- Filter type: [High Pass, Low Pass, Band Pass, Band Cut. Default High Pass]
- High Pass Filter passes frequencies above its cutoff frequency and attenuates frequencies below its cutoff frequency. This effect can therefore be used to reduce low frequency noise.
- Low Pass Filter passes frequencies below its cutoff frequency and attenuates frequencies above its cutoff frequency. This effect can therefore be used to reduce high pitched noise.
- Band Pass filter passes frequencies within the specified frequency range and attenuates frequencies below and above this frequency band.
- Band Cut Filter attenuates frequencies within the specified frequency range and passes frequencies below and above this frequency band.
- Rolloff (dB per octave): [6 dB, 12 dB, 24 dB, 36 dB, 48 dB. Default 6 dB]
Sets the steepness of the attenuation. Higher rolloff values give a steeper attenuation of frequencies above the cutoff frequency. For example, with a rolloff of 6 dB per octave, the sound decreases by 6 dB in amplitude for each octave above the cutoff frequency (an octave above is double the frequency)
- Frequency (Hz): [1 to 20000 Hz. Default 1000]
For high pass and low pass filters, this is the -3 dB corner frequency. For band pass and band cut filters this is the centre frequency.
- Band filter width (Hz): [1 to 10000 Hz. Default 1000 Hz]
This applies only to band pass and band cut filters and is the width of the filter frequency band.
- Crossfade filter in/out (ms): [0 to 100 milliseconds. Default 0]
When set greater than 0, the processed sound will be introduced with a short crossfade from the original audio at the beginning and end of the selection. This control sets the length of the crossfade.
http://wiki.audacityteam.org/wiki/Downl ... g_Plug-ins
After installation and restarting Audacity the effect will appear in the Effect menu and is called "Filter..."
The "Crossfade filter in/out" control:
When applying a filter to a section of audio, there is likely to be a "glitch" (pop) occur at the start / end of the selection unless the selection can be made such that the waveform is close to silence at the start/end of the selection. Sometimes it is not possible or not practical to make the selection start and end close to silence and this is the purpose of the "Crossfade filter in/out" feature. If this control is set greater than zero, there will be a short crossfade automatically applied at the start and end of the selection, thus avoiding clicks at the start or end. A setting of around 20 milliseconds is usually sufficient to avoid clicks.
If required for a special effect, the crossfade duration may be set longer than 100 ms by typing the required value in the text box. The crossfade duration cannot be more than half of the length of the selection.
Filter frequency range:
Filter frequencies cannot be greater than half the sample rate. Note that for a band pass or band cut filter the filter frequency will be higher than the "centre frequency". for example, for a centre frequency of 10000 Hz (10 kHz) and a band-width of 15000 Hz (15 kHz) the audio will be filtered between 500 Hz and 20000 Hz (10000 Hz is the centre of this range on a logarithmic scale).