I am using Audacity to filter out certain frequencies from a click and see if there is preferred freq that does not interfere with a task. when I apply a low pass filter at say 2000Hz and plot the spectrum there is still low freq present. I know the click is a broadband noise is this possibly a reason why?
A high-pass filter allows frequencies above the “filter frequency” to pass, virtually unaffected, while lower frequencies are progressively reduced in level.
It is never a total “cut off” at the filter frequency, but always a progressive “roll off”.
The “steepness” of the roll-off is called the “roll-off rate” and is often described in “dB per octave”.
An octave is equivalent to a halving, or doubling of the frequency.
With a 6 dB per octave high pass filter, with a filter frequency of 1000 Hz,
frequencies of 500 Hz (one octave below the filter frequency) will be reduced by 6 dB,
frequencies at 250 Hz (2 octaves below the filter frequency) will be reduced by 12 dB
frequencies at 125 Hz (3 octaves below the filter frequency) will be reduced by 18 dB
and so on.