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Wrong, a power ratio of 1000:1 is 30db.Well if you look at it that way, a microwave oven is say 1000 watts, to make it safe outside the microwave you need to drop that to 1watt, that's 1/1000th the power. Only 10db attenuation required.
About screened enclosures,
where there are holes in the enclosure, for a fixed hole size the effectiveness of the screening is lower at higher frequencies.
The effect is known as "Waveguide Below Cutoff" (Google it), essentially the attenuation of an electromagnetic wave travelling along a waveguide or through an aperture increases greatly below some frequency.
As mentioned earlier, the effect is well demonstrated by the effects of radio reception in a vehicle passing under a bridge.
Reception of VHF signals (about 100Mhz) is relatively unaffected as the aperture under the bridge is large compared with the wavelength of the radio signal.
Reception of MF (0.5 to 1.5Mhz) on the other hand, is severely affected because aperture under the bridge is small compared to the wavelength of the radio signal.
In this context the bridges I am thinking of are small, less that 10 metres high, bridges like the Forth Bridge, Humber Bridge or Golden Gate Bridge are an order of magnitude larger.
I have just had a look at my microwave oven, the flange on the door is about 3.5cm wide.
The oven runs at 2400Mhz or so, which gives a wavelength of 12.5cm.
The door flange is a quarter wave choke at 2400Mhz, at 1900Mhz it will be much less effective, hence some of the effects observed with the mobile phone.