what is the meaning of "x dB of isolation" for Faraday cage box? I need a Faraday-cage box that block the device from transmitting or receiving any signals when it is in it.
When I search for such a box I always see the companies that are making describe it is performance like:

10 kHz to 6 GHz - Up to 70 dB plane wave RF shielding isolation (typical 50 db @ 1 GHz))

OR the other company says: 

the isolation is >90dB @ 2GHz.

Please could some one explain for me in plain English what they mean? 
 A: Definition of dB
Fist we must understand the meaning of "dB".
The symbol "dB" means "decibel" and is la dimensionless unit of measure.
Given a quantity $N$, the value in the dB scale is
$$N_\text{dB} \equiv 10 \times \log_{10} N \, .$$
For example, the quantity $N = 1000$ becomes
$$N_\text{dB} = 10 \times \log_{10}(1000) = 10 \times 3 = 30$$
in the dB scale.
The dB scale is useful because it allows expression of vary wide range of numbers without having to write many zeros.
This is simply due to the use of the $\log_{10}$.
Electromagnetic isolation
When building a Faraday cage, the goal is to prevent external electromagnetic radiation from coming inside the cage.
Of course, it is never possible to completely eliminate all radiation from entering the cage.
The isolation is a measure of how much the cage removes incoming radiation.
Suppose we shoot some microwave frequency (1 GHz) radiation at the Faraday cage.
The incoming radiation has a certain power $P_\text{in}$.
As the radiation travels through the walls of the cage, most of it is reflected; that's the point of the cage.
However, some amount, which we call $P_\text{transmitted}$, gets through.
The isolation is defined as
$$\text{isolation} \equiv  P_\text{in} / P_\text{transmitted} \, .$$
For example, if we shoot $ 1 \text{W}$ toward the cage and $1 \text{mW}$ gets through, the isolation is 1000.
You can say "the cage has an isolation of one thousand".
Rather than specify the isolation in this way, it is more common to specify it in the dB scale.
Therefore, rather than saying we have an isolation of 1000 we say we have "30 dB of isolation".
The ratio of the incoming to transmitted power depends on frequency of the radiation.
That is why the companies are telling you the isolation at a specific frequency.
We can now understand the specifications given by the companies

10 kHz to 6 GHz - Up to 70 dB plane wave RF shielding isolation (typical 50 db @ 1 GHz))

This means


*

*For radiation at a frequency from 10 kHz to 6 GHz you can expect isolation.

*The highest isolation over this range is 70 dB (but they don't tell you at what frequencies this is achieved).

*At 1 GHz frequencies the cage usually gets about 50 dB of isolation. They say "typical" meaning that's what they usually get, but they do not guarantee it.


Normally things like this come with a minimum guaranteed isolation, but the sentences you provided do not.

the isolation is >90dB @ 2GHz.

This simply says that for radiation at 2 GHz frequency, the isolation is better than 90 dB i.e. better than $10^9$.
