I’ve been playing around with the high and low pass filters to try to remove unnecessary parts of audio files. I’ve noticed in the spectrogram that the high and low pass filters don’t remove frequencies below or above the cutoff frequency; they only reduce the amplitude of those frequencies. Is there any way to remove ALL of the sound with frequencies specified in a filter rather than make them quieter like the high/low pass filters do?
I’m running Audacity 2.0.5 on Lubuntu 14.10.
Thanks in advance!
ALL high pass/low pass filters have a “roll-off slope”. This is the “rate at which frequencies are attenuated beyond the filter frequency”.
This graph shows the response of a “6 dB/octave high pass filter”:
The "filter frequency) (also called the “cutoff frequency” or “corner frequency”) is usually defined as the -3 dB point - that is, the frequency at which the signal is reduced by 3 dB.
Different filters will “roll-off” at different rates (often quoted as “dB per octave” or “dB per decade”). The Audacity High Pass / Low Pass filters provide a range of roll-off settings from 6 dB/octave to 48 dB per octave. For more details see these pages in the manual:
It is not possible for a filter to cut off totally at a specified frequency, though some types of filter can roll off extremely steeply.
If you apply the High Pass or Low Pass filters, set at 48 dB per octave, twice to the same audio, then you will achieve approximately 96 dB per octave, but note that the -3 dB point will now be -6 dB, so for a high pass filter you may want to set the corner frequency a little lower, or vice verse for a low pass filter.
For extremely steep filters, try using the Equalization effect (http://manual.audacityteam.org/o/man/equalization.html)
Here is an example of an extremely steep low pass filter. The blue line shows my settings and the green line shows the actual filter behaviour:
(note all of the slider settings as well as the graph).