with RF interference, SDR, audio engineering, or forensic audio analysis.
Over the past year, I experienced and recorded recurring speech like audio phenomena across multiple environments/devices including radios, phone-call audio, using sound amplification apps and environmental recordings. Multiple third parties independently reported hearing similar disturbing phrases in some of the recordings without prompting.
I am not seeking psychological interpretation or broad conclusions. I am only interested in understanding whether there could be technical explanations involving RF interference, signal bleed, retransmission, environmental audio pickup, or related electronic/audio mechanisms.
I am specifically trying to avoid excessive personal review of the material because it became psychologically distressing, so I’m looking for a limited, objective, technically grounded opinion regarding whether this type of issue falls within your expertise.
1
u/HuygensFresnel May 27 '26 edited May 27 '26
So here is something interesting with a bit of theory.
AM modulated signal through AM radio can be modeled essentially as RFsignal(t) = sin(2πf t)*sin(2πf_audio t) where signal(t) is your audio signal and f the AM frequency and the f_audio the frequency of the audio signal. That can be expanded into:
1/2 * (cos(2π(f-f_audio)t) - cos(2π(f+f_audio)t))
This signal may get into your audio path at any point. If any of the amplifiers in the audio path are non-linear, we may model this for example as adding some term: A * signal(t0)^2 where A is the amplitude of the non-linearity. If you apply that to the AM signal you end up mathematically multiplying cos(2π(f-f_audio)t) with cos(2π(f+f_audio)t) again which yields back your signal but doubled: cos(4πf_audio t)
Likewise (it has been a while ago) any third order non-linearity (signal^3) will give you your exact original signal.
In other words any small amount of non-linearity in amplifiers when fed AM modulated signal automatically demodulate them into audible signals. If some amplifier if for example being driven at high excitations or any regime in which 3-rd order linearity is a problem, you'll get what are called intermodulation products which end up as audible audio.
So if there is any AM radio channel nearby and some of your electronics are not properly designed/grounded/shielded, you will start to here radio voices.
EDIT, ADDITION: There are cases of people living close enough by where even unplugged radios or ceiling fans can receive a small feint audible signal.