# Why atteunation of the cut-off frequency is 6dB in MATLAB filter Designer

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YanBo Liu on 8 May 2022
Answered: Jonas on 8 May 2022
I am new to FIR filter deign. I have a question about the MATLAB filter designer. I notice that the atteunation at the cut-off frequency is fixed at 6dB. So if I want my atteunation to be 3dB, what should I do?

Jonas on 8 May 2022
sounds like you have a filter ehich has a attenuation slope of 6dB/octave (doubling of frequency). Assuming you want to keep the -6dB per octave slope, you want to shift you nominal cutoff frequency such that the frequency you are interested in is in the middle of the octave (half octave -> 0.5*-6dB=-3dB). half octave is Factor 3/2, so the starting point is at 2/3 of the specific frequency.
example you have -6dB at 100Hz, so at 50Hz der should be the last frequency with 0dB and at 200Hz you have -12dB.
if you want -3dB at 100 Hz: last point at 0dB would be 2/3 * 100Hz. the middle if octave is then 1.5 * 2/3 * 100Hz=100Hz and you have -6dB at one octave= 2*2/3*100Hz=133.33Hz