"pretty well" means 40-ish dB attenuation - that's about 10000 times weaker signal, absorbing 99.99% of it. But that's not even close to sufficient for actuall

"pretty well" means 40-ish dB attenuation - that's about 10000 times weaker signal, absorbing 99.99% of it. But that's not even close to sufficient for actually isolating wireless transmissions. My microwave got 35-38 dB for WiFi signals when I measured it, and that got through a fairly reliable connection. You might want 90 dB or something like that if you want to ensure that you can't extract the data from the noise - so perhaps a starting point might be to put your device in a small microwave oven and put that oven in a large microwave oven :)