So how does it work?
If you had the through-metal technology now reinvented by Lawry, however, your intruder – inside mole or cleaner or pizza delivery, whatever – could stick an unobtrusive device to a suitable bit of structure inside the Faraday cage of shielding where it would be unlikely to be found. A surveillance team outside the cage could stick the other half of the kit to the same piece of metal (perhaps a structural I-beam, for instance, or the hull of a ship) and they would then have an electronic ear inside the opposition's unbreachable Faraday citadel, one which would need no battery changes and could potentially stay in operation for years.
Spooks might use such techniques even where there was no Faraday cage, simply to avoid the need for battery changes and detectable/jammable radio transmissions in ordinary audio or video bugs.
Naturally, if you knew how such equipment worked you might be able to detect or block it – hence the understandable plea from the British spooks to BAE to keep the details under wraps.
Unfortunately for the spooks, Lawry has now blown the gaff: his equipment works using ultrasound. His piezo-electric transducers send data at no less than 12 megabytes a second, plus 50 watts of power, through 2.5 inches of steel – and Lawry is confident that this could easily be improved upon. It seems certain that performance could be traded for range, to deal with the circumstances faced by surveillance operatives rather than submarine designers.
It also seems pretty much certain, now that they know what they're looking for, that counter-surveillance people will begin sticking transducers of their own onto the walls of their secure facilities and rooms. If they pick up ultrasonic vibrations – which will travel a long way if they're capable of carrying 50 watts of power – they'll know that they've been penetrated, and either hunt down the kit or just start transmitting jamming ultrasonics of their own.
Who knows, such countermeasures may already be routine in some circles, or the tech may well be in secret use for some completely different purpose. But the mere fact of the government suppression of BAE's technology tends to indicate that some sort of valuable trickery along these lines has been – or still is – going on.
The spooks will just have to hope that whoever-it-is doesn't watch this vid in which Lawry explains how his kit works ...
... or read this statement from the Rensselaer Polytechnic Institute, where he's working on his PhD. The through-barriers kit has put Lawry in the running to win a $30,000 student prize, which may be causing certain boffins in Blighty's secret labs to grind their teeth even more. ®
@"but what is the detection range of sonar"
Ask a Whale. They can hear sounds from over a 1000 miles away, but with all the noise in the oceans these days, that range is usually down to a few hundred miles.
@"More worrying is what 50W of ultrasound does to a solid structure over days/years"
More worrying is what it does to Whales and other marine life!
"Stop tunneling, jerry guards approaching!"
So this is just the 21st Century version of banging out morse code on water pipes in POW camps.
What carries for distances underwater is, I believe, the LOW end of the sonic 'spectrum" -- the bass notes of the whales' song. High-frequency sound, as with high-frequency light, is more easily scattered and dissipated in both air and water. The U.S. submarine service, for instance, uses Extremely Low Frequency (ELF) radio transmitters to send signals to vessels that remain on-station and submerged for weeks at a time.
The article here refers to "ultrasonic vibrations", so we're talking high-frequency, short-penetration waves (which probably explains the statement that "(i)t seems certain that performance could be traded for range," that is; that the frequency could be lowered, allowing greater penetration while lowering the amount of data that could be carried by those fewer cycles per second.
So, I suspect that, unless the whales are snuggling up to the sides of a nuclear "boomer", they're reasonably safe.