Hi
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?
Anyway any advice would be gratefully received.
Emma
Click the “Replot” button to refresh the spectrum plot.
i have tried this and it does not make a difference seen as I exit each spectrum plot after each hi/low pass filter an re-analyse.
Am I right in thinking that if I high pass filter the low freq should not be there at all?
Thanks for the reply
Emma
Not quite.
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.
For example:
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.
The manual for the next version of Audacity has more explanation about this: http://manual.audacityteam.org/man/High_Pass_Filter
If you have one particular thing you want to get rid of, Effect > Notch Filter may be better for you.
You are also warned that low frequencies are a little magic because once you get down below 100 or so, all the waves look the same and they’re really difficult to tell apart.
Here’s a software rumble filter I “built.”
http://kozco.com/tech/audacity/100Hz_Rumble.jpg
Note that there are green and blue lines. The blue one is the one I wanted and the green was the one that actually happened. The green one is sloppier.
Also note that the filter only goes down to -40dB. That’s really, really quiet, but not gone. you need to get into the -60dB range to make it mostly gone and it never completely goes away.
Fortunately, bass notes, hum, and rumble can’t be heard easily, so most people ignore them.
Koz