I used to be a an audio engineer before I joined the CF. Any reading taken within 3' is considered to be "At Source". They likely dropped it down a tube into a chamber and measured from the top of the tube. Thats how the dB SPL ratings are taken for a rifle (fireing into a bullet catch chamber and measuring out the top) so I'd imagine the process is similar for TFs as well. They'd likely use remote detonation too.
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