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Google’s self-designed office swallows Wi-Fi “like the Bermuda Triangle”
(arstechnica.com)
This is a most excellent place for technology news and articles.
To support MU-MIMO / beamforming (multipath signals for multiple devices) they could also just add more flat surfaces inside the ceilings to make radio reflections/echoes less complex so that the signal processing doesn't get overwhelmed when the source is some distance away.
Plain absorbing material removes interference but doesn't let you use MIMO tech as effectively, because the newer higher end routers can use those reflections to boost the signal
https://www.analog.com/en/resources/analog-dialogue/articles/massive-mimo-and-beamforming-the-signal-processing-behind-the-5g-buzzwords.html