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While editing a song, I tried to cut out the bass drum by applying a high-pass FFT filter, i.e. filtering out the low pitch of the bass drum. This resulted in the drum sound being severely quieted, rather than removed entirely. I know that higher overtones from the drum would still be present in the audio, but why isn't the lower pitch completely removed?

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Really muting sound below a certain frequency only happens when using a so-called 'brickwall' filter. Are you sure you are using such a type of filter? –  Pelle ten Cate Dec 9 '10 at 9:45
    
I believe I was, but I'm not entirely sure how Adobe Audition does it. –  Matthew Read Dec 10 '10 at 4:05
    
Can you see the low tones in a spectrum, or you just perceive that they're still there? Audition has settings in the FFT filter dialog to make the filter work better. Did you try adjusting them? –  endolith Dec 15 '10 at 16:43

2 Answers 2

up vote 7 down vote accepted

First of all, tell us more about the filter. Is it really a brickwall filter, or are you actually using a low roll-off or low cut-off filter? In the latter case, it is perfectly normal, when using a high cut-off filter, the low frequencies decrease with -6dB per octave below the specified frequency, and are not completely gone. Roll-off is the same, but with less dBs per octave. (either 4,5 or 3, can't really remember.)

If you did use a brickwall filter, there is another possible explaination:

A phenomenon in psycho-accoustics is the hearing of a low note where it is not there, but when the harmonics (= overtones) are there. Every base tone has different overtones. (Really, if multiple overtones are there, there is really not much ambiguity.) The human brain actually processes these overtones and adds the base tone.

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Hmm, psycho-acoustics sounds interesting ;) thanks! –  Matthew Read Dec 10 '10 at 4:06

In general a filter is not something that completely removes a given set of frequencies. Rather, it just attenuates them - cutting some percentage of them out. In extreme cases (kill EQs on DJ mixers come to mind) the attenuation is so much that it may as well be considered getting rid of it entirely.

Wikipedia has a pretty good graph of how a low pass filter cuts out higher frequencies in their article on Low-pass filters. A low-pass filter can be thought of as a "high-reducing" filter if that makes more sense.

This behavior shows up whether the filters are FFT-based, analog, whatever.

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Not entirely correct, you can actually get à real brickwall filter using FFT, as long as the FFT reaches the top frequency of your sample rate. That is, you can have an FFT filter that filters 5000Hz with 0dB and 4999Hz with -96dB. One of the advantages of a Fourier filter. –  Pelle ten Cate Dec 10 '10 at 6:57
    
@Pelle I didn't realize they could actually get that extreme! Thank you for the clarification :) –  Warrior Bob Dec 10 '10 at 10:36
    
Normal filters can't basically, since you always deal with 'filter components'. FFT filters are different, they can get any shape possible within the frequency and the dynamic range, and they can have any phase characteristics you wish. (That is why convolution reverb is based on FFT as well.) –  Pelle ten Cate Dec 10 '10 at 11:15
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with an FIR filter or FFT-filter (which is kind of the same thing in disguise) you can get an arbitrarily precise and sharp frequency response. See e.g. DSP Guide Chapter 17 –  Thies Heidecke May 22 '11 at 19:25

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