# Electronic – Cutoff frequency: exactly -3 db of power

cutoff frequencyfilter

I have a question which may sound strange, but still I'd like to ask community to explain that. Hope other people may find it interesting aswell.

As we know, cutoff frequency is the one frequency harmonics of which is being reduced by 3 db (or Vpp is sqrt(2) times lesser than harmonics Vpp before the filter).

This image gives more or less clear picture of that idea (consider the red line, cutoff frequency marked as fcp):

What that means: the filter passes all harmonics of the input signal through, but some of them are being reduced in power. Harmonics of some frequency are being reduced by 3 db, this frequency is a cutoff frequency of that particular filter.

This is the defininition and the explanation of my understanding of the term. I'd be grateful for corrections, if any ðŸ™‚

Now, the question itself: the word cutoff means that something is being cut. So, we may say that harmonics of particular frequency are filtered, if Vpp reduced by 3 db or more. The question is – why exactly 3 db? Saying exactly I mean

1. In a one-pole RC filter, the 3-dB frequency is conveniently found at a radian frequency of \$\dfrac{1}{RC}\$.