The following is from a tread involving a friend of mine. An ex-military fellow. Take it for what you want. I believe him.
This depends in large part on the strength of the degausser and how long it remains powered up. The one I have I used to "clean" 10.5 inch tape reels written at FM broadcast quality to a noise level -20db down from brand new tape - this back in my quadraphonic stereo days. It will not quite suck satellites out of orbit... TWA planes, maybe.
-20dB down from new would be a better spec if new tape was typically nulled rather than just blank low-level white noise.
However, even so, a SQUID is a very scary thing....
I've been out of commission for a week and found 276 messages in my in-box today. The tapes I was writing about were those already "prepared" for recording "items of interest", and had already been degaussed in the preparation procedure to FM broadcast standards then/there. My little hummer would take whatever whit noise remained even then to a further 20 dBA DOWN from the ridiculous levels then/there specified. In other words, it's pretty good. Of course, IF Big Brother wants, there are any number of ways for him to obtain whatever information he wishes. I recall long, long ago, I was privileged to watch a demo of equipment (1955 or 1956, I forget exactly) in a truck parked a MILE away from our CommCenter, in the years before Faraday cages, TEMPEST, etc., and watch these guys print out a 5x5 copy of our top secret traffic as it was read by the [elided upon request] one-time-tape encryption devices. Shortly after that demo, our CommCenter was moved into a cave (no joke) and modified to "low level" dc signalling voltages. And other measures were taken to screen the area... I _have_ led an interesting life... As for Squids, only if you read Jules Verne are they scary. Otherwise, cleaned and fried with a tangy dipping sauce, well, that's another story! --Lucky