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 f_{cp}):

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

## Best Answer

The 3-dB cutoff is just one commonly used way to describe a filter. For some applications you might want to specify the 10-dB cutoff or 60-dB cutoff instead. It is convention that if someone says "cutoff frequency" without being more specific, they are talking about the 3-dB cutoff. You should think of the 3-dB cutoff as the frequency where the filter begins to roll off, not where it is strongly attenuating the signal.

There are a couple of practical reasons to use the 3-dB point as the conventional cut-off frequency.

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

If you draw a piecewise linear approximation to your one-pole transfer function plot, using a horizontal line segment for the low frequencies, and a 20-dB per decade sloped line for the frequencies well above cut-off, these two approximate lines will meet at the 3-dB frequency. Often when we just want to sketch a frequency response we make a Bode plot, which uses exactly this approximation.

(image courtesy Wikimedia)