# Filtering a signal out of multiple frequencies without for loop

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ytzhak goussha on 17 Mar 2020
Answered: Raunak Gupta on 19 Mar 2020
Hello MATLABers,
I am trying to filter out a humming noise from my recordings. The noise has a frequency of 20kHz and it's harmonics (40,60,80 kHz).
I want to filter out all the noisy parts with one pass over the signal and the only solution I have managed to come up with is by removing each frequency with a for loop (see code). This takes a lot of time, I need to filter about 2 hours of audio recordings with 250k sampling rate.
My question is, is there a method to filter out all the desired frequencies in one pass instead of multiple passes to save computational time.
Fs = 250000; %Sampling rate
filtered =sig; %sig is a vector of the signal to be filtered
%The frequency of the noise to be filtered
onset = 19700;
offset = 20000;
times = [];
n=4; %Number of harmonics (n=1: 20, n=2: 40, n=3: 60, m=4: 80)
for k = 1:n
tic
%Is there a filter that can remove multiple bands at onces?
d = designfilt('bandstopiir','FilterOrder',2, ...
'HalfPowerFrequency1',onset*k,'HalfPowerFrequency2',offset*k, ...
'DesignMethod','butter','SampleRate',Fs);
filtered = filtfilt(d,filtered);
toc
times = [times,toc];
end

Raunak Gupta on 19 Mar 2020
Hi,
There is a similar question which is nicely explained by Star Strider, this can help you implement the multiband filter required here. It uses filtfilt.

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