## -3 dB point of a cutoff frequency

Mar 2, 2011 at 10:43pm

# -3 dB point of a cutoff frequency

HI,
working with a low-pass filter I realized the amplitude response at the cutoff frequency is not 0.707 but 0.98.
Could someone explain why this value is so different respect what we learned in many articles on filters???
Thanks
fv

#55245
Mar 2, 2011 at 10:49pm

How are you measuring that?

#198850
Mar 3, 2011 at 6:02pm

The peak amplitude (out) at a whatever cut frequency.

#198851
Mar 3, 2011 at 6:29pm

why should it be 0.707?

isnt 0.707that what we should exspect from a onepole filter
at one octave _above the cut frequency?

:)

#198852
Mar 3, 2011 at 7:50pm

My question is how are you isolating the frequencies at the cutoff so that you are only measuring their amplitude? If you’re using something like peakamp it will measure the amplitude of the entire signal.

#198853
Mar 3, 2011 at 8:24pm

what i’ve learned at school is -3dB at the center frequency and a -6/dB per octave slope for first order filter. So at twice the center frequency it should be -9dB…

But it seems, that these values are dependant of the center frequency for [onepole~] implementation…

Here’s something to play with…

– Pasted Max Patch, click to expand. –
#198854
Mar 4, 2011 at 10:00am

so, as @mudang showed in its patch, – 3dB point of the cutoff frequency is only around 1 Hz…
when we increase the fc values, the “-3dB point” decreases, becoming 2 dB, 1 dB and so on…

#198855

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