Audacity Parametric Equalizer: EQ Application Concepts in Audio Mixing

This is a tutorial about the best practices of EQ plugins built for Audacity or Ardour in Linux. The method in this tutorial is tested to work perfectly under Linux operating system, although it is not tested in Windows environment. This is based on LADSPA (Linux Audio Developer Simple Plugin API). The name of the plugin is swh-plugins or Steve Harris LADSPA plugins. To enable this EQ plugin feature in your Audacity installation, follow the steps below:

1.) Install swh-plugin in your Linux computer. Follow the installation steps of swh-plugin tutorial here.:. Follow the procedures under “Assuming you are using Ubuntu…, follow the rest of the steps”. Make sure you have successfully installed the plugin in your Linux computer.

2.) Launch Audacity and load a sample WAV audio file. The most important EQ effect is the “Triple band parametric with shelves”. To use this effect, go to Effect– Plugins 106 to 114 – Triple band parametric with shelves. This EQ effect can be used to do some parametric equalization(band-pass filtering) , high shelving and low shelving filtering tasks which are very important in shaping up the audio when doing mixing.

3.) To demonstrate the usage of this plugin, lets first define what needs to be set using this effect. This is very helpful for beginning users of this plugin who does not have strong background in audio engineering.

LOW SHELVING filter = this filter will allow frequencies ABOVE the “low shelving frequency” cutoff point to pass through unaffected. This filter will ONLY reduce or boost signals of frequencies BELOW the “low shelving frequency” cutoff. This works similarly to a high pass filter except that a high pass filter drastically cuts the frequencies below its cutoff. The amount of reduction and boost for a low shelving filter depends on the “low-shelving gain” and “low-shelving slope”. Combination of high amounts of low-shelving gain cutting decibels and high low-shelving slope drastically removes signficant amount of signals below the low shelving frequency cut-off.

HIGH SHELVING filter = this filter works in the opposite of low shelving filter. This will allow frequencies BELOW the “high shelving frequency” cutoff point to pass through unaffected while it ONLY reduce or boost signals of frequencies ABOVE the “high shelving frequency”. This behaves similarly to low pass filter only that low filter drastically cuts frequencies above its cut-off.

BAND-PASS filter – this filter does the actual work of a parametric equalization theory . This is defined in terms of Q (how wide or narrow are the gain/boost reduction, the center frequency and the amount of db to be cut of boost. This is called “triple band parametric with shelves” because it allows you to use this EQ for low shelving, high shelving and band-pass filtering tasks. Actually this EQ features: one low shelving filter, one high shelving filter and 3 band-pass filters. Practical example: Supposing you will use the triple band parametric to curve out the audio with the frequency spectrum below:

audio before applying tri-band parametric

audio before applying tri-band parametric

And you would like to apply the following EQ settings:

Low shelving filter (for removing signals below 200Hz)
Low shelving gain: -30dB
Low shelving frequency cutoff= 200Hz
Low-shelving slope= 1 (maximum)

Band pass filter1
Gain: Boost +12dB
Center frequency: 300Hz

Band pass filter2
Gain: Cut -12dB
Center frequency: 500Hz

Band pass filter3
Gain: Boost +12dB
Center frequency: 1000Hz

High shelving filter (for removing signals above 2000Hz)
Gain: Cut -24dB
High shelving frequency cutoff= 2000Hz
High shelving slope= 1 (max)

This is how the settings are done in the Triple band parametric EQ with shelves:

Tri-band parametric EQ settings in Audacity

Tri-band parametric EQ settings in Audacity

After EQ adjustment, when you check the frequency spectrum of the resulting audio, it will look like below:

Audio signal after adjustment

Audio signal after adjustment