Hi,
8 replies, but no comment from the OP...Are you satisfued with the answers or do you need more information?
3dB line:
Imagine a low pass RC filter.
From left to right you see a line.
At the beginning it is straight and horizontal, then around cutoff frequency it is bent, after that it is straight with a constant slope.
Now extend the horizontal line to the right, and the line with the constant slope to the left.
They cross each other at cutoff frequency. And you can see the gain of the bent line at this frequency is -3dB.
Klaus
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Added.
While a simple RC has an exact rate of 20.000dB/decade....
The commonly known 3dB are axactely 3,010299956dB.
Klaus