Might have been audio?
https://phys.org/news/2018-02-power-grid-fluctuations-hidden...
Sibling comment and article mentioning "Electric network frequency (ENF)" lead to further reading mentioning both audio and visual fluctuations being used.
Ah you’re right. Pretty cool.
Sibling comment and article mentioning "Electric network frequency (ENF)" lead to further reading mentioning both audio and visual fluctuations being used.
Ah you’re right. Pretty cool.