Audible XX.X kHz from PLC on power grid in Grimstad, Norway

Sine-spectrum before signal detected

Sine-spectrum when signal detected

The graph above shows a 20dB signal strength higher than the background noise. According to the equal loudness contour 20dB is at the lower range of hearing at this sine wave frequency.

Listening samples from professional microphone placed inside a room

Recording #002
Recording #002 downsampled
Recording #003

Client side equipment known to emit 12.5 kHz

Possible origin

Politerm 760 order in Norway - 25.000 pieces

Update October 2017

Agder Energi Nett has confirmed that all PLCs in my area has been removed. However there are still more PLCs running at 12.5 kHz.

Sine-spectrum when signal detected (post PLC shutdown)

Accumulated listening samples from professional microphone placed inside a room (post PLC shutdown)

Recording #004

Software used to detect and filter out only relevant signal



Can sound like a grasshopper. Signal actually comes from the power grid and propages through switched AC-DC power supplies via computers and audio amplifiers and becomes audio waves at the same pitch sine frequency. The source is difficult to locate unless you use a pair of microphones.

Last updated 9th of November 2017