To emulate a piano soft pedal (una corda pedal) effect in LMMS, you click on the track > the settings window opens > click FX > click EFFECTS CHAIN (so it turns green) > click the Add Effect button > a window opens with a list of effects > type the word equal in the filter field at the top > choose LADSPA Effect: TAP Equalizer
In the TAP Equalizer controls window, double click the following controls and set their values to -20 (or more, depending on your taste):
Band 5 Gain (dB)
Band 6 Gain (dB)
Band 7 Gain (dB)
Band 8 Gain (dB)
Earlier this morning, I had no clue about how to achieve this, but thanks to a post by steve on the audacity forums, I was alerted to equalization. Then it was just a matter of playing around with the LADSPA plugin until I figured it out.
See also my tutorial about how to create a sustain pedal effect in LMMS for a piano soundfont.
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