I believe that most jurisdictions don't take kindly to private individuals/corporations jamming mobile phone signals, not least because it blocks the ability to

I believe that most jurisdictions don't take kindly to private individuals/corporations jamming mobile phone signals, not least because it blocks the ability to make emergency calls. As we don't know what frequencies a cheater might use, or how powerful the antenna would be (and it only needs to produce a short signal once per match, potentially), any jamming would have to be very disruptive to be effective. You're right, though, that professional chess events need to become (Faraday) cage matches. There may be existing structures which could work, but in the worst case the tournament organisers could purchase some portable Faraday tents, which are apparently a real thing[0]. I'm not sure if ultrasound would work as a communications medium, but I'm guessing that it wouldn't be safe to allow spectators to watch the match even through a mesh window. A camera feed (over a fibre-optic connection) from inside the tent could be provided, but spectators would have to be kept in a separate room, possibly with the same 30 minute delay that the online viewers are subject to. [0] https://hollandshielding.com/EMI-RFI-shielded-Faraday-tent https://hollandshielding.com/EMI-RFI-shielded-Faraday-tent