[Johnythree]

The door seals in most microwave ovens consist of tuned slots (eg half-wave stubs) to prevent the RF escaping. These are ineffective at frequencies other than 2.4Ghz.

The reason for the tuned slots is that it's basically impossible to make a conventional metal joint which will seal sufficiently well.

[kurthr]

I encourage you to test a few. I found that the cheap ones blocked 2.45GHz ~60dB, but that they were still down 40dB at 600MHz... the real problems were at higher frequencies and 5GHz was only down ~20-30dB at the worst case polarization. I tried using RF absorbing tape, but while it's possible to improve another 10-20dB it's painful and not repeatable without testing, if you need to remove it.

I've also worked in RF Antenna test rooms with seals that are expensive, finicky, and easy to damage... they're also at least $10k and usually much much more. In a corporate environment that's not a problem, but for a home lab it's unrealistic.

If these bags were big enough to put a few phones and a small base antenna in they'd have other uses.